Explain how a single transistor amplifies a small signal, the need for biasing, and the meaning of voltage amplification

What this dot point is asking

SEAB wants you to explain how a single transistor amplifies a small signal, why it must be biased to do so, and what voltage amplification means. The central insight is that a small change in the base current produces a much larger change in the collector current, and that passing this changing collector current through a load resistor turns it into a large changing voltage, which is the amplified output.

The answer

Amplification from current gain

A transistor amplifies because a small base current controls a much larger collector current, related by the current gain $\beta = I_C / I_B$. When a small signal changes the base current slightly, the collector current changes by $\beta$ times as much. This current amplification is the starting point for making a transistor amplifier.

Turning current change into voltage change

To get an output voltage, a load resistor (the collector resistor) is placed between the collector and the supply. The collector current flows through this resistor, so by Ohm's law the voltage across it depends on the current. When the collector current changes, the voltage across the resistor changes a lot, and so the collector voltage swings by a large amount. This swinging collector voltage is the amplified output: a large copy of the small input.

Voltage amplification

Voltage amplification means the output voltage signal is larger than the input voltage signal. The voltage gain is the ratio of the change in output voltage to the change in input voltage. A single common-emitter transistor stage can give a useful voltage gain because a small base voltage change produces a large collector voltage change. The output is also inverted: as the input rises, the collector voltage falls.

The need for biasing

A transistor only conducts when its base-emitter voltage is above about $0.7\ \text{V}$. A small alternating signal swinging around zero could not turn it on during its negative half. Biasing fixes this: a steady base current (set by a bias resistor) holds the transistor part-way on, at an operating point in the middle of its active range. The signal then rides on top of this steady point, swinging the output both up and down. Without bias, only part of the signal gets through and the output is badly distorted, or clipped.

Examples in context

Example 1. A microphone pre-amplifier stage. The tiny voltage from a microphone is fed to the base of a biased transistor. The small signal causes a large swing in the collector voltage across the load resistor, producing an amplified output that later stages can use. The bias keeps the transistor in its active region so quiet and loud passages are both reproduced.

Example 2. Why op-amps took over. A single transistor stage works but its gain depends on the transistor and drifts with temperature. An operational amplifier with negative feedback gives a far more stable, predictable gain set by resistors, which is why most modern amplification uses op-amps. The transistor stage shows the principle that the op-amp then perfects.

Try this

• Cue. State what converts the changing collector current into an output voltage in a transistor amplifier. The collector (load) resistor; the changing current through it produces a changing voltage by Ohm's law.

• Cue. Explain why a transistor amplifier is biased. To set a steady operating point in the active region so the input signal can swing the output both up and down without clipping.

• Cue. A collector current change of $1.0\ \text{mA}$ flows through a $3.0\ \text{k}\Omega$ collector resistor. Find the output voltage change. $\Delta V = \Delta I_C \times R_C = 0.0010 \times 3000 = 3.0\ \text{V}$.