Programmable stabilization network

The present disclosure includes programmable stabilization circuits and methods. In one embodiment, a power amplifier in a wireless transmitter includes a transistor comprising a gate, a source, and a drain. Feedback from the drain to the gate is modified dynamically to stabilize the amplifier under changing operating conditions. In one embodiment, a series RC circuit is configured between the drain the gate and the RC circuit value is adjusted based on different power supply voltage modes.

BACKGROUND

The present disclosure relates to electronic circuits and methods, and in particular, to programmable stabilization circuits and methods.

The proliferation of wireless technologies and standards is placing ever increasing demands on electronic circuit technology. Circuits must be able to operate under drastically different operating conditions, and in some cases, must be able to perform well when a wireless system changes between various modes of operation. The complexities of this dynamic environment are particularly challenging for analog amplifiers, such as a power amplifier, in a wireless transmitter. A power amplifier, for example, may drive an antenna with enough output power to broadcast signals to a receiving system. Accordingly, the power amplifier must maintain stability when the wireless transmitter changes operating conditions during transmission, for example.

FIG. 1illustrates a typical transistor device110used in a power amplifier circuit100. One challenge with robust amplifier design pertains to the structure and parasitic effects of the transistors used to amplify signals in the transmission path. For example, some MOS transistor devices may have a very short gate length to compensate for high gate to drain capacitance (Cgd)120associated with some device structures. However, reducing the gate length can also cause the amplifier to become unstable under certain conditions. As illustrated by this example, some transistor devices and circuits designed to be stable for some operating conditions may be unstable for other operating conditions. However, conservative designs that are stable under all operation conditions may be highly inefficient in some situations, resulting in wasted power, for example.

SUMMARY

The present disclosure includes programmable stabilization circuits and methods. In one embodiment, a power amplifier in a wireless transmitter includes a transistor comprising a gate, a source, and a drain. Feedback from the drain to the gate is modified dynamically to stabilize the amplifier under changing operating conditions. In one embodiment, a series RC circuit is configured between the drain the gate and the RC circuit value is adjusted based on different power supply voltage modes.

DETAILED DESCRIPTION

FIG. 2illustrates a wireless transmitter200including a power amplifier (PA)210according to one embodiment. Wireless transmitter200may be part of a wireless communication system that can send and receive information using RF signals (receiver not shown). Wireless transmitter200may include a baseband controller201for receiving digital information to be transmitted as well as control information for controlling the operation of the transmitter. For example, transmitter200may operate in a variety of modes and may include transmit (Tx) mode control circuits202to configure the transmitter circuits to operating in different modes. Various modes may have different gain settings, power settings, or may pertain to different frequency bands and protocols, for example.

An output of baseband controller201is coupled to an input of an analog preprocessing block203. Various embodiments of analog preprocessing may include digital-to-analog conversion of data to be transmitted, filtering, gain control, and/or modulation (up-conversion), for example. An output of analog preprocessing203is coupled to an input of power amplifier210for amplifying an input signal, Vin, and producing an output signal, Vout, to drive antenna250.

Power amplifier210may include one or more gain stages comprising transistors. In this example, one gain stage is shown for illustrative purposes. In some embodiments, the gain stage may be at the output of a power amplifier where a drain of a gain stage transistor220is coupled to an antenna250. In other embodiments, the gain stage may be an intermediate stage, where the drain is coupled to the antenna through one or more additional gain stages. In some embodiments, multiple gain stages inside a power amplifier may include programmable feedback as described herein, for example. The transistors in each gain stage are typically powered by a power supply voltage, Vdd. In certain applications, different operational modes may change the power supply voltage, Vdd, received by power amplifier210. Since Vdd may change over time, it is sometimes denoted herein as Vdd(t). Example modes that effect the power supply voltage, Vdd, include an Average Power Tracking Mode (“APT mode”), an envelope tracking (ET) mode (e.g., where the power supply voltage corresponds to an envelope of a signal to be transmitted), or a low power mode (e.g., where a constant Vdd is reduced for lower power transmissions). The power supply voltage may also be impacted by the transmit power required when using different wireless transmission protocols, such as GSM, WCDMA, CDMA2000, or various LTE technologies including LTE5, LTE10, and LTE20, for example.

