GATE Electrical – Analog Electronics
Page 17: Frequency Response of Amplifiers
Frequency response describes how the gain of an amplifier changes with the frequency of the input signal. In practical amplifiers, gain is not constant for all frequencies.
Frequency Response Curve
The frequency response is usually represented by a graph of Voltage Gain (Av) versus Frequency (f).
The amplifier operates in three regions:
- Low Frequency Region
- Mid Frequency Region
- High Frequency Region
1. Low Frequency Region
At low frequencies, the gain decreases due to the effect of coupling capacitors and bypass capacitors.
Capacitive reactance becomes large, reducing signal transfer.
XC = 1 / (2Ï€fC)
Thus low-frequency gain drops.
2. Mid Frequency Region
In the mid-frequency range, capacitors behave like short circuits and the amplifier gain becomes approximately constant.
This region provides the maximum and stable gain.
Av ≈ Constant
3. High Frequency Region
At high frequencies, internal transistor capacitances affect the amplifier operation.
- Base-emitter capacitance
- Base-collector capacitance
These capacitances reduce gain at high frequencies.
Cut-off Frequencies
Two important frequencies define amplifier bandwidth:
- Lower Cut-off Frequency (fL)
- Upper Cut-off Frequency (fH)
At these frequencies, gain becomes:
Av = 0.707 × Av(max)
Bandwidth
Bandwidth is the range of frequencies where amplifier gain remains nearly constant.
Bandwidth = fH − fL
Important GATE Points
- Gain drops at both low and high frequencies.
- Mid-band gain is maximum and constant.
- Cut-off gain = 0.707 × maximum gain.
- Bandwidth = fH − fL.
Next Page → Bode Plot of Amplifiers
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