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maximum output from a transistor amplifier?

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andy257

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Hi Guys

Is there a general rule that in transistor amplifiers the output cannot exceed the input supply? (i thought this was just op amps)

ive been doing an experiment on multisim and the maximim output i allways seem to get is the supply voltage. looking at different gains and the maximum seems to be the supply when the calculated output should be higher.

cheers

andy
 
andy257 said:
Hi Guys

Is there a general rule that in transistor amplifiers the output cannot exceed the input supply? (i thought this was just op amps)

ive been doing an experiment on multisim and the maximim output i allways seem to get is the supply voltage. looking at different gains and the maximum seems to be the supply when the calculated output should be higher.

It applies to any directly coupled amplifier, the signal can only vary between the supply rails, if it tries to go above that where is the voltage going to come from?.

Your mistake is probably in assuming an amplifier 'amplifies', it doesn't really do any such thing - it provides a larger 'copy' of the input signal on the output - this larger 'copy' can't be any larger than it's supply rails.

Think of it as a photo-copier, if you use a photo-copier to enlarge a document you can only enlarge it as big as the output paper it will take - any more than that and you will clip the image and lose part of it. An amplifier works just the same - too much gain and you clip the output.

Perhaps a crude analogy, but hopefully it makes the point clear?.
 
I am barely comfortable with the basics but I look at a transistor as a switch that can be off, part way on or all the way on. A transistor in an amplifier behaves the same.

If I had a 12 volt bulb and a 12 volt battery I could control it with a single transistor by applying a very small current (relative to what the bulb needs) to the base of the transistor. The transistor would turn on or off. When on it behaves like a switch that is on. 12 volts appears at the bulb and nothing you do can make any more appear. Amplification did happen if the base current was less than the current thru the light bulb. Once enough base current is flowing so that the transistor is all the way on then more base current accomplishes nothing.

I had some friends who became confused about their car stereo amplifiers after doing a little math (Ohm's law). They reasoned that with 12 volts in and 4 ohm speakers that much less power was being delivered than was promised. In this situation they didn't realize that inside their amplifiers were power supplies that boosted the voltage available to the amplifiers so that when the transistors were all the way on that enough voltage was available across the load to get the current/power desired.

Nigel's use of the copier analogy is excellent.
 
A controller

Nigel, that's the first time I've ever heard someone explain an "amplifier" as I do -- as an audio signal controlling the output of a power supply. Good work! So many folks treat an amp as they would a magnifying glass.

Dean
 
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