Common Mode Voltage Gain of Op-Amp
Correct Answer: (c) < 1
Explanation:
Common mode voltage gain (Acm) of an op-amp is generally much less than 1, typically in the range of 0.1 to 0.0001 or even lower in practical op-amps.
Definition:
Common mode voltage gain refers to the gain when the same voltage is applied to both the inverting and non-inverting input terminals of an op-amp.
Key Characteristics:
1. Ideal Op-Amp Common Mode Gain:
- In an ideal op-amp, Acm = 0 (theoretically zero)
- This means no output should appear when identical signals are applied to both inputs
2. Practical Op-Amp Common Mode Gain:
- Real op-amps have small but non-zero common mode gain
- Typical values: 0.0001 to 0.1 (depending on the op-amp model)
- This is much less than 1 (< 1)
3. Common Mode Rejection Ratio (CMRR):
CMRR = Ad / Acm (in linear form)
or CMRR (dB) = 20 log10(Ad/Acm)
Where:
- Ad = differential gain
- Acm = common mode gain
For example:
- If Ad = 100,000 and Acm = 1, then CMRR = 100,000 (or 100 dB)
4. Why Less Than 1?
- The internal symmetry of op-amp circuits is designed to reject common mode signals
- Only differential signals should be amplified
- Due to internal mismatches and asymmetries, a tiny amount of common mode signal appears at output
5. Practical Values for Common Op-Amps:
- 741 Op-Amp: CMRR = 90 dB (Acm ≈ 0.0001)
- TL071: CMRR = 100 dB (Acm ≈ 0.00001)
- Modern op-amps: CMRR = 120 dB or higher
Why Option C is Correct:
(a) >1: Incorrect - Common mode gain is never greater than 1 in practical op-amps
(b) =1: Incorrect - This would mean 0 dB rejection, which is not acceptable
(c) <1: Correct! This is the expected behavior for all practical op-amps
(d) None of mentioned: Incorrect
Practical Significance:
- High CMRR (low Acm) is essential for noise rejection
- Differential amplifiers rely on this property to eliminate common noise
- Critical for precision measurement and instrumentation circuits
Conclusion:
The common mode voltage gain of an op-amp is generally less than 1 (typically 0.0001 to 0.1), making it an excellent component for rejecting common mode noise and interference while amplifying only the differential signal.