Abstract:
An output stage is disclosed wherein class AB bias is employed. The stage is quiescently biased by means of current mirrors so that the bias is controlled mainly by ratioed geometric elements. The output transistors are biased by means of unity gain common gate drivers that provide the desired level shifting. The output voltage can be swung from from close to the rail potential of the source of the n channel output transistor to close to the rail potential of the source of the p channel transistor. The circuit can drive relatively large load currents and can be fabricated using either CMOS or conventional bipolar integrated circuits.

Description:
BACKGROUND OF THE INVENTION 
     Complementary metal oxide semiconductor (CMOS) integrated circuits are well known in the semiconductor art. In fact, digital IC CMOS devices have captured a wide segment of the IC market. More recently the CMOS approach has been applied to linear circuit applications. In many cases an IC will require linear as well as digital functions on a single chip. It has been a desired goal to implement linear circuits in CMOS form without sacrificing the excellent performance that has been attained using bipolar device technology. In the present case it is desired to produce a class AB amplifier output stage using either bipolar or CMOS transistors that can act to drive relatively heavy loads and swing very close to the supply potentials. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to produce a transistor output stage having class AB characteristics. 
     It is a further object of the invention to operate a transistor amplifier output stage in class AB so that heavy loads can be driven in push pull with rail to rail drive capability and wherein the stage is insensitive to supply voltage variations, process variations and temperature. 
     These and other objects are obtained using a circuit configured as follows. A complementary output tran-sistor pair of relatively large size is coupled between the power supply rails with their drains (or collectors) providing the stage output. The gates (or bases) of the output pair are coupled together by a complementary pair of parallel connected drivers that act as common gate (or base) level shifters. Quiescent bias or the output pair is obtained from a complementary pair of constant current transistors that are operated as current mirrors. The current mirror inputs are obtained from relatively low current source and sink supplies. The transistor sizes are ratioed so that the output pair quiescent current is a relatively large multiple of current mirror inputs. A bipolar transistor emitter follower is employed to drive the gate (or base) of one of the output transistors. 
     While the output stage is described in detail in terms of CMOS devices, it can be constructed in bipolar transistor form. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a simplified schematic diagran of the circuit of the invention. 
     FIG. 2 is a CMOS schematic diagram of the circuit of the invention. 
     FIG. 3 is a bipolar transistor schematic diagram of the circuit of the invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     In FIG. 1 the circuit is operated from a power supply connected + to terminal 10 and - to ground. The output stage has an output at terminal 11 that responds to the input of terminal 12. A large area P channel output transistor 13 is coupled in series with a large area N channel output transistor 14 so that both conduct the same quiescent current. For this condition I 1  =I 2 . As will be shown hereinafter these output transistors are desirably scaled so that they have substantially the same gain. 
     A current source 15 conducts I 3  from terminal 10 to the gate of transistor 13. I 3  is split into two parts I 4  and I 5  which flow respectively in complemetary transistors 16 and 17 which are connected between the gates of transistors 13 and 14. 
     A current sink 18 conducts I 6  from the gate of transistor 14 to ground. Driver 19 modulates a current I 7  which is coupled to flow in sink 18 as a part of I 6 . I 6  =I 4  +I 5  +I 7 . A constant voltage source 20 maintains the gate of transistor 16 at two P channel transistors thresholds below the potential at terminal 10. Another constant voltage source 21 maintains the gate of transistor 17 at two n channel transistor thresholds above ground. This biasing arrangement causes transistors 16 and 17 to operate as common gate unity gain voltage level shifters. 
     As driver 19 modulates I 7  under small signal conditions, the potential at the gate of transistor 14 will be varied, thus varying I 2 . As I 7  increases, the potential at the gate of transistor 14 will rise and I 5  will decrease because I 6  is constant. This will raise the potential at the gate of transistor 13 and thereby decrease I 1  so that terminal 11 will sink current. Since the drain of transistor 17 looks into the source of transistor 16 the common gate connection will produce unity gain from the gate of transistor 14 to the gate of transistor 13. In the converse, as driver 19 reduces I 7 , I 5  will have to increase. This will lower the potential on the gate of transistor 14 and I 2  will fall. The action of transistor 17 will pull the potential at the gate of transistor 13 down thereby increasing I 1  so that terminal 11 will source current. Thus, transistors 13 and 14 are driven in push pull. 
