Q.With a circuit diagram explain how a transistor can amplify a signal.

A transistor amplifies by letting a weak signal in the low-resistance input (forward-biased emitter-base) circuit control a much larger current in the high-resistance output (reverse-biased collector-base) circuit, so the same signal is transferred from a low to a high resistance and appears magnified across the load.

[!ANSWER]

A small AC signal added to the forward-biased base-emitter loop produces small changes in base current, which cause $\beta$ times larger changes in collector current; this current driven through the collector load resistance $R_C$ develops a greatly enlarged output voltage — the mechanism of amplification.

[!TLDR]

A transistor amplifies because a small change in its forward-biased input (base-emitter) current controls a much larger change in its reverse-biased output (collector) current, and this output current develops a large voltage across a high load resistance.

Consider a single-stage CE amplifier: an npn transistor with the base-emitter junction forward biased and the collector-base junction reverse biased (active region). A DC bias fixes the operating point, and the AC signal to be amplified is coupled to the base through a capacitor $C_1$; the amplified output is taken from the collector through $C_2$ across the collector load resistor $R_C$.

Why amplification occurs: the input junction is forward biased, so it offers a low resistance, while the output junction is reverse biased and offers a high resistance. A small input signal voltage produces a small base current $i_b$. Because $i_c = \beta\, i_b$, the collector current change is $\beta$ times larger. This collector current flows through the large load resistance $R_C$, and since the same order of signal current now acts across a much higher resistance, the output voltage $v_o = i_c R_C$ is far greater than the tiny input voltage. The device thus transfers the signal from a low resistance to a high resistance (hence "transfer resistor", transistor), giving voltage and power gain.

During the positive half-cycle of the input the base current rises, collector current rises, the drop across $R_C$ rises and the collector voltage $V_C = V_{CC} - i_c R_C$ falls, so the output is an enlarged, 180° phase-shifted copy of the input.

[!ANSWER]

Amplification is the control of a large output (collector) current by a small input (base) signal: $i_c = \beta i_b$, and driving this current through a high collector load resistance $R_C$ yields an output voltage many times larger than the input — the transistor acts as a signal-controlled transfer of energy from the DC supply to the load.