Radio frequency (RF) amplifier

Embodiments of a device and method are disclosed. In an embodiment, an RF amplifier includes first and second RF signal paths having RF input interfaces, RF output interfaces, and corresponding transistors connected between the respective RF input interfaces and RF output interfaces, wherein control terminals of the transistors are connected to the RF input interfaces and current conducting terminals of the transistors are connected to the corresponding RF output interfaces. The RF amplifier including a conductive path between the current conducting terminal of the first transistor and the current conducting terminal of the second transistor, wherein the conductive path includes a first inductance, a second inductance, and a capacitance electrically connected between the first inductance and the second inductance.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority under 35 U.S.C. § 119 to European patent application no. 19306423.5, filed Nov. 4, 2019 the contents of which are incorporated by reference herein.

BACKGROUND

Radio Frequency (RF) amplifiers are used extensively in wireless communications systems, including in Base Transceiver Stations (BTSs). Such RF amplifiers typically include at least one RF transistor that is packaged into a discrete device that is then attached to a printed circuit board (PCB) to form an RF amplifier. When used in BTSs for wireless communications, it is desirable that RF amplifiers are compact, thermally efficient, and able to efficiently deal with modulated signals that exhibit a high Peak-to-Average Ratio (PAR).

SUMMARY

Embodiments of a device and method are disclosed. In an embodiment, an RF amplifier includes a first RF signal path having a first RF input interface, a first RF output interface, and a first transistor connected between the first RF input interface and the first RF output interface, wherein a control terminal of the first transistor is connected to the first RF input interface and a current conducting terminal of the first transistor is connected to the first RF output interface, a second RF signal path having a second RF input interface, a second RF output interface, and a second transistor connected between the second RF input interface and the second RF output interface, wherein a control terminal of the second transistor is connected to the second RF input interface and a current conducting terminal of the second transistor is connected to the second RF output interface, and a conductive path between the current conducting terminal of the first transistor and the current conducting terminal of the second transistor, wherein the conductive path includes a first inductance, a second inductance, and a capacitance electrically connected between the first inductance and the second inductance.

In another embodiment, a packaged RF amplifier device is disclosed. The packaged RF amplifier device includes a device substrate, a first RF signal path having a first RF input lead, a first RF output lead, and a first transistor coupled to the device substrate and electrically connected between the first RF input lead and the first RF output lead, wherein a control terminal of the first transistor is connected to the first RF input lead and a current conducting terminal of the first transistor is connected to the first RF output lead, a second RF signal path having a second RF input interface, a second RF output interface, and a second transistor coupled to the device substrate and electrically connected between the second RF input lead and the second RF output lead, wherein a control terminal of the second transistor is connected to the second RF input lead and a current conducting terminal of the second transistor is connected to the second RF output lead, and a microstrip transmission line between the current conducting terminal of the first transistor and the current conducting terminal of the second transistor, wherein the current conducting terminal of the first transistor is connected to the microstrip transmission line by at least one conductive wire bond and the current conducting terminal of the second transistor is connected to the microstrip transmission line by at least one conductive wire bond and further including a capacitance electrically connected to the microstrip transmission line.

In another embodiment, a push-pull RF amplifier is disclosed. The push-pull RF amplifier includes an input balanced unbalanced transformer (balun), an output balun, and an RF amplifier connected between the input and output baluns. The RF amplifier includes a first RF signal path having a first RF input interface, a first RF output interface, and a first transistor connected between the first RF input interface and the first RF output interface, wherein a control terminal of the first transistor is connected to the first RF input interface and a current conducting terminal of the first transistor is connected to the first RF output interface, a second RF signal path having a second RF input interface, a second RF output interface, and a second transistor connected between the second RF input interface and the second RF output interface, wherein a control terminal of the second transistor is connected to the second RF input interface and a current conducting terminal of the second transistor is connected to the second RF output interface, and a conductive path between the current conducting terminal of the first transistor and the current conducting terminal of the second transistor, wherein the conductive path includes a first inductance, a second inductance, and a capacitance electrically connected between the first inductance and the second inductance.

DETAILED DESCRIPTION

RF power amplifiers used in base station transmitters for the wireless infrastructure market (including the 4G and 5G markets) should be compact, configured to dissipate significant thermal energy generated by the amplifier transistors with relatively compact heat-dissipation structures, and should be able to handle modulated signals with a high Peak-to-Average Ratio (PAR). To address the thermal constraint of such amplifiers, efficient solutions that utilize gallium nitride (GaN) and silicon-based power transistor technologies have been considered. Additionally, to address the high PAR of the signals and to improve efficiency in back-off, advanced power amplifier techniques, including envelope tracking, have been considered. Envelope tracking involves continuously adjusting the power supply voltage applied to the RF amplifier to ensure that the RF amplifier operates at peak efficiency for the power required at each instant of transmission.

Conventional RF amplifiers are typically designed with a fixed supply voltage and operate most efficiently only when operating in compression. Such conventional RF amplifiers can become less efficient as the crest factor of the signal increases because the RF amplifier spends more time operating below peak power and, therefore, spends more time operating below its maximum efficiency. When envelope tracking is utilized, an RF amplifier is tuned close to its maximum efficiency at a low drain-source voltage (Vds), as the RF amplifier spends most of the time at a low Vds to maximize the efficiency. However, the RF amplifier should be able to deliver power at maximum Vds.

Thus, envelope tracking can increase RF amplifier efficiency not only at full BTS load (e.g., 8 decibels (dB) output power back-off (OBO)) but also at low loads (e.g., 18 dB OBO) where the BTS operates most of the time. Envelope tracking can therefore enable both capital expense savings due to higher efficiency at full load (e.g., smaller heat spreader needed so smaller remote radio head (RRH)), and operating expense savings for operation at medium to low loads.

