STK4131-② Internal circuit schematic diagram

The picture shows the internal circuit principle of the thick film integrated circuit ST K4131II. It consists of input stage, main voltage amplification and
output stage. The input stage consists of VTi and VTz to form a differential amplifier circuit. VT3 is a constant current source bias tube, and VDi
provides a constant current bias for VT3 and plays a temperature compensation role. VT4 is the main voltage amplifier stage and mainly provides voltage gain. VTs
is part of the VT4 load resistance, and the impedance changes with changes in signal amplitude. When the signal amplitude becomes larger, the impedance becomes smaller
to achieve compression of large-amplitude signals, expand the dynamic range of the circuit, and prevent distortion caused when the signal is too strong. VT6,
VT7, VTs, and VTg form a composite tube complementary push-pull power output stage. The double-tube multiplier circuit composed of VTs is used as
the bias circuit of the output stage to improve the bias accuracy of the quiescent current when the power supply voltage is different and stabilize the output stage operating point. VTio and
VTii are switching transistors that function as noise suppressors. In normal working conditions, pin 6 is above zero potential (≥1.2V),
v, rl 1 are not conducting, and VTio is conducting. Through the external resistor between pins 8 and 9, the VTlo base is at a negative potential, and the circuit is
powered and working normally. When pin 6 is at a negative potential, Vrl is conductive and VTlo is not conductive, so that VT3, VT2, and VTc
are not conductive. The emitter of VTio is connected to pin 4 through an external resistor, and the circuit does not work and is in a silent state. .