differential amplifier

THE DIFFERENTIAL AMPLIFIER

The Differential Amplifier is often called the Long Tailed Pair, the high value resistance R3 being the tail.

It has two inputs and one output.
It amplifies the difference between the two inputs.
(Input voltage 1, minus Input voltage 2) x gain = output voltage.

The idea is to keep the total collector current of the transistors (i.e. the current through R3) constant.
If Tr1 base voltage rises, then its collector current rises, its emitter voltage rises and its collector voltage falls. If at the same time the base voltage of Tr2 is held fixed, then the increase in emitter voltage will reduce its forward bias, and its collector current will fall and its collector voltage will rise.

If both input voltages are the same (called common-mode signals) then there is no output.
This means that if there is an interfering signal picked up by both inputs then it does not appear at the output.

If there is a rise in temperature it has no effect on the circuit since both transistors will be affected to the same degree.

The value of R3 should be as large as possible.

R3 can be replaced by an electronic equivalent, a CONSTANT CURRENT SOURCE.
This keeps the current in the circuit constant.

The base voltage of Tr3 is fixed by D1 and R4.
Therefore the current through R3 is fixed and constant and the emitter and collector voltages are fixed and constant.

The voltage across R3 is the diode voltage minus 0.7 volts (the base/emitter voltage).

The long tailed pair is shown here as a comparator in a stabilised power supply circuit.

A sample of the output from the power supply is compared with a reference voltage.

If they are the same then the output is correct and no control voltage is generated.

If the output is too high or too low, then since the two inputs are different, a control signal is produced which corrects the ouput.