A conventional RF amplifier structure used with envelope tracking includes multiple (e.g., two) transistors combined in quadrature with 3 dB couplers and no output matching. Although such an RF amplifier configuration may have desirable efficiency, such a configuration may exhibit peak-power (P2 dB) dispersion at a low Vds. Such dispersion can negatively impact an envelope tracking system. Such a system can be configured with in-package output impedance matching. However, large capacitors may be necessary. For example,FIG. 1depicts an RF amplifier100that includes a transistor assembly102having two transistors104-1and104-2that form portions of two RF signal paths110-1and110-2that have balanced RF power amongst the two paths. The transistors104-1,104-2produce amplified signals that are respectively split and combined in quadrature by two couplers106and108(with coupler106configured as a splitter and coupler108configured as a combiner). In particular, the RF power amplifier includes the input coupler106(configured as a splitter), first and second printed circuit board (PCB) input matching circuits112-1and112-2, first and second in-package input impedance matching circuits114-1and114-2, first and second transistors104-1and104-2, first and second in-package output impedance matching circuits120-1and120-2, first and second PCB output matching circuits122-1and122-2, and the second coupler108(configured as a combiner). In an embodiment, the couplers106and108are 3 dB hybrid couplers that provide balanced power coupling (e.g., splitting or combining). In an embodiment, the couplers106and108and transistor assembly102are standalone packaged devices that are mounted on a substrate (e.g., a PCB) and the PCB input matching circuits112-1and112-2and PCB output matching circuits122-1and122-2are components that may be, for example, integrated into the PCB and/or formed by discrete components (e.g., inductors and capacitors) that are attached to the PCB.

In the example shown inFIG. 1, each transistor104-1and104-2is shown to include a drain-source capacitance124-1and124-2, Cds. Equivalent circuits of the in-package output impedance matching circuits120-1and120-2in both RF signal paths110-1and110-2are shown and include a first inductance126-1and126-2, a second inductance128-1and128-2, a third inductance130-1and130-2(e.g., shunt inductance), and capacitance132-1and132-2(e.g., shunt capacitance). An RF power amplifier with the in-package output impedance circuit configured as shown inFIG. 1can help to control dispersion on the load pull (LP) contours. However, a relatively large capacitor (e.g., greater than 80 picofarads (pF) per side, dependent on frequency of operation) is needed for the shunt capacitor in both in-package output impedance matching circuits. For example, a capacitor on the order of 130 pF is used for the capacitor in both RF signal paths for a total capacitance seen by a modulator of 260 pF coupled to the amplifier output.

In a push-pull RF amplifier configuration, only one RF signal path is on (i.e., conducting current from input to output) at a time such that the total capacitance seen by the modulator is only about one-half (e.g., 130 pF), as opposed to the total capacitance (e.g., 260 pF), in the balanced RF amplifier configuration as shown inFIG. 1. Thus, as implemented in various embodiments of the inventive subject matter, the capacitance seen by a modulator coupled to an amplifier output can be reduced by combining two transistors in a push-pull configuration with balanced/unbalanced transformers (referred to as “baluns,” e.g., Marchand baluns that impart a 180 degree phase shift) and coupling the two RF signal paths at current conducting terminals (e.g., drains) of the transistors in a manner that shares a common capacitance and combines the inductances associated with the transistors to take advantage of a “virtual ground” that exists in a push-pull transistor configuration. Two examples of push-pull RF amplifiers that include a common capacitance are described with reference toFIGS. 2A and 2B. In contrast to the balanced RF amplifier depicted inFIG. 1, the push-pull RF amplifiers depicted inFIGS. 2A and 2Binclude a conductive path between the first and second RF signal paths that includes a common capacitance. In the embodiments ofFIGS. 2A and 2B, the common capacitance is located on the output side of the transistors and may be connected between the first and second RF signal paths in parallel or connected between the first and second RF signal paths in series. A push-pull RF amplifier configured with a common capacitor can significantly reduce the capacitance seen by a modulator at envelope frequencies while also exhibiting reduced dispersion of the peak power. Additionally, when using a push-pull RF amplifier as described herein, second harmonic power may be roughly 20 dB better in the compression region in the full band and at least 10 dB better than in the balanced configuration.

FIGS. 2A and 2Bdepict two embodiments of push-pull RF amplifiers200A and200B that include an RF transistor assembly202A and202B (having two transistors204-1and204-2) combined in a push-pull configuration by two baluns206and208. In the push-pull RF amplifiers depicted inFIGS. 2A and 2B, the first and second RF signal paths210-1and210-2include a conductive path between an output current conducting terminal (e.g., drain) of the first transistor and an output current conducting terminal (e.g., drain) of the second transistor, and a capacitance, which is electrically connected to the conductive path between a first inductance of the conductive path and a second inductance of the conductive path. In the embodiment ofFIG. 2A, the capacitor is connected in parallel between the first and second RF signal paths and in the embodiment ofFIG. 2B, the capacitor is connected in series between the first and second RF signal paths. The embodiments ofFIGS. 2A and 2Bare described below.

With respect to the various embodiments, the transistors204-1and204-2of the push-pull RF amplifiers200A and200B may be field effect transistors (e.g., III-V transistors, such as high electron mobility transistors (HEMTs)), which have a relatively low drain-source capacitance, Cds, when compared with other types of FETs (e.g., a silicon-based, laterally diffused metal oxide semiconductor (LDMOS) FET). InFIGS. 2A and 2B, the drain-source capacitance of the transistors204-1and204-2is represented with a capacitor224-1and224-2at the output terminal of the transistors. The capacitors224-1and224-2depicted inFIGS. 2A and 2Bare not physical components, but instead model the parasitic drain-source capacitance of the transistors. According to an embodiment, the transistors may have a drain-source capacitance that is less than about 0.2 picofarads/Watt (pF/W). Further, in some embodiments, the transistors may be GaN FETs, although in other embodiments, the transistors may be another type of III-V transistor (e.g., gallium arsenide (GaAs), gallium phosphide (GaP), indium phosphide (InP), or indium antimonide (InSb)), or another type of transistor that has a relatively low drain-source capacitance.

With reference toFIG. 2A, the push-pull RF amplifier200A includes the input balun206(configured as a splitter), first and second RF signal paths210-1and210-2that include first and second PCB input matching circuits212-1and212-2(i.e., input impedance matching circuits implemented on the PCB), first and second in-package input impedance matching circuits214-1and214-2, the first and second transistors204-1and204-2, an in-package output impedance matching circuit220A, first and second PCB output matching circuits222-1and222-2(i.e., output impedance matching circuits implemented on the PCB), and an output balun208(configured as a combiner). In the embodiment ofFIG. 2A, the input balun imparts about a 180 degree phase difference between the two RF signals that are conveyed through the first and second RF signal paths, and the output balun receives the 180 degree out of phase, amplified signals, and recombines the signals in phase. In an embodiment, the baluns206and208and the transistor assembly202A are standalone packaged devices that are mounted on a PCB and the PCB input matching circuits212-1and212-2and PCB output matching circuits222-1and222-2are components that may be, for example, integrated into the PCB and/or formed by discrete components (e.g., inductors and capacitors) that are attached to the PCB. InFIGS. 2A and 2B, the vertical dashed lines represent the transition between the PCB and a discrete packaged device that contains the components of the transistor assembly202A or202B.

