Abstract:
An amplifier for amplifying signals is presented. A cascode power amplifier includes two or more adjacent cascode amplifiers and at least one remote cascode amplifier. The adjacent cascode amplifiers are lined up adjacent each other with inputs of the adjacent cascode amplifiers connected to a common input line and outputs of the of adjacent cascode amplifiers connected to a common output line. The adjacent cascode amplifiers generally operate in parallel. The remote cascode amplifier is spaced apart from the adjacent cascode amplifiers. An input transmission line connects an input of the remote cascode amplifier to the common input line. An output transmission line connects an output of the remote cascode amplifier to the common output line. Amplified outputs of the adjacent cascode amplifiers and amplified outputs of the remote cascode amplifier are power combined and summed into a coherent amplified output signal that is output on the output transmission line.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from U.S. Provisional Application Ser. No. 61/701,888, filed Sep. 17, 2012; the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The current invention relates generally to apparatus, systems and methods for amplifying a signal. More particularly, the apparatus, systems and methods relate to amplifying a radio frequency (RF) power signal. Specifically, the apparatus, systems and methods provide for a power amplifier that uses multiple cascode amplifiers some of which are grouped together and some of which are spaced apart. 
     2. Description of Related Art 
     Amplifiers have long been used to amplify a variety of electrical signals. For example, amplifiers can be used to amplify voltage, current, power and the like. Early amplifiers used vacuum tubes to amplify signals. These tubes where large, used high power and often burned out. The invention of the silicon transistor greatly improved amplifier technology and quickly led to the extinction of vacuum tubes. Silicon transistors were much smaller, cheaper, could be more easily mass produced and had a much longer life span than vacuum tubes. Additionally, transistors consume much less power and generate less heat than vacuum tubes. 
     Because of a transistors small size, it has allowed for more sophisticated amplifiers to be designed. For example, operational amplifiers (Op Amps) contain two or more stages of amplification each with their own bias schemes all implemented with transistors and other discrete components. Op Amps provide excellent common mode rejection so that just a signal of interest is amplified. 
     One conventional approach to amplifying radio frequencies (RF) is to use a cascode amplifier that has a common gate transistor and a common source transistor. However, these types of amplifiers often have a small operational bandwidth (BW) and cannot handle higher currents/power. Therefore, what is needed is a better amplifier. 
     SUMMARY 
     The preferred embodiment includes a cascode power amplifier (PA). The cascode PA is an RF power amplifier (PA) that includes two or more adjacent cascode amplifiers and at least one remote cascode amplifier. The adjacent cascode amplifiers are lined up adjacent each other with inputs of the adjacent cascode amplifiers connected to a common input line and outputs of the of adjacent cascode amplifiers connected to a common output line. The adjacent cascode amplifiers generally operate in parallel. The remote cascode amplifier is spaced apart from the adjacent cascode amplifiers. An input transmission line connects an input of the remote cascode amplifier to the input line and to the common input line. An output transmission line connects an output of the remote cascode amplifier to the output line and the common output line. Amplified outputs of the adjacent cascode amplifiers and amplified outputs of the remote cascode amplifier are all power combined and summed into a coherent amplified output signal that is output on the output transmission line. 
    
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
       One or more preferred embodiments that illustrate the best mode(s) are set forth in the drawings and in the following description. The appended claims particularly and distinctly point out and set forth the invention. 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate various example methods, and other example embodiments of various aspects of the invention. It will be appreciated that the illustrated element boundaries (e.g., boxes, groups of boxes, or other shapes) in the figures represent one example of the boundaries. One of ordinary skill in the art will appreciate that in some examples one element may be designed as multiple elements or that multiple elements may be designed as one element. In some examples, an element shown as an internal component of another element may be implemented as an external component and vice versa. Furthermore, elements may not be drawn to scale. 
         FIG. 1  illustrates an example schematic of a preferred embodiment of a cascode radio frequency (RF) power amplifier (PA). 
