Bipolar transistors

In a common-emitter amplifier with transconductance gm = 40 mS and collector resistor RC = 5 kΩ, roughly what is the magnitude of the voltage gain? (Ignore the load.)

  1. AAbout 2,000
  2. BAbout 200
  3. CAbout 8
Show answer and explanation

Answer: B. About 200

The small-signal voltage gain of a common-emitter amplifier is Av ≈ −gm × RC, whose magnitude is 40 mS × 5 kΩ = 0.04 × 5,000 = 200. The minus sign indicates that the output is inverted relative to the input.

  • A. About 2,000 is an order of magnitude too large.
  • B. Correct. gm × RC = 0.04 S × 5,000 Ω = 200.
  • C. About 8 comes from converting the units of gm and RC incorrectly.
Practice this topic Back to topic

More Bipolar transistors questions

  1. What is the state of the junctions when a bipolar transistor is in the saturation region?
  2. When the output characteristics (IC-VCE) of a common-emitter transistor are plotted with base current IB as a parameter, what is characteristic of the active region?
  3. What does the load line drawn on the output characteristics of a transistor amplifier represent?
  4. Which value is closest to the maximum power efficiency of an ideal class B push-pull amplifier?
  5. In a circuit using a transistor as a switch, what is the main purpose of a resistor in series with the base?