As shown inFIG. 2A, the in-package output impedance matching circuit220A is modeled as a circuit that includes first inductances226-1and226-2, second inductances228-1and228-2, third inductances230-1and230-2(e.g., shunt inductance), and capacitance232(e.g., shunt capacitance) connected in parallel between the first and second RF signal paths210-1and210-2. In the embodiment ofFIG. 2A, there is a node236between the third inductances230-1and230-2, “connected in parallel,” as used in conjunction with the description of capacitance232, means that a first terminal of the capacitance232is coupled to the node236, and a second terminal of the capacitance232is coupled to a ground reference voltage or terminal of the packaged device.

With reference toFIG. 2B, the push-pull RF amplifier200B includes an input balun206(configured as a splitter), first and second RF signal paths210-1and210-2that include first and second PCB input matching circuits212-1and212-2(i.e., input impedance matching circuits implemented on the PCB), first and second in-package input impedance matching circuits214-1and214-2, first and second transistors204-1and204-2, an in-package output impedance matching circuit220B, first and second PCB output matching circuits222-1and222-2(i.e., output impedance matching circuits implemented on the PCB), and an output balun208(configured as a combiner). In the embodiment ofFIG. 2B, the input balun imparts about a 180 degree phase shift between the two RF signals that are output onto the first and second RF signal paths. In an embodiment, the baluns206and208and the transistor assembly202B are standalone packaged devices that are mounted on a PCB, and the PCB input matching circuits212-1and212-2and PCB output matching circuits222-1and222-2are components that may be, for example, integrated into the PCB and/or formed by discrete components (e.g., inductors and capacitors) that are attached to the PCB.

As shown inFIG. 2B, the in-package output impedance matching circuit220B is modeled as a circuit that includes first inductances226-1and226-2, second inductances228-1and228-2, third inductances230-1and230-2(e.g., shunt inductance), and capacitance238connected in series between the first and second RF signal paths210-1and210-2. In particular, “connected in series,” as used in conjunction with the description of capacitor238, means that one terminal of the capacitance238is connected to the first RF signal path210-1and the other terminal of the capacitance238is connected to the second RF signal path210-2.

In operation of either of the two push-pull RF amplifiers200A and200B, the input balun206splits an incoming RF signal into two components that are phase shifted by about 180 degrees with respect to each other. Because the RF amplifier has a push-pull configuration, when the common capacitance232is connected in parallel (FIG. 2A), the effective RF capacitance (e.g., that capacitance that corresponds to either one of the active RF signal paths) is one-half of the quadrature-combined amplifier, but the modulator still sees the entire magnitude of the common capacitance. In an embodiment, the magnitude of the capacitance232is much smaller than the magnitude of the capacitance132-1,132-2inFIG. 1. For example, the capacitance232may be on the order of 3-20 pF, dependent on operating frequency, and the capacitance132-1and132-2is on the order of 30-300 pF, also dependent on operating frequency-. When the common capacitance238is connected in series (FIG. 2B), the effective RF capacitance is still one-half of the quadrature-combined amplifier, but the modulator (at the envelope frequencies) now sees only the drain-source capacitance, Cds. In an embodiment, the magnitude of the capacitance238is much smaller than the magnitude of the capacitance132-1,132-2inFIG. 1. For example, the capacitance238may be on the order of 3-20 pF and the capacitance132-1and132-2is on the order of 30-300 pF. Series connection of the capacitance, as in the embodiment ofFIG. 2B, may provide other benefits with respect to an integrated passive device (IPD) standpoint as described below.

The RF amplifier assemblies202A and202B of the push-pull RF amplifiers200A and200B as depicted inFIGS. 2A and 2Brespectively, may be packaged into standalone/discrete devices that are attached and electrically coupled by leads of the packaged device to a substrate (e.g., a PCB) to produce the corresponding push-pull RF amplifiers. Examples of different embodiments of packaged RF amplifiers that may be incorporated into a push-pull RF amplifier are described below with reference toFIGS. 3A-10B.

FIGS. 3A and 3Bdepict an embodiment of a packaged RF amplifier302that includes a common capacitance in accordance with an embodiment of the invention. In particular,FIG. 3Adepicts a circuit-level layout of a configuration of the packaged RF amplifier andFIG. 3Bdepicts a corresponding component-level layout of the packaged RF amplifier. As shown in the circuit-level layout ofFIG. 3A, a capacitance332is electrically connected to an output terminal (e.g., the drain) of both of the transistors304-1and304-2such that the capacitance is electrically connected to both RF signal paths310-1and310-2, and as shown in the component-level layout ofFIG. 3B, a capacitance332(e.g., a capacitor) is electrically connected to a microstrip transmission line340on the output side (e.g., the drain side) of both of the transistors304-1and304-2such that the capacitor is common to both RF signal paths. In the embodiment ofFIGS. 3A and 3B, the common capacitance is connected to both RF signal paths in parallel.

With reference toFIG. 3A, the first and second RF signal paths310-1and310-2include (generally from left to right) first and second input leads342-1and342-2, first and second in-package input impedance matching circuits314-1and314-2, first and second transistor devices304-1and304-2, an in-package output impedance matching circuit320, and first and second output leads344-1and344-2. As shown inFIG. 3A, the first RF signal path310-1of the RF amplifier302includes the input lead342-1, the in-package input impedance matching circuit314-1, the transistor304-1, the in-package output impedance matching circuit320, and the output lead344-1, and the second RF signal path310-2of the RF power amplifier device302includes the input lead342-2, the in-package input impedance matching circuit314-2, the transistor304-2, the in-package output impedance matching circuit320, and the output lead344-2. Although the transistors304-1and304-2and various elements of the in-package input and output impedance matching circuits314-1,314-2, and320are shown as singular components, the depiction is for the purpose of ease of explanation only. Those of skill in the art would understand, based on the description herein, that the transistors and/or certain elements of the in-package input and output impedance matching circuits each may be implemented as multiple components (e.g., connected in parallel or in series with each other), and examples of such embodiments are illustrated in the other Figures and described later. Further, the number of input/output leads may not be the same as the number of RF signal paths and/or the number of transistors (e.g., there may be multiple transistors operating in parallel for a given set of input/output lead frames). The description of the transistors304-1,304-2and various elements of the in-package input and output impedance matching circuits314-1,314-2,320, thus are not intended to limit the scope of the inventive subject matter but only to the illustrated embodiments.

