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Common-Emitter Transistor Amplifier

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Recipe #7 — common-emitter transistor amplifier

This is the solid-state cousin of the common-cathode tube voltage amplifier.

COMMON-EMITTER TRANSISTOR AMPLIFIER +9 V IN C1 47 kΩ 10 kΩ B C E Q1 2N3904 RC 4.7 kΩ C2 OUT RE 1 kΩ

Illustrative values

  • Q1 = 2N3904
  • VCC = 9 V
  • RC = 4.7 kΩ
  • RE = 1 kΩ
  • base divider = 47 kΩ / 10 kΩ
  • C1, C2 = coupling capacitors appropriate to the frequency range
Follow the signal: IN → C1 → biased base → transistor current changes → voltage across RC changes → C2 → OUT.

Why bias matters

The transistor needs a DC operating point so the AC input can move the collector voltage both upward and downward without immediately running into cutoff or saturation.

Notice the family resemblance to the tube amplifier

Both circuits use a small control signal to vary current. A load resistor then converts that current change into a larger voltage change. Both stages also invert the signal.

Bias values are not universal

Transistor gain, load, temperature, desired output swing, and supply voltage all affect the best bias point. The values shown here are a teaching example, not a universal recipe for every signal source and load.

Try these yourself

What is the main purpose of a common-emitter transistor stage? COMMON-EMITTER TRANSISTOR AMPLIFIER +9 V IN C1 47 kΩ 10 kΩ B C E Q1 2N3904 RC 4.7 kΩ C2 OUT RE 1 kΩ
A properly biased common-emitter stage can provide useful voltage gain.
What do the two resistors connected to the base help establish?
The divider places the transistor base at a useful DC operating voltage.
Like the common-cathode tube stage, the common-emitter transistor stage normally:
Increasing base/emitter current increases the drop across RC, causing collector voltage to move in the opposite direction.
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