Why do we intend to obtain different gains at different places? In an emitter follower for example,

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There's no voltage gain, but there's current gain and therefore it has power gain. Okay, how's that different and useful from having an voltage gain? And what's the difference between current and power gain. In power amplifiers,

Since any more voltage amplification would lead to distortion, we amplify the power(current).

What effect does amplifying power bring with it?

  • \$\begingroup\$ First thing that comes to my mind would be audio speakers. Amplifying the current while retaining the same voltage level would allow you to drive larger speakers (hopefully) without distortion \$\endgroup\$
    – andowt
    Commented Apr 25, 2019 at 9:47
  • \$\begingroup\$ I think the idea is to transfer the power with least lost and then do the source transformation to get current or voltage at the end. I might be wrong. \$\endgroup\$
    – user1245
    Commented Apr 25, 2019 at 10:07

1 Answer 1


Gain is simply output/input, for example Vout/Vin. So for an emitter follower you cannot say that "it has no gain" because it does!

An emitter follower has a voltage gain, Vout/Vin of slightly less than 1. (and this is what you mean by "it has no gain", indeed the signal voltage isn't increased).

But an emitter follower can have a current gain Iout/Iin of much more, use the right transistor and the current gain can be a factor 500.

To achieve power gain Pout/Pin we must realize that Power = Voltage x Current so if we change the Voltage and/or Current such that output power is increased, we have power gain. In the case of the emitter follower, most power gain is due to the current gain.

For a common emitter circuit, both voltage and current gain can contribute to power gain of more than one.

  • \$\begingroup\$ So the ultimate aim is to get a power gain ? \$\endgroup\$ Commented Apr 25, 2019 at 12:52
  • \$\begingroup\$ Can you give a physical example, like what difference would I observe, when there's a current and voltage amplification. Will I see no difference? By physical I mean, for example(I don't know whether this is right), will the audio be louder in both the cases? \$\endgroup\$ Commented Apr 25, 2019 at 12:57
  • \$\begingroup\$ Is the ultimate aim to "get rich"? Not for everyone. So no, the ultimate aim is not to get power gain. The ultimate aim is the get what is needed. I might not need power gain. Aiming for "power gain always and everywhere" is short sighted. Always explain why you need (power) gain or not. \$\endgroup\$ Commented Apr 25, 2019 at 12:57
  • \$\begingroup\$ Put in another way, in audio amplifier, why don't we do the current amplification in the first instead of voltage amplification \$\endgroup\$ Commented Apr 25, 2019 at 13:01
  • \$\begingroup\$ Current amplification is done in the last stage/stages as a large signal current needs a large DC bias though a transistor. So many transistors would need to be biased at large currents, that results in large power dissipation. Also amplifying the voltage is needed to prevent noise and supply ripple to distort the signal. Also there is no advantage to first do current amplification and only voltage amplification after that. \$\endgroup\$ Commented Apr 25, 2019 at 13:11

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