Changes in output power and/or power supply voltage can produce drastically different stability constraints on a power amplifier. Accordingly, features and advantages of the present disclosure include a transistor device220in a gain stage having a programmable feedback circuit221configured between a drain and a gate of the transistor device220. Programmable feedback circuit221has an input terminal coupled to the drain of transistor220, an output terminal coupled to the gate of transistor220, and a control input to receive a control signal, Ctrl. Programmable feedback circuit221adjusts a series RC circuit between the gate and the drain of transistor220based on the control signal received on the control input. For instance, in particular embodiments, programmable feedback circuit221is configured to produce a first RC circuit value when the wireless transmitter200is configured to produce a first power supply voltage to the drain of transistor220, and programmable feedback circuit221is configured to produce another RC circuit value to change a negative feedback from the drain to the gate of transistor220when wireless transmitter200is configured to produce a different power supply voltage to the drain of transistor220. The control signal may be based on the mode of operation of transmitter200, for example, so that as different modes change the power supply voltage provided to the drain of transistor220, different feedback configurations from the drain to the gate may be programmed to maintain the stability and performance of the power amplifier. By way of example to illustrate advantages of the present disclosure, programmable feedback circuit221may be configured to produce a first RC circuit value between the drain and gate of transistor220when the wireless transmitter is configured to produce a variable power supply voltage corresponding to an envelope of the input signal (e.g., ET mode). However, programmable feedback circuit220may be configured to produce a second RC circuit value when the wireless transmitter is configured to produce a constant power supply voltage to the drain of the transistor (e.g., APT mode), where the second RC circuit value (e.g., in APT mode) is less than the first RC circuit value (e.g., in ET mode) to increase negative feedback from the drain to the gate to improve stability during APT mode, for example.

FIG. 3illustrates a wireless transmitter including a power amplifier according to one embodiment. In this example, a signal to be transmitted, Si, is provided to preprocessor302. A control circuit301(e.g., a baseband or other processor) may determine a mode of operation for the transmitter and may configure preprocessor302. For example, in some operating conditions the system may be configured to operate in a mode with a constant power supply voltage (e.g., APT mode or a low power mode). In this case, a power supply323may be configured to produce a constant Vdd. As another example, under other operating conditions the system may be configured to operate in a mode with a variable power supply voltage, such as an envelope tracking (ET) mode where the variable power supply voltage corresponds to an envelope of the input signal, Vin, of power amplifier310, for example. In this case, control circuit301may configure preprocessor302to produce an envelope signal, Ve, to power supply323and a corresponding input voltage, Vin, which in this example is coupled to the input of power amplifier310through a matching network303.

In this illustrative example, power amplifier310includes a transistor320having a gate configured to receive input voltage, Vin, a source coupled to ground, and a drain coupled to receive power supply voltage Vdd(t) through an RF choke (RFC)322. An output of the amplifier (e.g., in this example, at the drain) produces an output voltage, Vout, which is coupled through a matching network304to an antenna350. The power amplifier310is shown as having only a single MOS transistor stage for illustrative purposes only. It is to be understood that embodiments of the present disclosure may include multiple transistor stages, and in some embodiments the stages may include outputs arranged in cascade (e.g., stacked) or other typical power amplifier configurations. The features and techniques of the present disclosure are applicable to a wide range of amplifier topologies.

In this example, a programmable feedback circuit321has an input coupled directly to the drain of transistor320and an output coupled directly to the gate of transistor320. In this case, programmable feedback circuit321receives control signal, Ctrl, which is a logic signal. In other embodiments Ctrl may be a continuous signal, for example. Here, programmable feedback circuit321may be configured by control circuit301, which may be a baseband circuit, processor, or other logic circuit, for example, that may determine a mode of operation of the wireless transmitter and produce logic signal, Ctrl, for configuring the programmable feedback circuit321. As one example, when the wireless transmitter is in ET mode, stability may not be as critical because the gain may be reduced as the power supply voltage is reduced for small signals. Accordingly, control circuit301may configure the programmable feedback circuit321to produce lower negative feedback by increasing a feedback resistance between the drain and the gate of transistor320, for example. However, when the wireless transmitter is in APT mode, small signals processed by the amplifier may require additional feedback to maintain stability. Accordingly, control circuit301may configure the programmable feedback circuit321to produce higher negative feedback by decreasing a feedback resistance between the drain and the gate of transistor320, for example. Accordingly, as mentioned above, an RC circuit value in APT mode may be less than an RC circuit value in ET mode to increase negative feedback from the drain to the gate. In one example implementation described below, a corner frequency of a high pass filter from the drain to the gate is decreased in APT mode to increase negative feedback and stabilize the device. Control circuit301may send control signals to programmable feedback circuit321to change the feedback between the drain and the gate to produce optimized stability of transistor320for a wide range of other operating modes, for example. Accordingly, APT and ET modes described above are merely illustrative.

