Analog Electronics – Page 34
Low Frequency Analysis of Amplifiers
At low frequencies, the gain of an amplifier decreases due to the effect of capacitors present in the circuit.
The capacitors responsible for low frequency behavior are:
- Input Coupling Capacitor
- Emitter Bypass Capacitor
- Output Coupling Capacitor
1️⃣ Input Coupling Capacitor
The input capacitor blocks DC and allows AC signals to pass to the amplifier.
At low frequencies:
- Capacitive reactance increases
- Signal attenuation occurs
- Amplifier gain decreases
XC = 1 / (2πfC)
2️⃣ Emitter Bypass Capacitor
The emitter bypass capacitor provides AC ground for the emitter resistor.
At low frequencies:
- Capacitor reactance becomes large
- Emitter degeneration increases
- Voltage gain reduces
3️⃣ Output Coupling Capacitor
This capacitor transfers AC output to the load while blocking DC.
At low frequencies:
- Capacitive reactance increases
- Output signal reduces
Lower Cutoff Frequency
The lower cutoff frequency occurs when gain drops to 0.707 of midband gain.
fL = 1 / (2πRC)
Example Problem
If coupling capacitor C = 10 µF and resistance R = 1 kΩ.
Find lower cutoff frequency.
fL = 1 / (2πRC)
fL = 1 / (2π × 1000 × 10×10⁻⁶)
fL ≈ 15.9 Hz
Important GATE Points
- Low frequency roll-off caused by capacitors
- Three capacitors control low-frequency response
- Coupling capacitors block DC components
- Bypass capacitor increases voltage gain
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