         FIG. 2  illustrates an example view looking downward toward a gallium nitride (GaN) chip implementing a Non-uniform Distributed PA (NDPA). 
         FIG. 3  illustrates an example top view a preferred embodiment of a bias inductor formed in a metal layer and with air bridge connector devices. 
         FIG. 4  illustrates an example cross-section view a preferred embodiment an air bridge. 
         FIG. 5  illustrates an example top view of the metal layers of a FRAP circuit. 
     
    
    
     Similar numbers refer to similar parts throughout the drawings. 
     DETAILED DESCRIPTION 
       FIG. 1  illustrates the preferred embodiment of a cascode Power Amplifier (PA) cell  100  that uses a compound transistor. The compound transistors include a common source transistor X 1  and a common gate transistor X 2 . They are connected in series from a DC standpoint but in cascode configuration from an RF standpoint. The advantages to this compound transistor over a conventional single ended common source transistor is that first, it has a high efficiency. Secondly, it a higher voltage and lower current for a given power output which reduces certain power distribution loses both in the power module and in the chip itself due to reduced Ohmic losses operating at higher voltage and lower current. As a result of the higher voltage and lower current, a given power impedances are higher so that they can be matched over a wider bandwidth (BW). 
     The novelty of this embodiment of the PA cell  100  includes the bias network and how it stabilizes the cascade PA cell  100 . The two left-hand biasing legs of  FIG. 1  are the RF cascading and stabilization circuits. These two legs include R 1 -R 3 , C 1 , TL 1 , TL 6  and TL 7 . There is a resistive voltage divider formed with resistors R 1  and R 2  connected to the common gate transistor X 2  through a transmission line that sets the voltage of the compound transistor to half of Vdd across the common gate transistor X 2  and half of Vdd across the common source transistor X 1 . There also is a series RC formed by resistor R 3  and capacitor C 1  combination that allows cascading grounding of the common gate of transistor X 2 , that is essentially an RF ground. Ideally, a large capacitor C 1  is desired but this would require too much area and a small cascode cell is desired. Therefore, in the preferred embodiment, C 1  is still made as large as possible within a confined area and R 3  is connected in series with it. 
     The common gate transistor X 2  makes a very good oscillator configuration so stability can be controlled. The common gate transistor X 2  has its source connected to the drain of the common source transistor X 1  and its drain connected to the output P 1  and resistor R 1 . Common gate transistor X 2  has its gate connected to RF ground (capacitor C 2 ). The common transistor X 1  has a gate connected to an input line and has its drain connected to resistor R 2  and capacitors C 1 , C 2  and has its source connected to ground. 
     In the preferred embodiment, the value of the components in  FIG. 1  are now provided. R 1  and R 2  are each 5000 Ohms and have widths of 10 micro meters (um) and lengths of 123 um. Resistor R 3  has a value of 320 Ohms, a width of 12.5 um and a length of 10 um. Capacitor C 1  has a value of 1.0 pF and capacitor C 2  has a value of 0.085 pF. Transmission line TL 1  has a width of 8 um and a length of 155 um, transmission line TL 2  has a width of 15 um and a length of 105 um, transmission line TL 3  has a width of 10 um and a length of 40 um, transmission line TL 4  has a width of 8 um and a length of 185 um, transmission line TL 5  has a width of 8 um and a length of 58 um. Transmission line T 6  has a width W 1  of 14, a width W 2  of 14 um and a width W 3  12 um, transmission line T 7  has a width W 1  of 10 um, a width W 2  of 10 um, a W 3  18 um and transmission line T 8  has a width W 1  of 14 um, a width W 2  of 14 um and a width W 3  12 um. 
