Q.With circuit diagram explain the working of CB amplifier.

In a common-base amplifier the base is common to input and output, the input signal is applied to the forward-biased emitter and the amplified output is taken from the reverse-biased collector, with input and output in phase.

[!ANSWER]

A small change in emitter current produced by the input signal causes a nearly equal change in collector current ($i_c \approx \alpha i_e$, $\alpha<1$); driven through the large collector load $R_L$ this gives an amplified, in-phase output voltage — high voltage gain, high power gain, but current gain less than unity.

[!TLDR]

A common-base amplifier applies the signal to the emitter and takes the output from the collector; a small emitter-current change produces a nearly equal collector-current change that develops a large, in-phase output voltage across the collector load.

Circuit: the transistor is connected with the base as the common terminal. The emitter-base junction is forward biased by supply $V_{EE}$ and the collector-base junction is reverse biased by $V_{CC}$, fixing the active-region operating point. The input signal is applied between emitter and base; the output is taken between collector and base across the load resistor $R_L$.

Working: the signal adds to the emitter-base bias. During the positive half-cycle the forward bias increases, so the emitter current $i_e$ increases; since $i_c = \alpha i_e$ with $\alpha$ just less than 1, the collector current increases almost equally. This increased collector current flows through the large load $R_L$, and because $R_L$ is large the output voltage $v_o = i_c R_L$ is much greater than the small input voltage — hence voltage and power amplification. As emitter current and collector current rise and fall together, the output is in phase with the input (0° phase shift).

Characteristics: current gain $\alpha<1$ (no current gain), but high voltage gain, moderate power gain, low input impedance and high output impedance; the good high-frequency response makes it useful in RF stages.

[!ANSWER]

In the CB amplifier the input at the forward-biased emitter varies the emitter current, producing a nearly equal collector-current change ($i_c=\alpha i_e$) that, driven through a large collector load, gives an amplified output voltage in phase with the input — high voltage and power gain with current gain below unity.