Preamplifiers

497

05:53:40

Preamplifiers are designed to deliver the needed current/voltage gain to a Power Amplifier (which I'll explain in a different tutorial). There are 3 distinct configurations to do that. (from left to right) Common collector, common emitter,common base.

-Common Collector - typically has a voltage gain below 1 but with a few readjustments can have some voltage gain no more then 20-30. This preamp specifies itself with a high current gain and its purpose is to act as a buffer between unmatched stages by having a high input impedance and low output one.

-Common Emitter - this preamp works in a class A mode but since its a preamp the efficiency should not bother us. It has a medium high and output impedance. Has a very high voltage gain and could have a high current gaim too but that would require more power and since it's in a class A mode the efficiency levels will drop sharply with the increaae of power. Simce this amp has a cap for more gain the signal becomes more distorted and ive decided to add a simple negative feedback to negate some of the distortion.

-Common base- Now this topology is a little more difficult and is not widely used because it typically has a very low input impedance and the fact that high amounts of DC passes through the input source. This topology is mostly used in high frequency amplifiers because it has the best frequency response characteristics from the 3 topologies. The gain may be limited by either decreasing the base dividers capacitor value. It doesn't provide current gain comparing input/output but typically has very high voltage gain restricted only by the transistor hfe.

published 11 years ago

Sine_eyed

11 years ago

Nice Professor B.! All three have a 1Volt peak to peak output correct? How/why did you decide on the input voltages?..

thebugger

11 years ago

I haven't decided anything. That's just how I made them. They all have different input/output ratio but the outputs are all around the same value

Sine_eyed

11 years ago

Maybe I didn't word that right- I'm curious if the input voltages are meant to represent something, like the output of a small radio for instance. I mean, 510mV and 6mV are specific values so I assumed you set those values intentionally. Just trying to find out why that's all..

thebugger

11 years ago

No I chose them randomly. Since the first stage has no voltage gain thats why I chose the input to be so high. To match the others. And the other two have a high voltage gain so I needed a small signal to amplify==>6mv.

Sine_eyed

11 years ago

Oh ok then. Bear with me bugger I'm still wet behind the ears with transistor amplifiers! So ok- did you bias the circuits first, then do the math/saw how much gain it would have, and then choose the input voltage in order to match the 1V p/p like the others?..

thebugger

11 years ago

This is how I do it. Build the amplifier without the source and any capacitors. Then vary the base voltage divider until I get half the power supplies voltage on the output. Remember with the voltage divider go as higb as you can with the values for higher input impedance. After that plug the signal source all the caps if needed and pick it up from there. Of the gain is too high either decrease the inputs voltage or add a feedback of some sort. Another way do decrease the gain is to either increase the emitter resistance or decrease the collectors resistance but then you must modify the base voltage divider. If you have a hard time following me just ask me again and I'll be glad to try and rephrase myself ;)

Sine_eyed

11 years ago

Hmm, ok- now I'm getting a better picture, and I follow what you're saying just fine, thanks bugger! I'm sure along the way I'll come up with plenty more questions for you though.. ;•)

wyoelk

11 years ago

Thanks @bugger I was just working on a 3 transistor amplifier with that question

thebugger

11 years ago

3 transistor? What's the purpose. Very high voltage gain or a single voltage gain stage with 2 buffer amps?

wyoelk

11 years ago

It is an old 3 transistor op amp design from an old sony amp schematic with some values faded out. My output is too low both V and I . I hooked an ipod too it but cant get the output up without overheating the input transistor from the DC source . I will post the circuit as Transistor OpAmp, in awhile with the values I used.

thebugger

11 years ago

Ok id sure like to look it up.

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