       FIG. 2  illustrates the preferred embodiment of how some components and cells are positioned and laid out on a GaN chip to create a RF PA. The chip can be implemented with GaN or with another type of semiconductor material as understood by one of ordinary skill in the art.  FIG. 2 , illustrates both halves  3 A,  3 B of cascaded RF PA  1  that is symmetrical about centerline CL 1  that cuts the RF PA  1  into two halves  3 A,  3 B. Because it has a lot of symmetry, only one half  3 A will be described and that description and labeling will equally apply to the second half  3 B. The PA  1  is a non-uniform distributed PA for two reasons. First the widths of the transmission lines are different resulting in different impedances. Secondly, it is non-uniform because the cascode cells  100  are distributed with a cluster eight cascode cells  5  (e.g., eight amplifiers  100 ) clumped together at one location and with two other cascode cells  7 ,  9  distributed remotely away from the cluster of eight  5 . 
     The RF input enters the Non-uniform Distributed PA (NDPA) on transmission line TL 10  before passing by capacitor C 1  and onto a tapered transmission line TLT connected to the bank of eight cascaded cells  5  (e.g., eighth amplifiers  100 ). Transmission line TLT is generally tapered so that it becomes smaller in width until the last cascode amplifier  100  of the cluster of eight cascaded cells  5  receives the RF input signal. 
     Transmission line TL 11  is formed with transmission lines TL 11 A and TL 11 B. Transmission line TL 11 A is connected to the end of the tapered transmission line TLT and is also connected to the remote cascode amplifier  7 . Transmission line TL 11 A includes a generally semicircle portion  21  that is included to increase the length of transmission line TL 11  to make it a proper length. Transmission line TL 11 B is connected between remote cascode amplifier  7  and remote cascade amplifier  9 . Transmission line TL 11 B is straight between remote cascade amplifier  7  and remote cascode amplifier  9  and has a constant width between these two amplifiers. 
     Transmission line  13  is formed with transmission lines TL 13 A-C. Transmission line  13 A is connected to the outputs of the cluster of eight cascaded cells  5 . This transmission line TL 13 A is slightly tapered beginning at the first cascode amplifier  100  of the bank of eight cascaded cells  5  until it reaches the last cell  100  of the bank of cascaded cells  5 . Transmission line TL 13 B is connected to transmission line TL 13 A at the last cell  100  of the bank of cascaded cells  5  and transmission line TL 13 B is routed from here to the output of remote cascade amplifier  7 . This transmission line TL 13 B is jogged way from transmission line  11 A for shielding reasons. Transmission line  13 C is connected between the outputs of remote cascade amplifier  7  and remote cascade amplifier  9 . This transmission line  13 C is straight with a constant width. 
     Output transmission line TL 14  is connected between the output of remote cascade amplifier  9  and an output capacitor C 6 . It is also connected to a biasing inductor I 1 . This transmission line TL 14  includes a somewhat semicircular portion  23  that extends its length a desired amount for optimal operation. Bias inductor I 1  is connected/wired to capacitors C 2  and C 3 . The mirrored cascode RF PA  1  contains other capacitors C 4 , C 5  and other components that are not discussed in detail here as they are not the primary novelty of the preferred embodiment of the cascode RF PA  1 . 
       FIGS. 3 and 4  illustrated the bias inductor I 1 . The bias inductor I 1  has two levels of metal. One layer of metal is a transmission strip  25  layer of metal in combination with a spiraling octagonal shape metal  31  and the another layer of metal includes metallic air bridge metal structures  27  that air bridge over the transmission strip of  25  metal passing under the air bridge metal  27 . There is actually a gap  41  between the air bridge metal  27  and the transmission strip  25 . This gap can be filled with air, another gas or another material as understood by those of ordinary skill in the art. The air bridge metal  27  can include tab ends  29 A,  29 B that are used to connect it to ends  31 A,  31 B of the spiraled metal  31 . The air bridge metal  27  actually arches upward from the first end  31 A of the spiral metal  31  and has a curved arch that later curves downward toward the second end  31 B of the spiral metal  31 . A central portion  33  of the spiral of the bias inductor I 1  is free of metal. In the preferred embodiment, the spiraled metal  31  almost makes five complete spirals around the central portion  33 . Of course, in other embodiments, a different number of completed spirals may be desired. 
