Question

For the differential amplifier given below, determine the Vx and RF value. Assume that the circuit is initially nulled.

(a) Vx = -8v, RF  = 9.9kΩ

(b) Vx = 8v, RF = 9.9kΩ

(c) Vx = -8v, RF  = -9.9kΩ

(d) Vx = 8v, RF = -9.9kΩ

The question was asked in my homework.

My query is from Differential Amplifiers with Multiple Op-Amp in division Operational Amplifier Characteristics of Linear Integrated Circuits

Answer 1

The correct answer is (b) Vx = 8V, RF = 9.9kΩ.

For a differential amplifier circuit that is initially nulled:

Vx (Null Voltage) = 8V

In a nulled differential amplifier, the null voltage is the voltage applied to one input to null out the offset voltage of the op-amp. Given the circuit parameters, this value is approximately 8V.

RF (Feedback Resistor) = 9.9kΩ

The feedback resistor value is calculated based on the gain requirements and the input resistor values. The 9.9kΩ value is the standard feedback resistance needed to achieve the desired gain and stability characteristics.

In differential amplifier circuits, proper nulling ensures that:

- Input offset voltage is minimized

- Output zero voltage is accurate

- Common-mode rejection is maintained

The combination of Vx = 8V and RF = 9.9kΩ provides the necessary conditions for proper circuit operation with minimal offset errors.