Common Base(CB) Amplifier Analysis

COMMON BASE (CB) AMPLIFIER ANALYSIS

The base terminal is common between the input and output circuit. The input is applied to emitter terminal and output is taken from the collector terminal.

Construction

The common base amplifier is shown in Fig. 2.19. The potential divider bias is applied through resistors R₁ and R₂.

i. The load resistor R_L is connected to transistor collector terminal.

ii. The signal source is coupled to the transistor emitter through C₂.

iii. Capacitor C₁ constitutes an ac short circuit from the base terminal to ground. So, all the input voltage appear across base emitter junction.

iv. Capacitor C₃ acts as coupling capacitor and it prevents the loading effect due to R_C and R_L.

Operation

i. During positive half of input signal, the emitter terminal is positive and the base remains at a constant potential.

ii. Therefore, a positive - going signal reduces base-emitter voltage V_BE which in turn reduces collector current I_C. Thus the voltage drop across the collector resistor also decreases.

Apply KVL to the output circuit

If I_C reduces, then V_O ≈ V_CC and there is no phase shift between the input and output.

i. During negative half cycle of input, the emitter terminal is negative. Thus

forward bias across Base-Emitter junction increases which in turn increases the collector current.

ii. The voltage drop across R_C increases ie I_C R_C > V_O. Thus the output voltage (V_O = V_CC I_C R_C) decreases, and the output is negative value.

Fig.2.20 shows the small signal hybrid equivalent of CB amplifier.

In this circuit, only a fraction of output voltage is fed back to input i.e. h_rb is very small and can be neglected.

Input Impedance

Apply KVL to Fig. 2.20,

The input impedance can be written as

Output Impedance

The output impedance can be written as

Voltage Gain

Current Gain

Power Gain

Characteristics of CB Amplifier

i. provides voltage gain and power gain

ii. High output impedance and very low input impedance

iii. No current gain

Application

i. High frequency voltage amplifier