Patent Publication Number: US-2019187733-A1

Title: HIGH-VOLTAGE, HIGH-SPEED GaN DRIVER CIRCUIT

Description:
BACKGROUND 
     Field 
     This disclosure relates generally to a driver circuit including a source amplifier and an active load and, more particularly, to a GaN driver circuit including a source amplifier having an amplifying field effect transistor (FET) device and an active load having a self-biasing load FET device and a resistor in the self-biasing line that provides high impedance and low capacitance. 
     Discussion 
     Power amplifiers that employ one or more FET devices for amplifying RF signals are well known in the art. Power amplifiers have many applications including, but not limited to, low noise amplifiers (LNA), intermediate frequency (IF) amplifiers, local oscillator (LO) amplifiers, etc. FET devices are well known in the transistor art and come in a variety of well known types, such a HEMT, MOSFET, MISFET, FinFET, etc., and can be integrated as horizontal devices or vertical devices. A typical FET device will include various semiconductor layers, such as silicon, gallium arsenide (GaAs), indium gallium arsenide (InGaAs), gallium nitride (GaN), indium phosphide (InP), etc. Sometimes the semiconductor layers are doped with various impurities, such as boron, to increase the population of carriers in the layer, where the higher the doping level of the layer the greater the conductivity of the particular semiconductor material. An FET device will also include a source terminal, a drain terminal and a gate terminal, where one or more of the semiconductor layers is designated a channel layer and is in a electrical contact with the source and drain terminals. An electrical potential provided to the source terminal allows electrical carriers, either N-type or P-type, to flow through the channel layer to the drain terminal. An electric signal applied to the gate terminal creates an electrical field that modulates the carriers in the channel layer, where a small change in the gate voltage can cause a large variation in the population of carriers in the channel layer to change the current flow from the source terminal to the drain terminal. Some FET devices are depletion mode devices that are on with a zero volt gate bias and are off with a negative potential gate bias. 
     Gate driver circuits that control a gate terminal of an FET switch or other device often employ power amplifiers. For some applications, driver circuits need to be high-voltage, high-speed circuits that are able to generate a switch control signal that has very fast pulse edges, i.e., a square wave signal having a very fast rise time, and has a large voltage swing to turn the switch on and off quickly. The speed of known driver circuits can be improved for high-speed, high-voltage applications. 
     SUMMARY 
     The present disclosure describes a high-voltage, high-speed GaN driver circuit that is operable to control a switch. The driver circuit includes a source amplifier having an amplifying FET device with a drain terminal, a gate terminal and a source terminal, where the amplifying FET device receives a square wave control signal at its gate terminal and outputs an amplified square wave control signal at its drain terminal. The driver circuit also includes an active load having a self-biasing load FET device with a drain terminal, a gate terminal and a source terminal, where the drain terminal of the load FET device is coupled to a power supply, the source terminal of the load FET device is coupled to the drain terminal of the amplifying FET device, and the source and gate terminals of the load FET device are electrically coupled together by a self-biasing line. The active load includes a load resistor provided within the self-biasing line that causes a low and high frequency response of the active load that provides high impedance and low capacitance. 
     Additional features of the present disclosure will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a schematic diagram of a high-voltage, high-speed driver circuit including an active load and a source amplifier. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following discussion of the embodiments of the disclosure directed to a GaN driver circuit including a self-biased active load is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. 
       FIG. 1  is a schematic diagram of a driver circuit  10  including a source amplifier  12  having an FET device  14  and an active load  16  having a self-biased FET device  18 . In this non-limiting embodiment, the FET devices  14  and  18  are N-type depletion mode GaN FET devices, and the circuit  10  controls a switch  50 , such as a high impedance GaN FET switch, where the driver circuit  10  would be a gate driver circuit. However, the driver circuit  10  can control other devices, such as a second stage amplifier. Further, the amplifier  12  can be any amplifier suitable for the purposes discussed herein that may include multiple amplifier stages and may or may not include FET devices. 
     A control signal to be amplified is provided as a bias to the gate terminal G of the FET device  14  on line  20  through an RC circuit including a capacitor  22 , a resistor  24  and a power supply  30 . The control signal can be any control signal suitable for the purposes discussed herein, such as a square wave digital control signal where each square wave pulse represents a bit. An amplified output of the control signal is provided at the drain terminal D of the FET device  14  on line  26  through capacitor  28  to the switch  50 , where the source terminal S of the FET device  14  is coupled to a reference potential. In one embodiment, the square wave control signal is amplified by the FET device  14  to a much larger square wave signal having fast rising and falling edges and a slew rate similar to the input control signal. 
     The impedance of the active load  16  multiplied by the drain current from the amplifier  12  forms the output signal. The active load  16  plus a relatively small external resistance has a minimal voltage drop so that minimal headroom and power are consumed. The drain terminal D of the FET device  18  is coupled to a power source  34  and the source terminal S of the FET device  18  is coupled to the drain terminal D of the FET device  14 . The source terminal S and the gate terminal D of the FET device  18  are tied together by a self-biasing line  40  so that the FET device  18  is self-biasing, and a resistor  38  is provided in the line  40 . When the control signal is low, the FET device  14  conducts because it operates in the depletion mode, and current flows through the devices  14  and  18  allowing the FET device  14  to amplify the control signal. By including the resistor  38  in the self-biasing line  40 , a voltage drop is provided across the resistor  38  that provides the large voltage swings necessary to allow the FET device  14  to conduct very quickly, where the voltage drop across the resistor  38  can be properly calibrated for a certain performance by selecting a proper resistance. 
     By including the resistor  38  in the self-biasing line  40 , the low frequency response looking into the load  16  from the line  36  provides high impedance, which provides high gain, and the high frequency response looking into the load  16  from the line  36  provides low capacitance, which provides a broad-band response and faster transitioning of the edges of the square wave, i.e., higher slew rate. Derivations of the low and high frequency responses of the active load  16  illustrating these features are provided below, where gm is the transconductance of the FET device  18 , R is the resistance of the resistor  38 , r 0  is the intrinsic impedance of the FET device  18 , v x  is an imaginary voltage looking into the active load  16 , i x  is an imaginary current looking into the active load  16 , |v x /i x | is the impedance looking into the active load  16 , v g  is the gate terminal voltage of the FET device  18 , v s  is the source terminal voltage of the FET device  18 , v gs  is the gate/source terminal voltage (self-bias) of the FET device  18 , Cds is the drain/source terminal capacitance of the FET device  18 , and ω is angular frequency. The advantages of providing the resistor  38  in the self-biasing line  40  referred to herein is generally provided by the multiplied gmR affect. 
     The derivation of the low frequency response is provided in equations (1)-(5) below and shows that the resistor  38  has the effect of increasing the impedance r 0  by a gmR factor, which increases the load impedance. 
     
