Electronics Principles V11

Clive W. Humphris

OP-AMP THEORY: Inverting Long Tail Pair.  

The long-tailed pair transistor configuration forms the basis of the Operational Amplifier. In this example it consists of two NPN transistors with their emitters connected together and having a common emitter resistor. The voltage developed across the emitter resistor through either transistor conducting will be reflected as a change in collector current in the other.

The amplifier has two inputs, inverting and non-inverting, with a single output.

INVERTING INPUT. A signal applied to the (-) input is phase inverted between base and collector of T1. Increasing the base current causes the collector current to also increase, thereby the pulling down the collector voltage, as a greater voltage is dropped across the collector load resistor R1.

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