Referring toFIG. 3A, the input leads342-1and342-2and output leads344-1and344-2of the packaged RF amplifier302each include a conductor, which is configured to enable the packaged RF amplifier to be electrically coupled with external circuitry (not shown). More specifically, the input and output leads are physically positioned to span between the exterior and the interior of the device package. The in-package input impedance matching circuits314-1and314-2are electrically coupled between the input leads342-1and342-2and a first terminal (or input terminal) of the corresponding transistor304-1and304-2(e.g., the gate). Similarly, the in-package output impedance matching circuit320is electrically coupled between a second terminal of the transistors304-1and304-2(e.g., the drain terminal) and the output leads344-1and344-2.

According to an embodiment, the transistors304-1and304-2are the primary active component of the packaged RF amplifier302. The transistors include a control or input terminal (e.g., a gate terminal) and two current conducting terminals (e.g., source and drain terminals), where the current conducting terminals are spatially and electrically separated by a variable-conductivity channel. For example, the transistors may be field effect transistors (FETs) (as described above with reference toFIGS. 2A and 2B), which each include a gate (control terminal), a drain (a first current conducting terminal), and a source (a second current conducting terminal). For example, the transistors may include GaN HEMTs or silicon LDMOS FETs. Alternatively, the transistors may be bipolar junction transistors (BJTs). Accordingly, references herein to a “gate,” “drain,” and “source,” are not intended to be limiting, as each of these designations has analogous features for a BJT implementation (e.g., a base, collector, and emitter, respectively). According to an embodiment, and using nomenclature typically applied to MOSFETs in a non-limiting manner, the gate of each of the transistors is coupled to the corresponding in-package input impedance matching circuit314-1and314-2, the drain of each of the transistors is coupled to the in-package output impedance matching circuit320, and the source of each of the transistors is coupled to ground (or another voltage reference). Through the variation of control signals provided to the gate of the transistors, the current between the current conducting terminals of the transistors may be modulated.

The below-provided description is provided with regard to the first RF signal path310-1as depicted inFIG. 3Aalthough the description applies as well to the second RF signal path310-2as depicted inFIG. 3A. With reference to the first RF signal path, the in-package input impedance matching circuit314-1is coupled between the input lead342-1and the control terminal (e.g., gate) of the transistor304-1. The in-package input impedance matching circuit314-1is configured to raise the impedance of the packaged RF amplifier302to a higher (e.g., intermediate or higher) impedance level (e.g., in a range from about 2 to about 10 Ohms or higher). This can be advantageous in that it allows the printed circuit board level (PCB-level) matching interface from a driver stage to have an impedance that can be achieved in high-volume manufacturing with minimal loss and variation (e.g., a “user friendly” matching interface). In the embodiment ofFIG. 3A, the in-package input impedance matching circuit314-1includes first and second inductive elements346-1and348-1(e.g., two sets of bond wires347-1,349-1,FIG. 3B) and a shunt capacitor350-1(e.g., capacitor315-1,FIG. 3B). The first inductive element346-1(e.g., a first set of bond wires) is coupled between the input lead342-1and a first terminal of the capacitor350-1, and the second inductive element348-1(e.g., a second set of bond wires) is coupled between the first terminal of capacitance350-1and the control terminal of transistor304-1. The second terminal of capacitance350-1is coupled to ground (or another voltage reference). The combination of the inductive elements346-1and348-1and the shunt capacitor350-1functions as a low-pass filter. According to an embodiment, the series combination of inductive elements346-1and348-1may have a value in a range between about 50 picohenries (pH) to about 3 nanohenries (nH), and the shunt capacitor350-1may have a value in a range between about 5 picofarads (pF) to about 80 pF, although the inductance value may be lower or higher, as well.

The in-package output impedance matching circuit320is coupled between both the first and second RF signal paths310-1and310-2. In particular, the in-package output impedance matching circuit is coupled between the first current conducting terminal (e.g., drain) of the transistor304-1and the output lead344-1in the first RF signal path310-1and between the first current conducting terminal (e.g., drain) of the transistor304-2and the output lead344-2in the second RF signal path310-2. The in-package output impedance matching circuit320is configured to match the output impedance of the packaged RF amplifier302with the input impedance of an external circuit or component (not shown) that may be coupled to the output leads344-1and344-2. According to an embodiment, the in-package output impedance matching circuit320includes first inductive elements328-1and328-2, second inductive elements330-1and330-2(e.g., shunt inductance), and a common (or shared) capacitance332. The capacitance332may be referred to herein as the “shunt” capacitance, or Cshunt. The first inductive elements328-1and328-2(e.g., two sets of parallel bondwires329-1,329-2,FIG. 3B), which may be referred to herein as a “series inductor” or Lseries, are coupled between the first current conducting terminals (e.g., the drain) of the transistors304-1and304-2and the corresponding output leads344-1and344-2. The second inductive elements330-1and330-2(e.g., two sets of parallel bondwires331-1,331-2,FIG. 3B), which may be referred to herein as a “shunt inductor” or Lshunt, are coupled between the first current conducting terminals (e.g., the drain) of the transistors304-1and304-2and the shunt capacitance332. As is described below with reference toFIG. 3B, the transistors are connected to the capacitance by a microstrip transmission line, portions of which are represented inFIG. 3Aby elements340-1and340-2.

As is described in more detail below, various embodiments of the packaged RF amplifier302may include at least one integrated passive device (IPD) that forms all or a portion of the in-package input impedance matching circuits314-1and314-2and/or the in-package output impedance matching circuit320. In an embodiment, each IPD includes a semiconductor substrate and one or more integrated passive components and/or passive components coupled to the semiconductor substrate. In a particular embodiment, each IPD may include a shunt capacitance, and in some embodiments, a shunt inductance. In other embodiments, some or all of the portions of the in-package input impedance matching circuits and/or the in-package output impedance matching circuit may be implemented as distinct/discrete components or as portions of other types of assemblies (e.g., a silicon based passive circuit, a low-temperature co-fired ceramic (LTCC) device, a small PCB assembly, and so on). In still other embodiments, some or all of the portions of the in-package input impedance matching circuits and/or the in-package output impedance matching circuit may be coupled to and/or integrated within the semiconductor die that includes the corresponding transistor or transistors. The below, detailed description of embodiments that include IPD assemblies should not be taken to limit the inventive subject matter, and the term “passive device substrate” means any type of structure that includes a passive device, including an IPD, a LTCC device, a transistor die, a PCB assembly, and so on.

