Internal circuit principle of LM386

LM386 is a variety of National Semiconductor's series of power amplifier integrated circuits. It is widely used in communication equipment, recorders and various playback equipment due to its low power consumption,
wide operating power supply voltage range, and easy adjustment of peripheral components and devices.
middle. The following is an introduction to its internal circuit composition, performance parameters and usage methods.
    Figure 2-50 is the internal circuit schematic diagram of LM386. The whole circuit consists of 10 transistors, 2 diodes and 7 resistors
.
    VTi ~ VT6 form a PNP composite differential input stage, in which VTs and VT6 are mirror constant current sources and serve as
active loads for VT3 and VT4, so that the input stage has stable gain. The voltage amplification stage is played by VT7 connected in a common emitter configuration, and
its load also uses a constant current source. The open-loop gain of the entire circuit is mainly determined by this stage. VT8 and VTg form a composite
PNP tube, and VTloNPN tube forms a complementary push-pull output stage. VDj and VDj form the bias of the output stage, so that the final stage bias is
in the Class A and B working state orRs^-R7 form an internal feedback circuit.
    LM386 has a wide range of applications. Due to its low voltage and low power consumption characteristics, it is particularly suitable for
electronic devices that use dry batteries as power sources. Figure 2-51 is a typical application of LM386 . In the figure, there is no resistor-
capacitor component connected between pin 1 and pin 8. At this time, the circuit gain is only determined by the internal resistors R7 and R5+R6, which is 20 times. It can be seen from the internal circuit analysis that
connecting different resistor-capacitance components in series between pin 1 and pin 8 can change the AC negative feedback amount of the amplifier
, thereby changing the closed-loop gain of the amplifier. When only a capacitor of tens of microfarads is connected between pin 1 and pin 8, the internal resistor R6 is bypassed by AC, and
the amplifier gain reaches the maximum value, which is about 200 times (2R7/Rs). If you want to change the low end of the amplifier Frequency response, an RC network can be connected in series between pin 1 and
pin 5 of the output terminal. The resistance and capacitance value of the series RC network should be adjusted according to the specific requirements. When pin 8 is open,
R is 10kfl, and C is 0.033t.-eF, the 50-200 ratio low-end frequency response of the amplifier can be improved by about 24%. This is obviously beneficial to improving
the poor low-frequency response caused by the use of small-diameter speakers in portable radios.