Voltage amplifier model.

The diagram below shows a voltage amplifier.  It is defined by its input impedance, Ri, its output impedance, Ro, and its open circuit voltage gain, Avo.

wpe4.gif (2204 bytes)



Single Transistor (Bipolar) Amplifiers

The circuits shown are some of the more common single transistor amplifier models.  Also shown are the amplifier parameters given in terms of the small signal model of the amplifier.  Note that Avo=Vo/Vi (not Vo/Vs), when Rl is infinite.

Common Emitter Amplifier

wpe1.gif (2549 bytes) wpeE.gif (1826 bytes)


  • Relatively high voltage gain
  • Relatively low input impedance
  • Relatively high output impedance



Common Emitter Amplifier with Emitter Resistor

wpe2.gif (2605 bytes) wpeA.gif (2761 bytes)


Compared to Common Emitter without Emitter Resistor:

  • Higher input impedance.

  • Bypass capacitor can be smaller.

Other notes

  • Voltage gain can be made relatively independent of b.


Common Collector Amplifier (Emitter Follower)
wpe3.gif (2397 bytes) wpeD.gif (2757 bytes)


  • High input impedance
  • Low output impedance
  • Gain close to 1