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Explain use of constant current source in diff amps. Give description of any one type.

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Need of Constant Current Source:

For dual input balanced output, CMRR is given by following formula,

$CMRR=\frac{Ad}{Ac}=\frac{(re+2Re)}{re}$

Hence we can see that CMRR dependent on Re. To improve CMRR we need to increase Re. But Re cannot be increased to a great extent, since the dc current will decrease which will change the Q point of the circuit.

Since, we want constant current in the emitter terminal and also very high resistance in the emitter terminal. Hence Re is replaced by constant current source which provide very high resistance.

The different Constant Current Source are:

Using BJT :

i. Cascode Current Source

ii. Current Mirror Constant Current Source

iii. Wilson Current Mirror

iv. Wilder Current Mirror

Using MOSFET :

i. Wilson Current Mirror

ii. Current Mirror

iii. Cascode Current Source

Current Mirror Constant Current Source using BJT:

It is a circuit in which output current is made equal to input current called as constant mirror circuit. In this output current is mirror image of input current.

Figure below shows the circuit diagram for current mirror constant current source,

Apply KVL,

VBE3=VBE4

Hence

IB3=IB4

IC3=IC4

But,

I2=IC4+I by KCL

I=IB3+IB4

I=2 IB3

I2=IC4+2 IB3

But, IC3=IC4

I2=IC3+2 IB3

$I2=IC3+\frac{2IC3}{\beta}$

$I2=IC3{(1+\frac{2}{\beta})}$

Since,$\beta\gt\gt2 then 2/\beta≈0$

I2≈IC3

The current mirror circuit requires less components than constant current source. Hence packing density of differential amplifier increases.