     It is to be understood that while driver 19 is shown coupled directly to the gate of transistor 14 it could be coupled to the gate of transistor 13. If this connection alternative is employed, the driver current, I 7  would flow as components of I 3 , I 4  and I 5 . 
     Capacitor 22 which is coupled between the gates of transistors 13 and 14 frequency compensates the amplifier. In effect it acts as a feed-forward element that is particularly useful under large signal conditions. In such a circumstance, terminal 12 is taken very low causing I 4  to decrease to nearly zero as the gate of transistor 13 falls. A high impedance then exists at the drain of transistor 17 causing it to exhibit voltage gain and excessive phase shift. Capacitor 22 feeds the signal from driver 19 forward of the potentially troublesome gain stage that transistor 17 represents. 
     FIG. 2 is a schematic diagram of the operating circuit. Where the parts function as described in FIG. 1, the same numerals are used. Transistors 13, 14, 16 and 17 function the same. 
     Current source 15 is achieved by P channel transistor 15&#39; which has its gate coupled to current mirror transistor 23 which has its gate coupled to its drain. P channel transistor 24 also has its gate returned to its drain and is coupled in series with transistor 23. Current sink 25 causes I 8  flow in transistors 23 and 24. These transistors when conducting develop a combined voltage drop of two P channel transistor thresholds. Transistors 23 and 15E&#39; can be ratioed so that I 3  is a multiple of I 8 . In addition, it can be seen that transistor 13 will mirror the current in transistor 15&#39;. Thus, a ratio between these transistors can make I 3  a fraction of I 1 . This means that the quiescent value of I 1  can be well controlled as a result of the geometric transistor ratios. As will be shown hereinafter transistors 23, 15&#39; and 13 can be ratioed so that I 1  is twenty times I 8 . 
     Current sink 18 is achieved by n channel transistor 18&#39; which is current mirror driven by transistor 26 which has its gate returned to its drain. Series connected transistor 27 also has its gate returned to its drain. Current source 28 causes I 9  to flow in transistors 26 and 27 which therefor develop a voltage drop of two n channel transistor thresholds. These transistors are ratioed so that the ratio of I 2  /I 9  is the same as I 1  /I 8 . Thus, relatively modest bias currents set up the amplifier quiescent current for class AB operation. 
     Driver 19 is functionally accomplished by emitter follower transistor 19&#39;. This transistor has its collector dedicated to the supply terminal 10 which is the CMOS IC substrate. This device has a large current gain so that the input current supplied to terminal 12 is a very small fraction of I 7 . 
     FIG. 3 is a schematic diagram of the bipolar transistor version of the FIG. 2 circuit. The various transistors are labeled with the same numerals as the FIG. 2 designations with a prime sign added to distinguish the bipolar devices. The circuit performs in substantially the same way. The x designation relate to the PNP transistor areas and the y designations relate to the NPN transistor areas. It will be noted that the same area ratios are employed and the same bias currents can be employed. Thus, a 10 microampere source 28 will bias the output transistors at 200 microamperes. It was found that the bipolar transistor version did not require the feed forward capacitor 22. 
     EXAMPLE 
     The circuit of FIG. 2 was constructed using conventional CMOS p well IC construction. The p wells for the n channel transistors were all grounded. The following transistor ratios were employed: 
     
         ______________________________________Device    (Drawn Size W/L Microns)______________________________________13        2200/714        1500/915&#39;       440/716, 17    150/1118        600/923        110/724, 27    75/1126        75/9______________________________________ 
    
     Current sink 25 and current source 28 were both operated at 10 microamperes and capacitor 22 was 10 picofarads. The quiescent were 200 microamperes. The output swing with a 2K load was within 200 millivolts of each rail when using a 4 to 16 volt power supply and the circuit was stable when driving a 100 picofarad load capacitor. Using a 10-volt supply, the power supply rejection ratio was in excess of 80 db, the circuit could source and sink up to 50 ma in class B operation, and had a slew rate of 1.7 volts per microsecond. 
     The invention has been described and a working example detailed. When a person skilled in the art reads the foregoiong description, alternatives and equivalents, within the spirit and intent of the invention, will become apparent. Accordingly, it is intended that the scope of the invention be limited only by the following claims.