JNTU B.Tech II Year ELECTRONIC CIRCUITS Previous Question Paper (October - 2020 - R16)

A Common Base (CB) BJT amplifier is designed by connecting the base terminal to a common reference (ground) for both input and output signals. The input signal is applied to the emitter, and the output is taken from the collector.

Design Considerations:

• Biasing: A voltage divider or fixed bias circuit is used to set the operating point (Q-point) in the active region.
• Coupling Capacitors: Input and output capacitors are used to block DC bias while allowing AC signals to pass.
• Bypass Capacitor: The base is connected directly to ground or through a capacitor to ensure it remains at AC ground potential.

Analysis:

Using the hybrid-pi model, where r_e is the AC emitter resistance (r_e = 26mV / I_E):

The voltage gain is high and non-inverting (0° phase shift).

Since α is slightly less than 1, the current gain is approximately unity.

The input impedance is very low, making it suitable for low-impedance signal sources.

The output impedance is high, determined primarily by the collector resistor R_C.

The CB amplifier is characterized by low input impedance, high output impedance, and high voltage gain, making it ideal for impedance matching applications and high-frequency operations due to the absence of the Miller effect.

The frequency response of a BJT amplifier is divided into three regions: the low-frequency region, the mid-band region, and the high-frequency region. The overall gain remains constant in the mid-band, while it rolls off at lower and higher frequencies due to reactive components.

Low-Frequency Response: In this region, the gain decreases due to coupling capacitors (C_C1, C_C2) and the emitter bypass capacitor (C_E). At low frequencies, the reactance of these capacitors (X_C = 1 / (2πfC)) becomes significant, causing them to act as open circuits rather than short circuits. This results in signal attenuation and a reduction in voltage gain. The lower cutoff frequency (f_L) is defined as the frequency where the gain drops by 3 dB from its mid-band value. The dominant pole is typically determined by the largest time constant associated with these capacitors.

High-Frequency Response: In this region, the gain decreases due to the internal parasitic capacitances of the BJT, specifically the base-emitter junction capacitance (C_be) and the collector-base junction capacitance (C_bc). As frequency increases, the reactance of these capacitances decreases, effectively shunting the signal to ground or providing negative feedback. The Miller effect significantly multiplies the impact of C_bc, as it appears as a large capacitance at the input. The upper cutoff frequency (f_H) is the frequency where the gain again drops by 3 dB from the mid-band value.

Important Parameters:

• Mid-band Gain (A_mid): The maximum stable gain achieved when all coupling and bypass capacitors act as short circuits and parasitic capacitances act as open circuits.
• Lower Cutoff Frequency (f_L): The frequency at which the gain is 0.707 of the mid-band gain, caused by external coupling and bypass capacitors.
• Upper Cutoff Frequency (f_H): The frequency at which the gain is 0.707 of the mid-band gain, caused by internal transistor capacitances.
• Bandwidth (BW): Defined as BW = f_H - f_L. It represents the range of frequencies over which the amplifier provides significant gain.