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How does a single-transistor common-emitter amplifier work, and how is it biased to amplify a signal?

Transistors as amplifiers: the common-emitter voltage amplifier, biasing for a quiescent point, voltage gain, the role of the load and emitter resistors, and coupling capacitors.

A focused answer to WJEC A-Level Electronics transistor amplifiers, covering the common-emitter voltage amplifier, biasing to set the quiescent operating point, voltage gain from the collector and emitter resistors, coupling capacitors, and why the output is inverted.

Generated by Claude Opus 4.812 min answer

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
  4. Try this

What this dot point is asking

Beyond switching, the transistor's other classic role is to amplify a small signal. WJEC expects you to understand the common-emitter voltage amplifier: how it is biased to a quiescent point so the signal can swing both ways, how the collector and emitter resistors set the gain, why coupling capacitors are used, and why the output is inverted. The gain estimate and the explanation of inversion are standard exam content.

The answer

Why biasing is needed

A transistor only amplifies linearly when it is held in its active region. If it is biased too close to cut-off or saturation, one half of the signal is clipped. A potential divider on the base sets the quiescent base voltage, and the emitter resistor stabilises the operating point against temperature and gain variation.

The common-emitter configuration

Voltage gain

With an unbypassed emitter resistor the voltage gain is set mainly by the ratio of the collector resistor to the emitter resistor. The negative sign is the inversion. (Bypassing the emitter resistor with a capacitor raises the gain but reduces stability, a standard trade-off.)

Why the output is inverted

As the input rises, the base current rises, the collector current rises, and more voltage is dropped across the collector resistor. Since the output is the collector voltage (supply minus the drop), the output falls. So a rising input gives a falling output: a 180∘180^\circ phase inversion.

Examples in context

Example 1. A microphone pre-amplifier
A common-emitter stage raises the tiny signal from a microphone to a usable level before it reaches a power amplifier. The input coupling capacitor keeps the microphone's DC conditions separate from the bias network, while the gain set by RC/RER_C/R_E lifts the millivolt signal to hundreds of millivolts.
Example 2. Cascading stages for more gain
A single common-emitter stage might give a gain of ten; two in series, coupled by a capacitor, multiply to a gain of a hundred. Each stage is biased independently, and the coupling capacitor between them passes the AC signal while keeping each stage's quiescent point undisturbed.
Example 3. Trading gain for stability
Adding a bypass capacitor across the emitter resistor raises the AC gain sharply, but the stage then drifts more with temperature because the stabilising effect of the emitter resistor is removed for AC. Designers choose between a high, less stable gain and a lower, rock-steady one depending on the application.

Try this

Q1. A common-emitter amplifier has RC=10 kΩR_C = 10\,\text{k}\Omega and an unbypassed RE=1.0 kΩR_E = 1.0\,\text{k}\Omega. Estimate its voltage gain. [2 marks]

  • Cue. Av≈−RCRE=−101.0=−10A_v \approx -\frac{R_C}{R_E} = -\frac{10}{1.0} = -10 (magnitude 10, inverting).

Q2. Explain why the emitter resistor improves the stability of the amplifier's operating point. [2 marks]

  • Cue. If the collector current rises (for example with temperature), the emitter voltage rises, reducing the base-emitter voltage and so reducing the current again. This negative feedback holds the quiescent point steady.

Exam-style practice questions

Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

WJEC Eduqas 20194 marksA common-emitter amplifier has a collector (load) resistor of 4.7 kΩ4.7\,\text{k}\Omega and an emitter resistor of 470 Ω470\,\Omega. Estimate its voltage gain, and explain why the output signal is inverted relative to the input.
Show worked answer →

For a common-emitter stage with an unbypassed emitter resistor, the voltage gain is approximately the ratio of the collector resistor to the emitter resistor.

Gain: Av≈−RCRE=−4.7×103470=−10A_v \approx -\dfrac{R_C}{R_E} = -\dfrac{4.7 \times 10^3}{470} = -10.

The minus sign shows inversion. The output is inverted because as the input voltage rises, the base and collector currents rise, so more current flows through the collector resistor, the voltage drop across it increases, and the collector voltage (the output) falls. A rising input therefore gives a falling output, a phase inversion of 180 degrees.

Markers reward the gain magnitude of about 10, the negative sign, and the rising-current-falling-collector-voltage explanation of inversion.

WJEC Eduqas 20223 marksExplain the purpose of the input and output coupling capacitors in a common-emitter amplifier.
Show worked answer →

The coupling capacitors block DC while passing the AC signal.

The input coupling capacitor stops the DC bias voltage at the base from being disturbed by, or leaking into, the previous stage or signal source, while letting the AC input through to the base.

The output coupling capacitor stops the DC collector voltage reaching the load, passing only the amplified AC signal.

This keeps the carefully set bias (quiescent) point of the transistor undisturbed while still coupling the signal in and out.

Markers reward blocking DC and passing AC, and protecting the bias point at the input and the load at the output.

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