The frequency response of a transistor amplifier refers to how its voltage gain changes with signal frequency. Ideally, the gain should remain constant over the desired frequency range. However, in practice, the gain deteriorates at both low and high frequencies due to various circuit components and effects.
The two main factors that cause deterioration are:
1. Coupling and Bypass Capacitors (Affecting Low-Frequency Response):
At low frequencies (below f₁, the lower cut-off frequency), the following capacitive components contribute to the drop in gain:
- Input Coupling Capacitor (Cin) and Output Coupling Capacitor (Cout):
- These capacitors have high reactance at low frequencies, which restricts the AC signal from passing through effectively. As a result, only a small part of the signal reaches the amplifier, reducing the voltage gain.
- Emitter Bypass Capacitor (Cb):
- At low frequencies, the reactance of this capacitor is also high, and it cannot effectively bypass the emitter resistor (Re). This causes more of the input signal to drop across Re, reducing the output.
Effect on Performance:
- Causes reduced signal transfer through the amplifier.
- Leads to drop in amplifier gain at frequencies below the lower cut-off frequency (f₁).
2. Diffusion and Stray Capacitances (Affecting High-Frequency Response):
At high frequencies (above f₂, the upper cut-off frequency), internal capacitances cause deterioration:
- Base-Emitter Diffusion Capacitance:
- At high frequencies, this capacitance presents low reactance, increasing base current. This results in reduced current amplification (β) and thus lowers the voltage gain.
- Collector-to-Earth Stray Capacitance:
- Also decreases in reactance with frequency, introducing a loading effect and further lowering the amplifier’s output gain.
- Cout at High Frequencies:
- Acts as an additional load to the next stage, further reducing the voltage gain.
Effect on Performance:
- Results in loss of gain due to internal capacitance effects.
- Leads to drop in amplifier gain at frequencies above the upper cut-off frequency (f₂).
Conclusion:
- At low frequencies (< f₁): Gain drops due to high reactance of coupling and bypass capacitors (Cin, Cout, Cb).
- At high frequencies (> f₂): Gain drops due to low reactance of base-emitter diffusion capacitance and stray capacitances.
The maximum, stable gain is maintained in the mid-frequency range, and the cut-off frequencies f₁ and f₂ are defined where the gain falls to 70.7% of its maximum value. For optimal performance, the amplifier should be designed to operate within this mid-frequency range.