Question:
In the ideal case, what is the input impedance of an operational amplifier?
In the ideal case, what is the input impedance of an operational amplifier?
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Ideal Op-Amp parameters to memorize:
- Input Impedance = \(\infty\) (Draws zero current, prevents source loading).
- Output Impedance = \(0\) (Can supply unlimited current to any load without a voltage drop).
Updated On: Jun 23, 2026
- Infinite
- \( 1\text{ M}\Omega \)
- Zero
- \( 100\ \Omega \)
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The Correct Option is A
Solution and Explanation
Concept:
An operational amplifier (op-amp) is a high-gain differential voltage amplifier. The structural performance attributes of an ideal op-amp are modeled using specific boundary conditions:
• Input Impedance (\(Z_{\text{in}}\)): Tends to infinity (\(\infty\)).
• Output Impedance (\(Z_{\text{out}}\)): Tends to zero (\(0\)).
• Open-loop Voltage Gain (\(A_v\)): Tends to infinity (\(\infty\)).
• Bandwidth (\(BW\)): Tends to infinity (\(\infty\)).
• Common Mode Rejection Ratio (CMRR): Tends to infinity (\(\infty\)).
Step 1: Understanding the physical significance of infinite input impedance.
The input impedance is the internal opposition encountered by an electrical signal source looking into the differential input terminals (between the inverting and non-inverting terminals). By Ohm's law, the current drawn by the op-amp input pins is given by: \[ I_{\text{in}} = \frac{V_{\text{in}}}{Z_{\text{in}}} \]
Step 2: Evaluating the limit for the ideal case.
In the ideal limit, we substitute \(Z_{\text{in}} \to \infty\): \[ I_{\text{in}} = \lim_{Z_{\text{in}} \to \infty} \frac{V_{\text{in}}}{Z_{\text{in}}} = 0\text{ A} \] An input impedance of infinity ensures the op-amp draws zero current from the driving signal source. This prevents loading effects, meaning the amplifier can read the input signal voltage without attenuating or distorting the original signal source. This matches Option (A).
• Input Impedance (\(Z_{\text{in}}\)): Tends to infinity (\(\infty\)).
• Output Impedance (\(Z_{\text{out}}\)): Tends to zero (\(0\)).
• Open-loop Voltage Gain (\(A_v\)): Tends to infinity (\(\infty\)).
• Bandwidth (\(BW\)): Tends to infinity (\(\infty\)).
• Common Mode Rejection Ratio (CMRR): Tends to infinity (\(\infty\)).
Step 1: Understanding the physical significance of infinite input impedance.
The input impedance is the internal opposition encountered by an electrical signal source looking into the differential input terminals (between the inverting and non-inverting terminals). By Ohm's law, the current drawn by the op-amp input pins is given by: \[ I_{\text{in}} = \frac{V_{\text{in}}}{Z_{\text{in}}} \]
Step 2: Evaluating the limit for the ideal case.
In the ideal limit, we substitute \(Z_{\text{in}} \to \infty\): \[ I_{\text{in}} = \lim_{Z_{\text{in}} \to \infty} \frac{V_{\text{in}}}{Z_{\text{in}}} = 0\text{ A} \] An input impedance of infinity ensures the op-amp draws zero current from the driving signal source. This prevents loading effects, meaning the amplifier can read the input signal voltage without attenuating or distorting the original signal source. This matches Option (A).
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