CE Amplifier and its Frequency Response

CE amplifier or common emitter amplifier is a type of bipolar junction transistor (BJT) amplifier that is widely used in electronic circuits. It provides high voltage gain, high input impedance, and low output impedance. The frequency response of a CE amplifier describes its behavior at different frequencies of the input signal.

Gain of a CE Amplifier

The gain of a CE amplifier is the ratio of the output voltage to the input voltage. It is usually expressed in decibels (dB). The gain of a CE amplifier depends on various factors such as biasing, load resistance, and frequency of the input signal.

Frequency Response of a CE Amplifier

The frequency response of a CE amplifier is the plot of gain versus frequency. It shows how the gain of the amplifier varies with the frequency of the input signal. The frequency response of a CE amplifier can be divided into three regions:

- Low-frequency region: In this region, the gain of the CE amplifier is relatively constant and is given by the DC gain. The DC gain is the gain of the amplifier at zero frequency. The low-frequency region extends from 0 Hz to a certain frequency called the cutoff frequency (f_c).
- Mid-frequency region: In this region, the gain of the CE amplifier starts to decrease with increasing frequency. The mid-frequency region extends from the cutoff frequency to a certain frequency called the mid-band frequency (f_m).
- High-frequency region: In this region, the gain of the CE amplifier decreases rapidly with increasing frequency. The high-frequency region extends from the mid-band frequency to the upper limit of the frequency response.

Why gain is highest at mid-frequencies?

The gain of a CE amplifier is highest at mid-frequencies because of the following reasons:

- In the low-frequency region, the gain of the CE amplifier is limited by the capacitive coupling between the input and output circuits. This coupling results in a high-pass filter that attenuates low-frequency signals. Therefore, the gain of the amplifier is relatively low in this region.
- In the high-frequency region, the gain of the CE amplifier is limited by the capacitive coupling between the transistor and the load resistor. This coupling results in a low-pass filter that attenuates high-frequency signals. Therefore, the gain of the amplifier is relatively low in this region.
- In the mid-frequency region, the gain of the CE amplifier is not limited by any capacitance. Therefore, it is relatively high in this region.

Conclusion

The gain of a CE amplifier is highest at mid-frequencies because it is not limited by any capacitance in this region. The frequency response of a CE amplifier depends on various factors such as biasing, load resistance, and frequency of the input signal. It is important to choose the appropriate values of these factors to achieve the desired frequency response of the amplifier.