Dual Input, Unbalanced Output Differential Amplifier

DUAL INPUT, UNBALANCED OUTPUT DIFFERENTIAL AMPLIFIER

The dual input unbalanced output differential amplifier is shown in Fig.3.20. It has two inputs and the output voltage is measured only at one collector with respect to ground.

DC Analysis

As the biasing of this circuit is identical to that of the dual input balanced output- circuit, the output of the dc analysis will be identical.

Thus the values of I_CQ and V_CEQ are obtained as

AC Analysis

The result of ac analysis using h-parameters can be obtained in a similar method as that for dual input balanced output amplifier.

Differential gain 

Input impedance 

Output impedance R_O = R_C

The differential gain is reduced to 50% compared to dual input balanced output amplifier. The input and output impedances remain unchanged.

SINGLE INPUT, BALANCED OUTPUT DIFFERENTIAL AMPLIFIER

The input signal is applied only to the base of Q₁ transistor with the other input connected to ground. The output is taken between the collector terminals C₁ and C₂. Since both the collector terminals are at the same dc potential, the output is called as "balanced output". The single input balanced output differential amplifiers is shown in Fig. 3.21.

DC Analysis

The same procedure is followed for the dc analysis because the dc equivalent circuit for all these configurations is the same.

AC Analysis

The parameters of ac analysis of single input balanced output differential amplifier are as follows.

Differential gain 

Input impedance 

Output impedance R_O = R_C

The gain is same as that of dual input balanced output configuration.

SINGLE INPUT UNBALANCED OUTPUT DIFFERENTIAL AMPLIFIER

The single input unbalanced output differential amplifier is shown in the Fig.3.22. The input is applied to the base of Q₁ transistor and the output is measured at collector terminal C₂ with respect to ground. This configuration is rarely used in practice.

DC Analysis

The dc equivalent circuit for this configuration is identical to that of the dual input, balanced output configuration. Thus the expressions for I_CQ and V_CEQ are obtained as

AC Analysis

The result of analysis of single input unbalanced output differential amplifier is obtained as follows

The gain is same as that of dual input unbalanced output configuration.

Improving CMRR

CMRR is the figure of merit of a differential amplifier. CMRR should be as high as possible. We know that CMRR is the ratio of differential voltage gain A_d and common mode voltage gain A_c. Thus CMRR can be increased by

i. Increasing A_d

ii. Decreasing A_c

Effect of Emitter Resistor R_E

R_E will not introduce any negative feedback for differential mode operation. Hence A_d does not depend on the value of R_E. But R_E introduces a negative feedback in common mode operation, this inturn reduces the common mode gain.

If R_E increases, negative feedback is also increased and the common mode gain is reduced. Hence CMRR will increase.

If R_E value is changed, then Q point will be shifted. Thus we have to go for other methods to improve CMRR, such as

i. Constant current bias

ii. Current mirror circuit

iii. Active load.