Operational Amplifiers – Complete Theory
Page 3 – Open Loop vs Closed Loop Operation
An operational amplifier can operate in two modes:
- Open Loop Operation
- Closed Loop Operation
1. Open Loop Operation
In open loop operation, no feedback is used.
Vo = A (V+ − V−)
Where- A = Open loop gain (very large)
- V+ = Non-inverting input
- V− = Inverting input
The open loop gain of an op-amp is extremely high:
A ≈ 10⁵ to 10⁶
Because of this large gain, even a very small input difference produces a large output voltage.
Applications
- Comparator circuits
- Zero crossing detectors
- Switching circuits
2. Closed Loop Operation
Closed loop operation uses negative feedback.
Negative feedback stabilizes the gain and makes the amplifier linear.
Af = A / (1 + Aβ)
Where- A = Open loop gain
- β = Feedback factor
- Af = Closed loop gain
Advantages
- Stable gain
- Improved bandwidth
- Reduced distortion
- Better linearity
Comparison
| Parameter | Open Loop | Closed Loop |
|---|---|---|
| Feedback | No feedback | Negative feedback used |
| Gain | Extremely high | Controlled gain |
| Stability | Poor | Good |
| Applications | Comparators | Amplifiers |
GATE Important Points
- Open loop gain ≈ 10⁵ – 10⁶
- Closed loop gain controlled using feedback
- Negative feedback improves stability
- Most op-amp circuits use closed loop configuration
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