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Transistor usage in amplifier circuits

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Resistanceisfutile

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So a transistor is like an electronic variable resistor - as you increase the current going to the base, more current is allowed through from the emitter to the collector? (Please correct me if I'm wrong).

I know they are sometimes used in amplifier circuits and op amps, and I was just wondering if someone could explain to me how.
1. Am I right to think that you have to provide all the power that you want the output signal to have, and you have to use the transistor (with appropriate biasing to prevent saturation or cutoff) to control the power added to the signal?
2. Also, since you can't (so far as I know) just feed a battery directly to an audio signal to make it more powerful, what does a transistor amplifier circuit do differently to increase the power of the signal?
3. Does an amplifier circuit actually amplify the power of the signal, or just the voltage (or the current)?
 

Well, to be honest, I don't know how to start... I guess the fundamentals. A transistor is not quite exactly like a controllable resistor, because it is not linear in the characteristics (voltage/current relationship is not like a resistor's).

To avoid writing a whole book worth text here, I would ask you to look at for example https://www.electronics-radio.com/a...-does-a-transistors-works-basics-tutorial.php

In addition, I would say very briefly a few words to your numbered questions:
1. Yes, you have to provide all power from outside, and then some more - because some (often large) part will be lost as heat. And regarding biasing: Yes. You need to have a correct "operating point".
2. The input power source is used to power the amplifier components. Usually DC is used - and the audio output is AC. The amplifier is generating the output AC power from the DC power, and is controlled by the input signal (which defines waveform, frequency, and amplitude of the output AC)
3. The output power is larger - either because both voltage and current are larger, or at least one of those is larger. Depending on amplifier type and application, the voltage and current amplifications vary a lot!
 
ted said:
Well, to be honest, I don't know how to start... I guess the fundamentals. A transistor is not quite exactly like a controllable resistor, because it is not linear in the characteristics (voltage/current relationship is not like a resistor's).

To avoid writing a whole book worth text here, I would ask you to look at for example https://www.electronics-radio.com/a...-does-a-transistors-works-basics-tutorial.php

In addition, I would say very briefly a few words to your numbered questions:
1. Yes, you have to provide all power from outside, and then some more - because some (often large) part will be lost as heat. And regarding biasing: Yes. You need to have a correct "operating point".
2. The input power source is used to power the amplifier components. Usually DC is used - and the audio output is AC. The amplifier is generating the output AC power from the DC power, and is controlled by the input signal (which defines waveform, frequency, and amplitude of the output AC)
3. The output power is larger - either because both voltage and current are larger, or at least one of those is larger. Depending on amplifier type and application, the voltage and current amplifications vary a lot!
Click to expand...
Thanks, this is really useful!
 

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