Abstract:
A linear Radio Frequency power amplifier with improved efficiency and bandwidth is provided using two power amplifier devices, operating in a Class-G mode of operation.

Description:
TECHNICAL FIELD 
     The present disclosure generally relates to the field of Radio Frequency power amplifiers, and more particularly to a circuit for a modified Class-G Radio Frequency power amplifier. 
     BACKGROUND 
     Linear Radio Frequency (RF) power amplifiers are commonly utilized in radio transmitters, particularly radio transmitters utilizing amplitude modulation. Power losses in linear RF power amplifiers have been identified as significant contributors to the overall power loss of radio transmitters. Various amplifier designs have attempted to improve efficiency at the expense of increasing circuit complexity or poor operational bandwidth. 
     SUMMARY 
     A circuit includes, but is not limited to: a first and a second power amplifier transistor, the first and the second power amplifier transistor each having a gate, a drain, and a source, the source of the first power amplifier transistor coupled to the drain of the second power amplifier transistor, the source of the second power amplifier transistor coupled to ground; a diode, the diode having an anode and a cathode, the anode of the diode coupled to the source of the first power amplifier and the drain of the second power amplifier transistor, the cathode of the diode coupled to a first voltage source; a decoupling capacitor, the decoupling capacitor having a first and a second end, the first end coupled to the cathode of the diode and the first voltage source, and the second end coupled to ground; a Radio Frequency choke, the Radio Frequency choke having a first and a second end, the first end coupled to the drain of the first power amplifier transistor, the second end coupled to a second voltage source; an output network coupled to the first end of the radio frequency choke and the drain of the first power amplifier transistor; a first bias voltage source coupled to the gate of the first power amplifier transistor; a second bias voltage source coupled to the gate of the second power amplifier transistor; a Radio Frequency source for driving the amplifier, the Radio Frequency source coupled to the gate of the second power amplifier transistor via the second bias voltage source; and a transformer, the transformer having an input cathode and anode and an output cathode and anode, the input cathode coupled to the Radio Frequency source, the input anode coupled to ground, the output cathode coupled to the gate of the first power amplifier transistor via the first bias voltage source, and the output anode coupled to the source of the first power amplifier transistor, the drain of the second power amplifier transistor, and the anode of the diode. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not necessarily restrictive of the present disclosure. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate subject matter of the disclosure. Together, the descriptions and the drawings serve to explain the principles of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The numerous advantages of the disclosure may be better understood by those skilled in the art by reference to the accompanying figures in which: 
         FIG. 1  is a simplified block diagram of a linear Radio Frequency power amplifier in accordance with an embodiment of the present disclosure; and 
         FIG. 2  is a circuit diagram of a linear Radio Frequency power amplifier circuit in accordance with an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Radio frequency (RF) power amplifiers capable of generating complex output waveforms are generally linear power amplifiers. Typically, Class-A, Class-B, or Class-AB power amplifier types are utilized in such applications. However, when generating waveforms with a high peak voltage to average voltage ratio, the efficiency of such power amplifier types is low. 
     Linear amplification with nonlinear components (LINC) power amplifier designs provide higher efficiency while generating complex output waveforms, but require complex drive signal processing circuitry. Envelope Envelope elimination and restoration (EER) provides higher efficiency, but requires a high bandwidth power modulator to create the envelope signal. Doherty, Cheirex, and Taylor power amplifier designs also provide high efficiency, but are limited to narrow band frequency operation. 
     Reference will now be made in detail to the subject matter disclosed, which is illustrated in the accompanying drawings. 
     Referring to  FIG. 1 , a simplified block diagram of an improved radio frequency (RF) signal power amplifier  100  in accordance with an exemplary embodiment of the present invention is shown. Power amplifier  100  may include RF source  110 . RF source  110  may provide an RF signal to low level RF drive  120 . Low level RF drive  120  may provide an input for the low level power amplifier  130  of power amplifier  100 . Low level power amplifier  130  may be configured to amplify the RF signal for the entire output voltage range of power amplifier  100 . 