With reference toFIG. 3B, the packaged RF amplifier302includes a flange354(or “device substrate”), upon which the other components are attached, and which may provide a ground reference node for the amplifier302. The first RF signal path310-1includes an input lead342-1(e.g., input lead342-1,FIG. 3A), an input impedance matching circuit (e.g., the in-package input impedance matching circuit314-1,FIG. 3A), which includes first and second sets of wirebonds347-1,349-1(e.g., inductances346-1,348-1,FIG. 3A), and an input IPD315-1(including capacitance350-1,FIG. 3A), a transistor304-1(e.g., transistor304-1,FIG. 3A), an output impedance matching circuit (e.g., the in-package output impedance matching circuit320,FIG. 3A), which includes third and fourth sets of wirebonds329-1,331-1(e.g., inductances328-1,330-1,FIG. 3A), a transmission line inductance340(e.g., inductance340-1,FIG. 3A), and an output IPD360(including capacitor332,FIG. 3A). The first RF signal path310-1also includes an output lead344-1(e.g., the output lead344-1,FIG. 3A). The second RF signal path includes an input lead342-2(e.g., input lead342-2,FIG. 3A), an input impedance matching circuit (e.g., the in-package input impedance matching circuit314-2,FIG. 3A), which includes fifth and sixth sets of wirebonds347-2,349-2(e.g., inductances346-2,348-2,FIG. 3A), and an output IPD315-2(including capacitance350-2,FIG. 3A), a transistor304-2(e.g., transistor304-2,FIG. 3A), the output impedance matching circuit (e.g., the in-package output impedance matching circuit320,FIG. 3A), which includes seventh and eighth sets of wirebonds329-2,331-2(e.g., inductances328-2,330-2,FIG. 3A), a transmission line inductance340(e.g., inductance340-2,FIG. 3A), and the output IPD360(including capacitor332,FIG. 3A). The second RF signal path310-2also includes an output lead344-2(e.g., the output lead344-2,FIG. 3A). All of the above-listed components may be packaged together as parts of the packaged RF amplifier. In the example packaged RF amplifier302ofFIG. 3B, the two RF signal paths310-1and310-2essentially function in parallel, although another semiconductor device may include more than two RF signal paths. The output IPD360includes the microstrip transmission line340(e.g., a conductive strip) that extends between the drain side of both transistors304-1and304-2of the two RF signal paths, and the drain side of both transistors is electrically connected to the microstrip transmission line via bond wires331-1and331-2, which correspond to the inductances330-1, and330-2, respectively, as described with reference toFIG. 3A. More specifically, a first end of the microstrip transmission line340is connected to bondwires331-1, and a second end of the microstrip transmission line340is connected to bondwires331-2. A capacitor332is connected in parallel to a point along the microstrip transmission line340between the two ends, and that connection point corresponds to node336,FIG. 3A. More specifically, a first portion of the microstrip transmission line340between the connection point and bondwires331-1corresponds to inductance340-1(FIG. 3A), and a second portion of the microstrip transmission line340between the connection point and bondwires331-2corresponds to inductance340-2(FIG. 3A). Capacitor332may be, for example, a metal-insulator-metal (MIM) capacitor that is integrally formed within IPD360, although capacitor332may be implemented using other technologies, as well. More specifically, a first terminal of capacitor332is electrically connected to the microstrip transmission line340, and a second terminal of capacitor332is electrically connected to a ground reference node (e.g., to flange354with conductive through substrate vias (TSVs) that extend from the second terminal to the bottom surface of the IPD360). In other embodiments, capacitor332may be implemented with multiple MIM or other types of capacitors coupled in parallel between microstrip transmission line340and the ground reference node (e.g., flange354). The components of the packaged RF amplifier may be attached to the flange354(e.g., a conductive metal substrate) and electrically connected to each other by bondwires, including, for example, sets of parallel bondwires. For purposes of clarity, the input leads, the input IPDs, the transistors, the output IPD, and the output leads each may be referred to in the singular sense, below, as will analogous components in other, later-described Figures. It is to be understood that the description of a particular device component in the singular sense applies to the set of all such components.

As shown inFIG. 3B, the components of the packaged RF amplifier302device are electrically connected by conductive bondwires347-1,347-2,349-1,349-2,329-1,329-2,331-1, and331-2(referred to herein simply as bondwires) as is known in the field. For example, with regard to the first RF signal path310-1(a similar description applies also to the second RF signal path), multiple parallel bondwires347-1are connected between the input lead342-1and the input IPD315-1, multiple parallel bondwires349-1are connected between the input IPD315-1and the transistor304-1(e.g., the gate of the transistor), and multiple bondwires329-1are connected between the transistor304-1(e.g., the drain of the transistor) and the output lead344-1. Additionally, multiple bondwires331-1are connected between the transistor304-1(e.g., the drain of the transistor) and the microstrip transmission line340of the output IPD360. As shown in the example ofFIG. 3B, eight bondwires329-1are connected in parallel between the transistor304-1and the output lead344-1and three bondwires331-1are connected between the transistor304-1and the microstrip transmission line340of the output IPD360. The bondwires correspond to inductive elements as indicated in the circuit-level layout ofFIG. 3A.

According to an embodiment, the packaged RF amplifier302is incorporated in an air cavity package, in which the transistors304-1and304-2and the in-package input and output impedance matching circuits314-1,314-2,320are located within an enclosed air cavity. Basically, as will be described in more detail below, the air cavity is bounded by the flange354, an isolation structure (not shown), and a cap (not shown) overlying and in contact with the isolation structure and the input/output leads342-1,342-2,344-1, and344-2. In other embodiments, the RF amplifier device may be incorporated into an over-molded package (i.e., a package in which the electrical components within the active device area are encapsulated with a non-conductive molding compound, and in which portions of the leads, and all or portions of the isolation structure also may be encompassed by the molding compound).

