Electronics > Beginners

Would an "one transistor amplifier" be a "class A" amp?

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Beamin:
If you had a simple BJT circuit where a (audio) signal was fed into the base and a load was put on the collector would that be considered a class A amp since it would be amplifying all the time? But doesn't a class A amp also pull the same current whether or not there is a signal on it, and this would pull almost none when there is no signal, like in AM modulation with no audio on it,  Maybe I'm trying to over simplify.  :-//

oPossum:
The circuit has to be biased in a way that allows current flow through all transistors all the time. It is possible to bias a one transistor (or one tube) circuit to meet this requirement. It is also possible to use a 'sliding bias' that greatly reduces idle current. Some say that this is not 'true class A'.

oPossum:
A simple one transistor Class A amp can be made with a depletion mode MOSFET (IXTH20N50)... http://www.firstwatt.com/pdf/art_delite.pdf

ejeffrey:
Yes, a single ended amplifier is an example of class A since the lone transistor is always in conduction.

It needs to be biased such that the transistor is conducting about 50% max current at 0 volts input because it can't swing past zero.  This is normally accomplished by capacitively coupling the audio input and adding a DC bias circuit to the base which could be as simple as a couple of diodes forward biased, or a more complicated servo.

rstofer:
W2AEW does an excellent job of explaining this stuff:


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