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In-depth analysis of differential amplifier circuits (features/role/circuit diagram)

In circuit design, it is essential to master the basic circuit, and in the basic circuit, the differential amplifier circuit is a very important circuit, but in many hardware learners may not have a particularly firm grasp of it, here to study and analyse it in depth.

It consists of two symmetrical basic amplification circuits (with opposite input voltages) and a common resistive coupling of the output poles, also known as the emitter stage.

Differential amplifier circuit characteristics.

Symmetrical on both sides of the circuit: both sides of the circuit are identical in terms of element composition and current-voltage magnitude, but in different directions.

Common emitter resistor: the emitter currents are superimposed, i.e. twice the current of a single amplifier

With two signal inputs: this is the same as a common-mode circuit, with two input signal terminals

The signal can be either double-ended or single-ended: there are four types: single-ended input and single-ended output; single-ended input and double-ended output; double-ended input and single-ended output; double-ended input and double-ended output.

The role of differential amplification

To understand the role of differential amplification circuits, it is necessary to first understand the scenarios in which they are used.

During the operation of a circuit, the surrounding environment, including temperature, is not constant, so when some factors change, the characteristics of the components will change, causing zero drift and changes in the static operating point.

Therefore, in a direct-coupled amplifier circuit, the error drift voltage and the useful signal are amplified and transmitted to the next stage of the amplifier circuit due to possible changes in the operating environment. A differential amplifier circuit, on the other hand, does a good job of avoiding this and effectively suppressing zero drift.

The usual way to overcome temperature zero drift is to introduce negative DC feedback, or temperature supplementation.

In specific operation, differential amplification circuits can be very good at suppressing zero drift. For example, for single-ended output differential amplification circuits, the emitter influence is the same in both directions at the output because the emitter resistors are shared, and this does not lead to different effects being produced (for the emitter resistor R). The current flowing through the emitter resistor R at this point is 2i, so the static operating point can be stabilised.

For differential amplification circuits with double-ended outputs, because the voltage drops at both ends are equal, for example, if the temperature increases by T, the collector currents of the two tubes will change accordingly, but the difference between the two changes is the same, so the final voltage at the two collectors is also the same. This effectively suppresses the zero drift phenomenon (static operating point away from the original position).

Double-ended input and double-ended output differential amplifier circuit diagram (below)

Half-sided equivalent circuit diagram (below)

So when there is an additional common mode signal input, the differential amplifier circuit can effectively suppress its interference and suppress the common mode signal.

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