       
         
           
             
               
                 
                   
                     
                       v 
                       x 
                     
                     + 
                     
                       
                         i 
                         x 
                       
                        
                       R 
                     
                     + 
                     
                       gm 
                        
                       
                           
                       
                        
                       
                         v 
                         gs 
                       
                        
                       
                         r 
                         o 
                       
                     
                   
                   = 
                   0 
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       v 
                       x 
                     
                     + 
                     
                       i 
                       x 
                     
                     + 
                     
                       gm 
                        
                       
                           
                       
                        
                       
                         i 
                         x 
                       
                        
                       
                         R 
                         · 
                         
                           r 
                           o 
                         
                       
                     
                   
                   = 
                   0 
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       v 
                       x 
                     
                     + 
                     
                       
                         i 
                         x 
                       
                        
                       
                         ( 
                         
                           R 
                           + 
                           
                             gm 
                              
                             
                                 
                             
                              
                             
                               R 
                               · 
                               
                                 r 
                                 o 
                               
                             
                           
                         
                         ) 
                       
                     
                   
                   = 
                   0 
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
             
               
                 
                   
                      
                     
                       
                         v 
                         x 
                       
                       
                         i 
                         x 
                       
                     
                      
                   
                   = 
                   
                     R 
                      
                     
                       ( 
                       
                         1 
                         + 
                         
                           gm 
                            
                           
                               
