An Op-amp Circuit

*An operational amplifier is usually composed of: 1 - A differential amplifier stage; 2 - Additional amplifers stages to increase the voltage gain; 3 - An emitter follower amplifier (class AB push-pull) to the output stage. *In this circuit, the input stage is a differential amplifier that uses current mirrors (active load) instead of RC and RE, providing greater gain; *The gain stage is a Darlington transistor pair. The first transistor (emitter follower) provides a large input impedance, minimizing the loading effects on the input differential stage. The other transistor, that uses an active load (current mirror), provides an additional gain; *The output stage is a class AB amplifier. The first transistor (that uses a active load) provides a high gain and input impedance, minimizing the loading effects on the gain stage. The diodes are used in the biasing of the push-pull configuration formed by the 2 last transistors, reducing the cross-over distortion; *The capacitor provides a variable negative feedback (according to the signal frequency) that stabilizes the opamp in feedback configurations. *The final voltage gain is: Av = |Ad|*|A2|*|A3| -Ad -> Differential stage gain -A2 -> Second stage gain -A3 -> Output stage gain *This gain is extremely high (over 100,000).