LM internal circuit principle

LM12 is a new high-performance high-power operational amplifier launched by National Semiconductor. This operational amplifier has extremely high technical
indicators and excellent performance. When the power supply voltage is ±40V, it can output about 150W sinusoidal power to 40 loads with
a distortion rate of only 0-ol%. When a 60Hz square wave and a 7kHz sine wave signal are mixed at a ratio of 4:1, the transient mutual distortion
rate is only 0.015%. The shape of this op amp is like a metal-encapsulated high-power triode. Under the voltage
of 10A, it can output a driving current of 10A. It is not only an audio power amplifier device with excellent performance, but also an ideal
servo motor driver.
    LM12 is available in a 3 package with only 4 leads and the shell is connected to V1. Figure 2-100 is its internal circuit principle .
    In Figure 2-100, VTi and VTz form an emitter output device as the input stage. The input impedance is very high, and the maximum bias current is
1. VT3 and VT4 are voltage amplification stages, and VTs and VT6 are mirror constant current source loads. VB and VT8 constitute a composite tube
amplifier stage. The other transmitter of the double emitter transistor VT7 is used as the bias of the mirror constant current source circuit. VTg is the output push
stage. VTio~VT13 form a composite complementary push-pull output stage. VT14 and VTis form a clamping output protection circuit. When the negative
When the load is an inductive load, if the back electromotive force exceeds the safe voltage, the two tubes will be connected to protect the output tube.
    The input wall of the circuit is equipped with protective resistors Rt, R2 and protective diodes vDl, VD2, so that
the circuit will not be damaged even if the input signal is very strong. When the power supply voltage is less than 14V (±7V), the output is disconnected. When the LM 12 case temperature exceeds
150°C, the output is automatically disconnected, but it automatically recovers when it drops below 145°C. When the power supply voltage reaches
the VCEO of the output tube, the output is automatically disconnected. At any output voltage, if the output current reaches 10A, it has a complete overload protection
function.