In an embodiment, the flange354includes a rigid electrically-conductive substrate, which has a thickness that is sufficient to provide structural support for electrical components and elements of the RF amplifier device302. In addition, the flange may function as a heat sink for the transistors304-1and304-2and other devices mounted on the flange. In an embodiment, the flange has a top and bottom surface (only a top surface is visible in the figures), and a substantially-rectangular perimeter that corresponds to the perimeter of the device. In an embodiment, the flange is formed from a conductive material, and may be used to provide a ground reference for the device. For example, various components and elements may have terminals that are electrically coupled to the flange, and the flange may be electrically coupled to a system ground when the device is incorporated into a larger electrical system. At least the surface of the flange is formed from a layer of conductive material, and possibly all of the flange is formed from bulk conductive material. Alternatively, the flange may have one or more layers of non-conductive material below its top surface. Either way, the flange has a conductive top surface. The flange may more generally be referred to as a substrate or a PCB with a conductive surface.

As mentioned above, in an embodiment, an isolation structure (not shown) may be attached to the top surface of the flange354. The isolation structure is formed from a rigid, electrically insulating material, and functions to electrically isolate the leads342-1,342-2,344-1,344-2from the flange354. The term “isolation structure,” as used herein, refers to a structure that provides electrical isolation between conductive features of a device (e.g., between the leads, and the flange). The leads (e.g., leads342-1,342-2,344-1,344-2) may be coupled to the top surface of the isolation structure (e.g., using epoxy or other adhesive materials), and the bottom surface of the isolation structure may be coupled to the flange (e.g., using epoxy or other adhesive materials). In an embodiment, the isolation structure has a frame shape, which includes a central opening.

In an embodiment, the input and output leads342-1,342-2,344-1, and344-2are mounted on a top surface of the isolation structure on opposed sides of a central opening, and thus the input and output leads are elevated above the top surface of the flange354, and are electrically isolated from the flange. For example, the input and output leads may be soldered or otherwise attached to metallization on a top surface of the isolation structure. Generally, the input and output leads are oriented in order to allow for attachment of the bondwires between the input and output leads and components and elements within the central opening of isolation structure.

As described above, the transistors304-1and304-2and various elements of the in-package input IPDs315-1and315-2and the output IPD360are mounted on a generally central portion of the top surface of a flange354that is exposed through an opening in the isolation structure. According to an embodiment, the transistors are positioned within an active device area of packaged RF amplifier device302, along with the input and output IPDs. For example, the input IPDs, the transistors, and the output IPDs may be coupled to the flange using conductive epoxy, solder, solder bumps, sintering, and/or eutectic bonds.

The transistors304-1and304-2may be GaN-based transistors, in some embodiments, or silicon-based transistors, in other embodiments. Each transistor has a control terminal (e.g., a gate) and two current conducting terminals (e.g., a drain and a source). The control terminal of each transistor is coupled to the corresponding in-package input impedance matching circuit314-1and314-2. In addition, one current conducting terminal (e.g., the drain) is coupled to the in-package output impedance matching circuit320and to the corresponding output lead344-1and344-2. The other current conducting terminal (e.g., the source) is coupled to the flange (e.g., to ground), in an embodiment.

As described above with reference toFIGS. 3A and 3B, the common capacitance332is connected in parallel between the first and second RF signal paths310-1and310-2. Various other example embodiments of a packaged RF amplifier with common capacitance are described below with reference toFIGS. 4A-10B. Throughout the description ofFIGS. 3A-10B, similar reference numbers may be used to refer to similar elements.

In another embodiment, the common capacitance is connected in series between the first and second RF signal paths.FIGS. 4A and 4Bdepict an embodiment of a packaged RF amplifier402in which a common capacitance338(e.g., capacitance238,FIG. 2B) is connected in series between the first and second RF signal paths310-1and310-2in accordance with an embodiment of the invention. In particular,FIG. 4Adepicts a circuit-level layout of a configuration of a packaged RF amplifier, andFIG. 4Bdepicts a corresponding component-level layout of the packaged RF amplifier.

With reference to the circuit-level layout ofFIG. 4A, capacitance338is electrically connected to the output terminal (e.g., the drain terminal) of both of the transistors304-1and304-2such that the capacitance is electrically connected to both RF signal paths in series. As depicted inFIG. 4A, one terminal of the capacitance338is connected to the first RF signal path310-1, and the other terminal of the capacitance338is connected to the second RF signal path310-2.

With reference to the component-level layout ofFIG. 4B, the packaged RF amplifier402includes an output IPD460that includes a first microstrip transmission line440-1(e.g., inductance440-1,FIG. 4A), a second microstrip transmission line440-2(e.g., inductance440-2,FIG. 4A), and a capacitor338. The capacitor338is electrically connected in series between the outputs (e.g., drain terminals) of transistors304-1and304-2as follows. A first terminal of capacitor338is electrically connected to a first end of transmission line440-1, and a second end of transmission line440-1is electrically connected to the output terminal of transistor304-1through wirebonds331-1. A second terminal of capacitor338is electrically connected to a first end of transmission line440-2, and a second end of transmission line440-2is electrically connected to the output terminal of transistor304-2through wirebonds331-2.

Capacitor338may be, for example, a MIM capacitor that is integrally formed within IPD460, although capacitor338may be implemented using other technologies, as well. More specifically, a first terminal of capacitor338is electrically connected to the microstrip transmission line440-1, and a second terminal of capacitor338is electrically connected to microstrip transmission line440-2, as mentioned above. In other embodiments, capacitor338may be implemented with multiple MIM or other types of capacitors coupled in parallel between microstrip transmission lines440-1,440-2.

In an embodiment, series-connected capacitance in an IPD does not need to have TSVs, which can reduce component cost by avoiding TSV processing steps during fabrication. Additionally, the lack of TSVs may enable the thickness of a capacitor to be increased. For example, the transmission line Q factor may be significantly improved. Additionally, with no TSVs, the output IPD, which may be a relatively large component, may be less likely to experience micro-cracks during manufacturing, assembly and/or during temperature excursions while in use.

In an embodiment, common capacitance is connected at the input side of the transistors as well as at the output side of the transistors.FIGS. 5A and 5Bdepict an embodiment of a packaged RF amplifier502in which common capacitance is connected at the input side of the transistors304-1and304-2as well as at the output side of the transistors in accordance with an embodiment of the invention. In particular,FIG. 5Adepicts a circuit-level layout of a configuration of a packaged RF amplifier andFIG. 5Bdepicts a corresponding component-level layout of the packaged RF amplifier. Such a configuration can enable even-order harmonic termination.

