Class ab push-pull drive circuit, drive method therefor and class ab

Abstract

A class AB push-pull drive circuit has two NPN transistors Q.sub.1, Q.sub.2 and two PNP transistors Q.sub.3, Q.sub.4, the emitters of which are connected together. A constant voltage circuit maintains voltages between the bases of transistors Q.sub.1 and Q.sub.3 and between the bases of transistors Q.sub.2 and Q.sub.4 constant. A differential input voltage is applied across the bases of the transistors Q.sub.1 and Q.sub.2. The collector currents in the transistors Q.sub.1, Q.sub.3, Q.sub.2 and Q.sub.4 increase or decrease in an exponential and differential manner. When the collector currents are in their differential relationship, they are inverted and added to provide a class AB drive current and to increase the output amplitude. The symmetry can also be improved. Since any difference between signal amplification path lengths is eliminated, a differential phase between paths is less likely to be generated in the high-frequency region. Since there is no feedback or the like for class AB drive, it is difficult for any abnormal oscillation to be produced. Since the constant voltage circuits are formed by diodes, the temperature characteristics of the transistors Q.sub.1 to Q.sub.4 can be compensated. Furthermore, the class AB push-pull drive circuit can be driven by a relatively low voltage. The bipolar transistors Q.sub.1 -Q.sub.4 may be replaced by FETs.