Question

A differential amplifier has two inputs of 5 mV and 3 mV. The output is 300 mV. What is the amplifier gain? Also find the output voltage, if now inputs of 153 mV and 155 mV are applied?

• A. Gain=150, Output=2 mV
• B. Gain=300, Output=300 mV
• C. Gain=150, Output=300 mV
• D. Gain=300, Output=150 mV

Answer 1

The correct option is C Gain=150, Output=300 mV

Difference mode gain

$A_d=\frac{v_o}{v_2-v_1}=\frac{300\times {10}^{-3}}{(5-3)\times {10}^{-3}}=150$

Output voltage

$e_0=A_d(v_1-v_2)=150(155-153)=300mV$

Note that the inputs in the second case are much larger than what they are in the first case. Same output voltage is obtained since the difference of the two input voltages is the same i.e, 2mV.