     In further embodiments of the current disclosure, RF source  110  may also provide an RF signal to high level RF drive  140 . High level RF drive  140  may provide an input for the high level power amplifier  150  of power amplifier  100 . High level power amplifier  150  may be configured to amplify the RF signal for only a higher portion of the output voltage range of power amplifier  100 . The output of high level power amplifier  150  may pass through low level power amplifier  130 , adding to the output of low level power amplifier  130 . Low level power amplifier  130  may then provide the amplified signal to an output network  160  of power amplifier  100 . Low level power amplifier  130  may be connected to a higher voltage source than high level power amplifier  150 . 
     Referring to  FIG. 2 , a circuit for amplifying a RF signal in accordance with an exemplary embodiment of the present invention is shown. For example, the circuit may be utilized as a single-ended amplifier. Further, the circuit may be utilized as a push-pull amplifier. 
     In a current embodiment of the present disclosure, circuit  200  may include first power amplifier transistor  240  and second power amplifier transistor  245 . The source of first power amplifier transistor  240  may be connected to the drain of second power amplifier transistor  245 . Further, the source of second power amplifier transistor  245  may be connected to common connection  296 . First power amplifier transistor  240  and second power amplifier transistor  245  may be n-type power amplifier transistors. In another embodiment, first power amplifier transistor  240  and second power amplifier transistor  245  may be MOSFET-type transistors or HEMT-type transistors. The selection of technology for first power amplifier transistor  240  and second power amplifier transistor  245  may be related to the frequency of RF voltage source  210 . RF voltage source  210  may be the RF source  110 . 
     In further embodiments of the present disclosure, circuit  200  may include diode  250 . Diode  250  may be a low-capacitance diode. For example, diode  250  may be a Schottky diode. The anode of diode  250  may be connected to the source of first power amplifier  240  and the drain of second power amplifier transistor  245 . The cathode of diode  250  may be connected to first voltage source  265  and to a first end of decoupling capacitor  280 . A second end of decoupling capacitor  280  may be connected to common connection  297 . Decoupling capacitor  280  may be selected to provide RF decoupling of first voltage source  265 . However, other decoupling means may be selected to provide RF decoupling of first voltage source  265 . First voltage source  265  may be a voltage rail. 
     In further embodiments of the present disclosure, circuit  200  may include RF choke  270 . RF choke  270  may be an inductor. A first end of RF choke  270  may be connected to the drain of first power amplifier transistor  240 . A second end of RF choke  270  may be connected to second voltage source  260 . RF choke  270  may be selected to provide RF decoupling of second voltage source  260 . However, other decoupling means may be selected to provide RF decoupling of second voltage source  260 . Second voltage source  260  may be a voltage rail. Second voltage source  260  may be selected so the voltage of second voltage source  260  is higher than the voltage of first voltage source  265 . The voltage of second voltage source  260  may be a multiple of the voltage of first voltage source  265 . For example, the voltage of second voltage source  260  may be 24 Volts and the voltage of first voltage source  265  may be 12 Volts, resulting in a nominal 2:1 ratio for second voltage source  260  and first voltage source  265 . In another example, the voltage of second voltage source  260  may be at least twice the voltage of first voltage source  265 . 
     In a further embodiment of the present disclosure, the voltages of first voltage source  265  and second voltage source  260  may be varied dynamically to provide envelope tracking to optimize the efficiency of both first power amplifier  240  and second power amplifier  245 . For example, second voltage source  260  may be varied according to the peak voltage requirement required to reproduce the original RF signal without distortion. Further, second voltage source  260  may be varied so the voltage difference between second voltage source  260  and first voltage source  265  is related to the root mean square (RMS) voltage of the RF signal. 
     In a further embodiment of the present disclosure, varying second voltage source  260  and/or first voltage source  265  may result in minimum overall circuit dissipation. Further, varying second voltage source  260  and/or first voltage source  265  may result in evenly dividing circuit dissipation between first power transistor  240  and second power transistor  245 . 
