The signal from a synchro receiver or a variable resistive cam follower (potentiometer) is boosted by operational amplifier Ul, whose output swing is limited by back-to-back zeners D3 and D4. The signal is then applied to operational amplifiers U2 and U3, which drive the gates of Ql and Q2 respectively. The npn transistor (Q3) is a fast current limiter for the n-channel MTM8N10; a pnp transistor (Q4) performs the same function for the p-channelMTM8P10. Capacitors C3 and C4 eliminate the need for accurate dc offset zeroing.
T1 steps up the output voltage to 120 V for the 400 Hz servo motor.