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Transistor as an amplifier (common emitter) sheet
5-mark answer frame

With a labelled circuit diagram, explain how an npn transistor works as an amplifier in CE mode. Derive the expression for its voltage gain.

How to lay out the answer. Follow the frame, then write it in your own words.

  1. 1 · Define amplification. An amplifier increases the size of a signal; the voltage gain is AV = Vo/Vi.
  2. 2 · Draw the circuit. [Diagram: npn transistor with the emitter common; vs in series with VBB at the base; RL from the collector to VCC; output vo across collector and emitter]
  3. 3 · Explain biasing and action. VBB sets IB at the middle of the linear range. The signal changes IB by Δib = vs/ri, and IC changes by Δic = βΔib.
  4. 4 · Derive the gain.
    • VCC = VCE + IC RL, so Δvo = −Δic RL.
    • vs = Δic ri/β.
    • AV = −β RL/ri = −gm RL.
  5. 5 · Explain the phase. The minus sign shows that the output is 180° out of phase with the input.
  6. 6 · Real life. A loudspeaker system amplifies a microphone signal in this way.
  7. 7 · Close in one line. So, the power gain AP = β × AV, and the extra power comes from the DC supply.

Scoring tip: Write the Kirchhoff equation VCC = VCE + IC RL before the gain. It shows where the minus sign comes from.