Amplifier loads

[Michael Cotter]

My only experience with amplifiers is the ones that drive home entertainment equipment like CD players and whatnot. I'm studying elementary electronics and it looks to me like these simple little one transistor emitter amplifiers are all designed to drive high impedence loads, but I'm always thinking speakers. Speakers have got small impedence. What am I missing?

 

[Phasor]

Amplifiers aren't always used to drive speakers

You might use your one-transistor amp as a pre-amp for your sound system - your entertainment system probably has fairly high impedance inputs.

Or you might use it as part of some sort of control circuit.

I'm sure there are hundreds more uses, but I just can't think of more offhand :wink:

 

[mechie]

Transistors as Amplifiers

There are three basic configurations that you can use a transistor in :-

Common Emitter - good gain (but inverting), low impedance in, medium impedance out.

Common Collector - no voltage gain, high current gain, high impedance in, low impedance out.

Common Base - good voltage gain, no current gain, low impedance in, medium impedance out.

The common emitter circuit is the one you will see most of because of its gain.
Common collector (also called emitter follower) is very good at driving low impedance loads (loudspeakers).
Common base is rare unless you play with high frequencies (MHz).

If you look at the circuit inside an integrated circuit (linear or logic) you will see the majority of the transistors fall into one of these categories - there are a few 'cheats' though- connecting transistors backwards for such tricks as using a base-emitter junction as a zener diode or inter-electrode capacitance being exploited.

Stick to the first two and you won't go far wrong!