Question

What is the net output voltage for differential amplifier with one op-amp

(a) Vo = -(RF /R1)*Vx

(b) Vo = -(RF /R1)*(Vx -Vy)

(c) Vo = (1+RF /R1)*(Vx -Vy)

(d) None of the mentioned

I had been asked this question during an interview.

This question is from Differential Amplifiers with Multiple Op-Amp topic in portion Operational Amplifier Characteristics of Linear Integrated Circuits

Answer 1

The correct answer is:

(b) Vo=−(RFR1)(Vx−Vy)V_o = -\left(\frac{R_F}{R_1}\right)(V_x - V_y)Vo​=−(R1​RF​​)(Vx​−Vy​)

Explanation:

In a differential amplifier with one operational amplifier:

1. The output voltage (VoV_oVo​) is proportional to the difference between the input voltages (Vx−VyV_x - V_yVx​−Vy​).
2. The gain of the amplifier is determined by the ratio of RFR_FRF​ (feedback resistor) to R1R_1R1​ (input resistor).

Thus, the net output voltage is given by:

Vo=−(RFR1)(Vx−Vy)V_o = -\left(\frac{R_F}{R_1}\right)(V_x - V_y)Vo​=−(R1​RF​​)(Vx​−Vy​)

Why not the other options?

• (a) Vo=−(RFR1)VxV_o = -\left(\frac{R_F}{R_1}\right)V_xVo​=−(R1​RF​​)Vx​: This represents a single-input inverting amplifier, not a differential amplifier.
• (c) ( V_o = \left(1 + \frac{R_F}{R_1}\right)(V_x - V_y): This formula applies to some configurations of instrumentation amplifiers but not to a basic differential amplifier.
• (d) None of the mentioned: Incorrect, as option (b) is the correct formula.