     It is desired to have an RF PA that has high bandwidth which means that the bias inductor I 1  ideally has high impedances that don&#39;t interfere with the desired RF signal. Thus a large inductance is preferred, but a large inductance has a parasitic that is a shunt capacitance that limits the BW. However, the bias inductor of  FIGS. 3 and 4  has an overall good geometry that does well to balance these competing design constraints. The conductors are thick and wide enough to handle the high current of the PA  1 . In the preferred embodiment, the width (W) of the metal  31  used to form the octagonal shaped inductor I 1  is about 40 microns wide with about 10 microns of gap (G) between the metal spirals. Of course these measurements can be other values. 
       FIG. 5  illustrates the details of the fusible link resistive voltage divider “FRAP” device  70 . Before the invention of this FRAP  70  one needed to apply a gate voltage to each individual chip and each individual chip needed to be tracked and the proper voltage applied to power it when it was implemented in a circuit. The FRAP device  70  is used to adjust the bias point of biasing circuits at the time of wafer testing. In the preferred embodiment, the FRAP  70  is on a GaN wafer  71  with conductive electrical routing and pad components. Five resistors R 1 - 5  are provided and are connected to pad devices  77  that are connected to fusible links  73 . In the preferred embodiment, these five resistors can be used to create about 32 different voltages ranging from −9 volts to about −2 volts but other ranges and voltage could be created in other embodiments. Two more resistors R 6 - 7  are also provided that are always used to create a bias voltage. Resistor R 7  is connected by a pad  79  to a reference voltage that in the preferred embodiment is −9 volts. Resistor R 6  is connected to the other ends of the fusible links by a pad at a ground voltage and conductive routing  75 . In the preferred embodiment, the values of the resistors is as follows: R 1 =75 ohms, R 2 =150 ohms, R 3 =300 ohms, R 4 =600 ohms, R 5 =1200 ohms, R 6 =75 ohms and R 7 =80 ohms. Of course, in other embodiments the resistors can have different values and there may be fewer or more resistors used to implement the FRAP  70 . 
     At the time of wafer testing, the bias voltage of the RF PA  1  is measured while it being applied to the RF PA circuits themselves. Next, a determination is made as to how much the bias voltage needs to be changed so that the RF PA  1  is biased to a proper value. A calculation is performed to determine which of the five resistors R 1 - 5  connected to the fusible links  73  need to be used to create the proper bias voltage. Once that is determined, the fusible links  73  connected to just the unneeded resistors are broken so that just the required resistors participate in creating the proper bias voltage. In the preferred embodiment, the FRAP is a voltage divider circuit formed by resisters R 1 -R 5 . The fusible links  73  can be broken on the GaN wafer  71  by any method as understood by those of ordinary skill in the art. For example, one way they can be broken is applying a strong enough voltage across them to create the breakage. 
     The related and co-owned U.S. Applications entitled “TILE ARRAY PA MODULE USING QUADRATURE BALANCED PA MMICS,” “DIGITALLY CONTROLLED POWER AMPLIFIER,” and “METHOD OF OPERATING A POWER AMPLIFIER IN CLASS F/INVERSE CLASS F,” which are filed contemporaneously herewith, are incorporated as if fully rewritten. 
     In the foregoing description, certain terms have been used for brevity, clearness, and understanding. No unnecessary limitations are to be implied therefrom beyond the requirement of the prior art because such terms are used for descriptive purposes and are intended to be broadly construed. Therefore, the invention is not limited to the specific details, the representative embodiments, and illustrative examples shown and described. Thus, this application is intended to embrace alterations, modifications, and variations that fall within the scope of the appended claims. 
     Moreover, the description and illustration of the invention is an example and the invention is not limited to the exact details shown or described. References to “the preferred embodiment”, “an embodiment”, “one example”, “an example”, and so on, indicate that the embodiment(s) or example(s) so described may include a particular feature, structure, characteristic, property, element, or limitation, but that not every embodiment or example necessarily includes that particular feature, structure, characteristic, property, element or limitation. Furthermore, repeated use of the phrase “in the preferred embodiment” does not necessarily refer to the same embodiment, though it may.