With reference to the circuit-level layout ofFIG. 5A, a first capacitance332is electrically connected in parallel to the drain of both of the transistors304-1and304-2such that the first capacitance is electrically connected to both RF signal paths310-1and310-2and a second capacitance333is electrically connected in parallel to the gate of both of the transistors such that the second capacitance is electrically connected between both RF signal paths. As depicted inFIG. 5A, on the output side there is a node336between the inductances330-1and330-2and a first terminal of the first capacitance336is coupled to the node336and a second terminal of the first capacitance is coupled to ground. Additionally, on the input side there is a node337between the inductances366-1and366-2and a first terminal of the second capacitance is coupled to the node337and a second terminal of the second capacitance is coupled to ground.

With reference to the component-level layout ofFIG. 5B, the packaged RF amplifier502includes an output IPD360that includes a first microstrip transmission line340and a first capacitor332connected to the first microstrip transmission line in parallel and an input IPD370that includes a second microstrip transmission line372and a second capacitor332connected to the second microstrip transmission line parallel. InFIG. 5B, bondwires351-1and351-2connect the IPD370to the transistors304-1and304-2and correspond to inductance366-1and366-2,FIG. 5A. Additionally, microstrip transmission line372inFIG. 5Bcorresponds to transmission line inductance340(e.g., a microstrip transmission line) corresponds to elements372-1and372-2inFIG. 5A.

In an embodiment, the common capacitance is connected in parallel between the first and second RF signal paths in a spread shunt inductance configuration.FIGS. 6A and 6Bdepict an embodiment of a packaged RF amplifier602in which the common capacitance is connected in parallel in a spread shunt inductance configuration in accordance with an embodiment of the invention. In particular,FIG. 6Adepicts a circuit-level layout of a configuration of a packaged RF amplifier andFIG. 6Bdepicts a corresponding component-level layout of the packaged RF amplifier.

With reference to the circuit-level layout ofFIG. 6A, capacitance332is electrically connected to the drain of both of the transistors304-1and304-2such that the capacitance is electrically connected to both RF signal paths310-1and310-2in parallel and the shunt inductance is divided into three inductances630-1each corresponding to a portion of the microstrip transmission line, which portions are represented inFIG. 6Aby elements640-1. As depicted inFIG. 6A, node336is between the shunt inductances630-1of the first RF signal path310-1and the shunt inductances330-2of the second RF signal path310-2and a first terminal of the capacitance332is coupled to the node336and a second terminal of the capacitance332is coupled to ground.

With reference to the component-level layout ofFIG. 6B, the packaged RF amplifier602includes the output IPD660that has microstrip transmission line640and capacitor332connected in parallel to the microstrip transmission line. As depicted inFIG. 6B, the drain of the first transistor304-1is electrically connected to the microstrip transmission line640by a first set of parallel bondwires631-1that are separated from each other by bondwires329-1that electrically connect the drain of the first transistor to the first output lead344-1and the drain of the second transistor304-2is electrically connected to the microstrip transmission line640by a second set of parallel bondwires631-2that are separated from each other by bondwires329-2that electrically connect the drain of the second transistor to the second output lead344-2. As shown inFIG. 6B, pairs of IPD bondwires631-1are separated by two output lead bondwires329-1. Although an example of a bondwire distribution is provided inFIG. 6B, other distributions of bondwires are possible.

In an embodiment, the common capacitance is connected in parallel between the first and second RF signal paths in a spread shunt with phase compensation inductance configuration.FIGS. 7A and 7Bdepict an embodiment of a packaged RF amplifier702in which the common capacitance is connected in parallel in a spread shunt with phase compensation inductance configuration in accordance with an embodiment of the invention. In particular,FIG. 7Adepicts a circuit-level layout of a configuration of a packaged RF amplifier andFIG. 7Bdepicts a corresponding component-level layout of the packaged RF amplifier.

With reference to the circuit-level layout ofFIG. 7A, capacitance332is electrically connected to the drain of both of the transistors304-1and304-2such that the capacitance is electrically connected to both RF signal paths310-1and310-2in parallel and the shunt inductance is divided into three inductances730-1and730-2, each corresponding to a portion of the microstrip transmission line740, which portions are represented inFIG. 7Aby element741-1and741-2. In the example ofFIG. 7A, the inductances have different magnitudes, for example, an increasing magnitude from left to right. As depicted inFIG. 7A, there is a node336between the shunt inductances730-1of the first RF signal path310-1and the shunt inductances730-2of the second RF signal path310-2and a first terminal of the capacitance is coupled to the node and a second terminal of the capacitance is coupled to ground.

With reference to the component-level layout ofFIG. 7B, the packaged RF amplifier702includes an output IPD760that has a microstrip transmission line740and a capacitor332connected in parallel to the microstrip transmission line. As depicted inFIG. 7B, the drain of the first transistor304-1is electrically connected to the microstrip transmission line by a first set of parallel bondwires731-1(which corresponds to inductance730-1,FIG. 7A) that are separated from each other by bondwires329-1that electrically connect the drain of the first transistor to the first output lead344-1and the drain of the second transistor304-2is electrically connected to the microstrip transmission line by a second set of parallel bondwires731-2(which corresponds to inductance730-2,FIG. 7A) are separated from each other by bondwires329-2that electrically connect the drain of the second transistor to the second output lead344-2. As shown inFIG. 7B, pairs of IPD bondwires731-1and731-2are separated by two output lead bondwires329-1and329-2. Although an example of a bondwire distribution is provided inFIG. 7B, other distributions of bondwires are possible. Additionally, the microstrip transmission line includes tapered portions at each end. As shown, the tapered portions widen from an inner location to an end location. In an embodiment, the magnitude of the inductance increases as a function of the width of the microstrip transmission line at the point at with the corresponding bondwire is connected to the microstrip transmission line. Although an example of a microstrip transmission line with tapered ends is provided for phase compensation, other configurations of the microstrip transmission line may provide phase compensation.

In an embodiment, the common capacitance is connected in parallel between the first and second RF signal paths in a spread shunt with phase compensation inductance configuration.FIGS. 8A and 8Bdepict an embodiment of a packaged RF amplifier802in which the common capacitance is connected in parallel in a spread shunt with phase compensation inductance configuration in accordance with an embodiment of the invention. In particular,FIG. 8Adepicts a circuit-level layout of a configuration of a packaged RF amplifier andFIG. 8Bdepicts a corresponding component-level layout of the packaged RF amplifier. The embodiment ofFIGS. 8A and 8Bis similar to the embodiment ofFIGS. 7A and 7Bexcept that the configuration of the phase compensation is different.