                           
                            
                           
                             r 
                             o 
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
             
               
                 
                   
                      
                     
                       
                         v 
                         x 
                       
                       
                         i 
                         x 
                       
                     
                      
                   
                   = 
                   
                     
                       gm 
                        
                       
                           
                       
                        
                       R 
                        
                       
                           
                       
                        
                       
                         r 
                         o 
                       
                     
                     + 
                     R 
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     The derivation of the high frequency response is provided in equations (6)-(15) below and shows that the resistor  38  has the effect of reducing the capacitance Cds by a gmR factor. 
     
       
         
           
             
               
                 
                   
                     
                       v 
                       x 
                     
                     + 
                     
                       v 
                       gs 
                     
                     + 
                     
                       
                         gm 
                          
                         
                             
                         
                          
                         
                           v 
                           gs 
                         
                       
                       
                         ω 
                          
                         
                             
                         
                          
                         Cds 
                       
                     
                   
                   = 
                   0 
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       v 
                       x 
                     
                     + 
                     
                       
                         i 
                         x 
                       
                        
                       R 
                     
                     + 
                     
                       
                         gm 
                          
                         
                             
                         
                          
                         
                           v 
                           gs 
                         
                       
                       
                         ω 
                          
                         
                             
                         
                          
                         Cds 
                       
                     
                   
                   = 
                   0 
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       v 
                       x 
                     
                     - 
                     
                       v 
                       s 
                     
                   
                   = 
                   
                     
                       i 
                       x 
                     
                      
                     R 
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       v 
                       g 
                     
                     - 
                     
                       v 
                       s 
                     
                   
                   = 
                   
                     
                       i 
                       x 
                     
                      
                     R 
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
             
               
                 
                   
                     v 
                     gs 
                   
                   = 
                   
                     
                       i 
                       x 
                     
                      
                     R 
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       v 
                       x 
                     
                     + 
                     
                       
                         i 
                         x 
                       
                        
                       R 
                     
                     + 
                     
                       
                         gm 
                          
                         
                             
                         
                          
                         
                           i 
                           x 
                         
                          
                         R 
                       
                       
                         ω 
                          
                         
                             
                         
                          
                         Cds 
                       
                     
                   
                   = 
                   0 
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       v 
                       x 
                     
                     + 
                     
                       
                         i 
                         x 
                       
                        
                       
                         ( 
                         
                           R 
                           + 
                           
                             
                               gm 
                                
                               
                                   
                               
                                
                               R 
                             
                             
                               ω 
                                
                               
                                   
                               
                                
                               Cds 
                             
                           
                         
                         ) 
                       
                     
                   
                   = 
                   0 
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
             
               
                 
                   
                      
                     
                       
                         v 
                         x 
                       
                       
                         i 
                         x 
                       
                     
                      
                   
                   = 
                   
                     R 
                     ( 
                     
                       1 
                       + 
                       
                         gm 
                         
                           ω 
                            
                           
                               
                           
                            
                           Cds 
                         
                       
                     
                      
                     
                         
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       gm 
                        
                       
                           
                       
                        
                       R 
                     
                     
                       ω 
                        
                       
                           
                       
                        
                       Cds 
                     
                   
                   = 
                   
                     1 
                     
                       
                         ω 
                          
                         
                             
                         
                          
                         Cds 
                       
                       gmR 
                     
                   
                 
               
               
                 
                   ( 
                   14 
                   ) 
                 
               
             
             
               
                 
                   
                      
                     
                       
                         v 
                         x 
                       
                       
                         i 
                         x 
                       
                     
                      
                   
                   = 
                   
                     
                       1 
                       
                         ω 
                          
                         
                           ( 
                           
                             Cds 
                             gmR 
                           
                           ) 
                         
                       
                     
                     + 
                     R 
                   
                 
               
               
                 
                   ( 
                   15 
                   ) 
                 
               
             
           
         
       
     
     The foregoing discussion discloses and describes merely exemplary embodiments of the present disclosure. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the disclosure as defined in the following claims.