Operational Amplifiers – Complete Theory
Page 2 – Op-Amp Equivalent Circuit
The practical op-amp can be represented using an equivalent circuit model. This model helps analyze op-amp circuits easily.
Equivalent Circuit Components
An operational amplifier can be modeled using three main elements:
- Input resistance (Ri)
- Voltage controlled voltage source
- Output resistance (Ro)
Open Loop Gain Model
The output voltage of the op-amp is:
Vo = A (V+ − V−)
Where- A = Open loop gain
- V+ = Non-inverting input
- V− = Inverting input
The open loop gain of practical op-amps is extremely high:
A ≈ 10⁵ to 10⁶
Input Resistance
Input resistance is very high so that almost no current enters the amplifier.
Ri ≈ ∞
This impliesI+ ≈ 0 I− ≈ 0
Output Resistance
The output resistance of an ideal op-amp is approximately zero.
Ro ≈ 0
This allows the op-amp to drive loads efficiently.Ideal Op-Amp Model
- Infinite input resistance
- Zero output resistance
- Infinite open loop gain
- Infinite bandwidth
GATE Important Points
- Input current ≈ 0
- Open loop gain extremely large
- Output voltage proportional to input difference
- Equivalent model used for circuit analysis
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