With reference to the circuit-level layout ofFIG. 8A, capacitance332is electrically connected to the drain of both of the transistors304-1and304-2such that the capacitance is electrically connected to both RF signal paths310-1and310-2in parallel and the shunt inductance is divided into three inductances830-1and830-2, each corresponding to a portion of the microstrip transmission line840, which portions are represented inFIG. 8Aby element841-1and841-2. In the example ofFIG. 8A, the inductances have different magnitudes, for example, an increased magnitude in the center relative to the inductances on the left and right. As depicted inFIG. 8A, there is a node336between the shunt inductances830-1of the first RF signal path310-1and the shunt inductances830-2of the second RF signal path310-2and a first terminal of the capacitance is coupled to the node and a second terminal of the capacitance is coupled to ground.

With reference to the component-level layout ofFIG. 8B, the packaged RF amplifier802includes an output IPD860that has a microstrip transmission line840and a capacitor332connected in parallel to the microstrip transmission line. As depicted inFIG. 8B, the drain of the first transistor304-1is electrically connected to the microstrip transmission line840by a first set of parallel bondwires831-1(which corresponds to inductance830-1,FIG. 8A) that are separated from each other by bondwires329-1that electrically connect the drain of the first transistor to the first output lead344-1and the drain of the second transistor304-1is electrically connected to the microstrip transmission line840by a second set of parallel bondwires831-2(which corresponds to inductance830-2,FIG. 8A) are separated from each other by bondwires329-2that electrically connect the drain of the second transistor to the second output lead344-2. As shown inFIG. 8B, pairs of IPD bondwires831-1and831-2are separated by two output lead bondwires329-1and329-2. Although an example of a bondwire distribution is provided inFIG. 8B, other distributions of bondwires are possible. Additionally, the microstrip transmission line includes triangular portions at each end. As shown, the triangular portions have a base and a vertex and the base is electrically connected to the drain of the transistors and the vertex is electrically connected to a linear portion of the microstrip transmission line. In an embodiment, the magnitude of the inductance is a function of the width of the triangular portion of the microstrip transmission line at the point at with the corresponding bondwire is connected to the microstrip transmission line. Although an example of a microstrip transmission line with triangular ends is provided for phase compensation, other configurations of the microstrip transmission line may provide phase compensation.

In an embodiment, the common capacitance is connected in parallel between the first and second RF signal paths and the packaged RF amplifier further includes a low inductance modulator lead connected to the output IPD.FIGS. 9A and 9Bdepict an embodiment of a packaged RF amplifier902in which the common capacitance is connected in parallel and the packaged RF amplifier further includes a low inductance modulator lead connected to the output IPD in accordance with an embodiment of the invention. In particular,FIG. 9Adepicts a circuit-level layout of a configuration of a packaged RF amplifier andFIG. 9Bdepicts a corresponding component-level layout of the packaged RF amplifier.

With reference to the circuit-level layout ofFIG. 9A, capacitance332is electrically connected to the drain of both of the transistors304-1and304-2such that the capacitance is electrically connected to both RF signal paths310-1and310-2in parallel as described with reference toFIG. 3A. As depicted inFIG. 9A, a modulator lead980is electrically connected to the node336between the shunt inductance330-1of the first RF signal path and the shunt inductance330-2of the second RF signal path. An inductance982also exists between the modulator lead980and the node336.

With reference to the component-level layout ofFIG. 9B, the packaged RF amplifier902includes an output IPD360that has a microstrip transmission line340and a capacitor332connected in parallel to the microstrip transmission line as described above with reference toFIG. 3B. As depicted inFIG. 9B, the modulator lead980is electrically connected to the microstrip transmission line340of the output IPD360by bondwires983. In the example ofFIG. 9B, the bondwires are connected to the center of the microstrip transmission line and nearby to the capacitor. In an embodiment, the location of the bondwires983is selected to minimize phase difference between the wirebond closest to the center of the conductor (e.g., the microstrip transmission line340) and the end wires331-1and331-2. The distance from one set of bondwires to the other set of bondwires determines the overall inductance in the shunt path.

In an embodiment, the common capacitance is connected in series between the first and second RF signal paths and the packaged RF amplifier further includes a low inductance modulator lead connected to the output IPD.FIGS. 10A and 10Bdepict an embodiment of a packaged RF amplifier1002in which the common capacitance is connected in series and the packaged RF amplifier further includes a low inductance modulator lead connected to the output IPD in accordance with an embodiment of the invention. In particular,FIG. 10Adepicts a circuit-level layout of a configuration of a packaged RF amplifier andFIG. 10Bdepicts a corresponding component-level layout of the packaged RF amplifier.

With reference to the circuit-level layout ofFIG. 10A, capacitance332is electrically connected to the drain of both of the transistors304-1and304-2such that the capacitance is electrically connected to both RF signal paths310-1and310-2in series as described above with reference toFIG. 4A. As depicted inFIG. 10A, a modulator lead980is electrically connected to either side of the capacitance332. An inductance1082also exists between the modulator lead980and the node336-1and336-2.

With reference to the component-level layout ofFIG. 10B, the packaged RF amplifier1002includes an output IPD460that includes a first microstrip transmission line440-1, a second microstrip transmission line440-2, and a capacitor332as described above with reference toFIG. 4B. The capacitor is electrically connected in series between the first microstrip transmission line and the second microstrip transmission line. As depicted inFIG. 10B, the modulator lead980is electrically connected to the first microstrip transmission line440-1and to the second microstrip transmission line440-2of the output IPD460by bondwires1083-1and1083-2. In the example ofFIG. 10B, the bondwires are connected to either side of capacitor near the terminals of the capacitor. In an embodiment, the location of the bondwires1083-1and1083-2is selected to minimize phase difference between the wirebond closest to the center of the conductor (e.g., the microstrip transmission line440-1and440-2) and the end wires331-1and331-2. In an embodiment, a node corresponding to the capacitor338represents the RF cold point (e.g., essentially the combination of the series inductance and the capacitance338). The RF cold point is where the modulator or bias feed can be brought in without affecting the RF tuning. In such an embodiment, the series capacitance can be utilized because the push-pull circuit creates a virtual ground at RF frequencies.