In this circuit, R2 decreases the input impedance of the op-amp.

In this circuit, R2 decreases the input impedance of the op-amp.

• A. Decreases the input impedance of the op-amp.
• B. Increases the input impedance.
• C. Has no effect on input impedance.
• D. Increases the output impedance.

Answer: (A)

The input impedance is what the source sees looking into the input node. If a resistor is connected from that input node to ground, it provides a direct path for current to ground and loads the input. Since the op-amp’s own input impedance is extremely large, the overall input impedance becomes the parallel combination of the op-amp input and this resistor, which is essentially just the value of the resistor. So, placing R2 there lowers the input impedance to roughly R2. If R2 were removed, the input impedance would increase toward the op-amp’s high input impedance. This is why R2 decreases the input impedance in this circuit.

The input impedance is what the source sees looking into the input node. If a resistor is connected from that input node to ground, it provides a direct path for current to ground and loads the input. Since the op-amp’s own input impedance is extremely large, the overall input impedance becomes the parallel combination of the op-amp input and this resistor, which is essentially just the value of the resistor. So, placing R2 there lowers the input impedance to roughly R2. If R2 were removed, the input impedance would increase toward the op-amp’s high input impedance. This is why R2 decreases the input impedance in this circuit.