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[Moved]: Common collector, common emitter and common base

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Advanced Member level 2
Apr 17, 2011
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The 3 terminal transistor can be connected in multiple ways, how do we know which configuration of common collector, common emitter and common base we are supposed to use?

Besides this, does the same rules apply on BJT, FET as well as JFET?

When you learn about the applications of transistors then you will know which configuration to use.
Common collector has a voltage gain of 1 like a piece of wire but has a high input resistance.
Common emitter has up to a high voltage gain but has a fairly low input resistance.
Common base has up to a high voltage gain but has a very low input resistance.
Mosfets and Jfets are similar but their gate as an input has an extremely high input resistance.

The common collector (emitter follower) stage has a high frequency response, a high input impedance, and a low output impedance.
It has a voltage gain of slightly less than one but has a high current gain.
It is useful if you need to drive a low impedance load from a high impedance source.

The common emitter stage is the one most commonly used to provide a voltage gain.

The common collector stage has a higher frequency response than the common emitter stage but requires a low source impedance so is not often used.
It is sometimes used in RF systems since the signal impedance is often is 50Ω or 75Ω, low enough to readily drive the emitter input of a common collector amp.

See my reply in a recent thread:

You can pick up a lot about what transistors can do by looking through books on transistor circuits for the experimenter.

Every so often I see someone making unconventional use of some characteristic of transistors, which reveals possibilities to my mind.

I learned a great deal by playing with simulations.

Also examine the multitude of transistor circuits at the website below. It shows you the benefit in the philosophy that we learn more about what's inside an IC, by duplicating its function using discrete components.

Why bother about transistor amplifers? Why not just make one using op-amps?

Because an op amp can't do everything that a transistor can.

...and a transistor can`t do everything that an opamp can.
Hence, each device has its pros and cons and its own application area.

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