FIG. 4illustrates a wireless transmitter including a power amplifier according to yet another embodiment. Similar to the example inFIG. 3, a control circuit401may configure a preprocessor402for ET mode to produce an envelope signal, Ve, and input voltage Vin through matching circuit403(and possibly additional stages and circuitry in a signal path) to a gate of transistor420. In this example, power supply423produces a power supply voltage, Vdd(t), which is coupled to both the drain of transistor420(through RFC422) and to the control input of programmable feedback circuit421. Accordingly, in some embodiments, feedback from the drain to the gate of transistor420may be adjusted based on the power supply voltage, Vdd(t), for example. As illustrated in more detailed examples below, changes in the power supply voltage may produce changes in the feedback from the drain to the gate of transistor420to improve stability of the power amplifier over changing operating conditions that have correspondingly different power supply voltage requirements.

FIG. 5Aillustrates an example programmable feedback circuit according to one embodiment. In this example, a programmable feedback circuit includes a capacitor C500and a variable (or programmable) resistance Rp501, which are configured in series. A first terminal of capacitor500is coupled to the drain (D) of a transistor and the second terminal of capacitor500is coupled to a terminal of Rp501. A second terminal of Rp501is coupled to a gate (G) of the transistor. The programmable feedback in this example forms a series RC circuit having a value set by the product of resistance and capacitance (Rp*C). A signal passing from a gate to a drain of a transistor will produce an inversion, so the feedback is negative to improve stability. Capacitor C removes DC from the feedback path. Feedback is controlled by the resistance. As the resistance is increased, a feedback current is reduced, thereby reducing negative feedback. As the resistance is decreased, the feedback current is increased, thereby increasing negative feedback, lowering the gain, and stabilizing the circuit.

FIG. 5Billustrates an example programmable feedback circuit according to another embodiment. Embodiments of a programmable feedback circuit may include a plurality of resistors, a plurality of switches (e.g., MOS transistors configured as switches), and a capacitor configured between a drain and a gate of a transistor. Similar toFIG. 5A, a first terminal of capacitor500is coupled to the drain of a transistor and the second terminal is coupled to a parallel configuration of resistors and switches. For example, a second terminal of capacitor500is coupled to terminals of N switches SW1-SWN denoted510-512. The other terminal of each switch510-512is coupled to a terminal of a particular resistor R1-RN502-504. A second terminal of each resistor502-504is coupled to the gate of the transistor. Accordingly, the parallel configuration of series resistors and switches may be programmed to change the resistance in series with capacitor500. For example, when all switches SW1-SWN are open, there is no feedback from the drain to the gate. When SW1is closed and the other switches are open, R1502is in series with capacitor500to produce a first RC circuit value. As additional switches are closed, the parallel configuration of resistors acts to reduce the total resistance in series with capacitor500, thereby decreasing the RC circuit value. Switches may be opened and closed by a logic signal from a control circuit as mentioned above, for example. Embodiments of this particular example programmable feedback circuit may comprise resistors R1-RN having the same values or different values, for example, to tune the feedback according to the needs of a particular design.

FIG. 5Cillustrates an example programmable feedback circuit according to yet another embodiment. In this example, a terminal of capacitor500is coupled to the drain of a transistor and another terminal of capacitor500is coupled to a first terminal of a resistor R2506and a first terminal of a switch SW2514. A second terminal of resistor R2506is coupled to a second terminal of switch SW2514so that SW2is configured in parallel with R2to create a short circuit around R2when SW2is closed, and thereby remove R2from the feedback circuit. The second terminal of R2and the second terminal of SW2are coupled to a first terminal of switch SW1513. A second terminal of SW1513is coupled to a first terminal of resistor R1505. A second terminal of resistor505is coupled to a gate of the transistor. Accordingly, when SW1is open, there is no feedback from the drain to the gate. When SW1is closed and SW2is open, a series RC circuit is formed having an RC circuit value set by the sum of R1and R2multiplied by C (i.e., (R1+R2)*C). This configuration sets the highest RC circuit value with the highest feedback resistance and the lowest amount of negative feedback from the drain to the gate. Negative feedback may be increased by closing SW2and shorting R2, thereby reducing the resistance in the feedback path from the drain to the gate of the transistor. The resulting RC circuit value is less than the RC circuit value when SW2is open, and the negative feedback is increased to increase the stability of the circuit for this particular mode of operation. It is to be understood that various programmable feedback circuits according to other embodiments may have many numbers of discrete feedback resistances (or even a continuous range) available for programming corresponding to a wide variety of different modes of operation or transmission conditions, for example.

