written 5.8 years ago by | modified 2.7 years ago by |
written 2.7 years ago by |
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.
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