     In further embodiments of the present disclosure, circuit  200  may include an output network  160 . Output network  160  may be connected to the first end of RF choke  270  and the drain of first power amplifier  240 . Output network  160  may include first capacitor  284 . A first end of first capacitor  284  may be connected to the drain of first power amplifier transistor  240  and the first end of RF choke  270 . A second end of first capacitor  284  may be connected to common connection  298 . Output network  160  may include second capacitor  282 . A first end of second capacitor  282  may be connected to the drain of first power amplifier transistor  240 , the first end of RF choke  270 , and the first end of first capacitor  284 . Output network  160  may include inductor  275 . A first end of inductor  275  may be connected to the second end of second capacitor  282 . A second end of inductor  275  may be connected to a RF load  290  of circuit  200 . RF load  290  may also be part of output network  160 . The RF load  290  may then lead to a common connection  299 . The values of the inductor  275 , first capacitor  284 , and second capacitor  282  of output network  160  may be set for a generic Class-B power amplifier. 
     In further embodiments of the present disclosure, circuit  200  may include first bias voltage source  230 . First bias voltage source  230  may be connected to the gate of first power amplifier transistor  240 . First bias voltage source  230  may be set to maintain first power amplifier transistor  240  on the conduction threshold. First bias voltage source  230  may be variable. 
     In further embodiments of the present disclosure, circuit  200  may include second bias voltage source  235 . Second bias voltage source  235  may be connected to the gate of second power amplifier transistor  245 . Second bias voltage source  235  may be set to maintain second power amplifier transistor  245  on the conduction threshold. Second bias voltage source  235  may account for the peak of a RF voltage source also connected to the gate of second power amplifier transistor  245  when set to maintain second power amplifier transistor  245  on the conduction threshold. Second bias voltage source  235  may be variable. 
     In further embodiments of the present disclosure, circuit  200  may be connected to an RF voltage source  210 , which is connected to common connection  294 . Circuit  200  may amplify the RF voltage source  210 . The RF voltage source  210  may be coupled to the gate of second power amplifier transistor  245  via second bias voltage source  235 . The sum of the voltages of RF voltage source  210  and second bias voltage source  235  may drive second power amplifier transistor  245 . Second bias voltage source  235  may be included in high level RF drive  140 . 
     In further embodiments of the present disclosure, circuit  200  may include transformer  220 . The input cathode of transformer  220  may be connected to the RF voltage source. The input anode of transformer  220  may be connected to common connection  295 . The output cathode of transformer  220  may be connected to gate of first power amplifier transistor  240  via first bias voltage source  230 . The sum of the voltages of the output cathode of transformer  220  and first bias voltage source  230  may drive first power amplifier transistor  240 . The output anode of transformer  220  may be connected to the source of first power amplifier transistor  240 , the drain of second power amplifier transistor  245 , and the anode of diode  250 . Transformer  220  and first bias voltage source  230  may be included in low level RF drive  120 . Common connections  294 - 299  may be connections to ground. 
     In operation, first power amplifier transistor  240  may act as a class-B power amplifier for low levels of drive. Source current for first power amplifier transistor  240  may flow through diode  250  to first voltage source  265  for low levels of drive. High levels of drive occur when the sum of the peak voltage of RF voltage source  210  and the voltage of the second bias voltage source  235  is greater than the conduction threshold of second power amplifier transistor  245 . Second power amplifier transistor  245  will conduct beyond this point and add to the output of circuit  200 , reverse biasing diode  250 . Second bias voltage  235  may be variable to place second power amplifier transistor  245  at its conduction threshold as first power amplifier transistor enters compression. The efficiency of circuit  200  may equal a conventional Class-B power amplifier at full output and at −6 dBc. Circuit  200  may provide high efficiency amplification of broadband signals with low circuit complexity. 
     It is believed that the present disclosure and many of its attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction and arrangement of the components without departing from the disclosed subject matter or without sacrificing all of its material advantages. The form described is merely explanatory, and it is the intention of the following claims to encompass and include such changes.