FIG. 6Aillustrates an example programmable feedback circuit according to another embodiment. In this example, the power supply voltage is used as the control input signal to vary the resistance in the feedback path between the drain and gate of a transistor in a power amplifier circuit and thereby dynamically adjust the stability of the circuit. In this example, a shaper circuit623receives a power supply voltage Vdd(t) to control a value of variable resistance621. Variable resistance621is configured in series with capacitor622between a drain of transistor620and a gate of transistor620. Shaper circuit623may produce a variety of functional relations between Vdd(t) and resistance, for example, by receiving Vdd(t) and producing a control input signal that is a function of Vdd(t). For example, in some embodiments, the output of shaper circuit may be related to an nth power of Vdd (e.g., Vddn(t), such as 1<n<2) so that resistance is a function of Vddn(t). It is to be understood that a shaper circuit may use any other suitable function of Vdd (e.g., an exponential).

FIG. 6Billustrates an example programmable feedback circuit according to yet another embodiment. This example illustrates one particular implementation example of using the power supply voltage as the control input signal to vary the resistance in the feedback path between the drain and gate of a transistor in a power amplifier circuit. For instance, transistor620may receive an input signal Vin through a DC coupling capacitor C1650on a gate and produce an output signal Vout through a drain, for example. A power supply voltage is coupled to the drain of transistor620through an RF choke (RFC)610. A programmable feedback circuit in this example includes a capacitor622having a first terminal coupled to the drain of transistor620. A second terminal of capacitor622is coupled to a first terminal of resistor614. A second terminal of resistor614is coupled to a first terminal of resistor613, and a second terminal of resistor613is coupled to the gate of transistor620. Feedback resistance is varied in this example using a transistor612configured in parallel with resistor614. A first terminal of transistor612is coupled to the first terminal of resistor614and a second terminal of transistor612is coupled to the second terminal of resistor614. Power supply voltage Vdd is coupled to a control terminal of transistor612through gain stage611, which may translate a range of Vdd values into a corresponding range of values for controlling transistor612. For example, when Vdd is high, a voltage at the control terminal of transistor612may turn transistor612all the way on, thereby shorting out resistor614. Accordingly, when Vdd is high, the resistance in series with the capacitor622is low, and the RC circuit value is low, increasing negative feedback from the drain to the gate to improve stability. However, as Vdd decreases, transistor612forms an increasingly resistive path in parallel with resistor614. As the resistance in parallel with resistor614increases, the series resistance of the RC circuit increases, and the negative feedback decreases. When Vdd reaches some low threshold value, transistor612turns off, and a maximum feedback resistance is obtained with an RC circuit value equal to the sum of resistor613and resistor614multiplied by the capacitance622, thereby setting a minimum negative feedback from the drain to the gate for the power amplifier gain stage.

FIG. 7illustrates an example transistor feedback circuit configuration according to another embodiment. Embodiments of the present disclosure may segment a transistor in a gain stage and provide separate RC feedback circuits around different segments as shown. For example, a particular transistor may be segmented into transistor segments720A,720B, and720C (“segments”). Each segment may have a programmable RC feedback circuit coupled between a drain of the segment and the gate of the segment. For instance, segment720A includes a resistor R1711A and capacitor C1710A configured in series between the drain of segment720A and gate of segment720A, for example. Similarly, segment720B includes a resistor R2711B and capacitor C2710B configured in series between the drain of segment720B and gate of segment720B. Likewise, segment720C includes a resistor R3711C and capacitor C3710C configured in series between the drain of segment720C and gate of segment720C. Segments720A-C have gates, drains, and sources coupled together to form a single transistor device. Individual programmable feedback circuits for particular segments may improve uniformity over the transistor in the power amplifier stage, for example.