OTL low frequency power amplifier circuit

The OTL low-frequency power amplifier circuit eliminates the need for bulky
transformers and is an amplifier circuit with good performance.
The OTL low-frequency power amplifier circuit is shown in the figure.
    Transistor VT1 is 3DG6, VT2 is 3DG12, and
VT3 is 3CG12. The component parameter reference values
​​are shown in Figure 21-12.
  Among them, VT1 is a push stage (also called a preamplifier
stage), which works in Class A state. Its collector current
Ici is adjusted by the potentiometer RP1. Part of Icl flows
through potentiometer RP2 and diode VD. Provide bias voltage to VT2 and
VT3. VT2 and VT3 are a pair of
NPN and PNP transistors with symmetrical parameters, which form
a complementary push-pull transistor. Amplifier circuit. Since the VT2 and VT3
transistors are connected in the form of emitter output, the amplifier has the advantages of low output resistance and strong load capacity, and is suitable for use as a power output stage.
By adjusting RP2, VT2 and VT3 can obtain appropriate quiescent current and work in Class A and Class B states to overcome crossover distortion.
    When static, the potential of midpoint A of the output end is required to be "-VIr/2, which can be achieved by adjusting RP1. Since one end of RP1
is connected to point A, AC and DC voltage parallel negative feedback is introduced in the circuit, which can stabilize the amplifier on the one hand. The static operating point also improves the nonlinear distortion of the amplifier
. ,
    c2 and R constitute a bootstrap circuit, which is used to increase the amplitude of the positive half cycle of the output voltage to obtain a large dynamic range.