# Simple Audio amplifier Using Transistors

I would like to build a simple audio amplifier using transistors. I know there are IC designs specifically for the task. But i want to use transistors so i can learn how to use them for amplification.

How would I go about designing an audio amplifier from just discrete components.

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How basic? Just producing sound? Or some quality? What power supply do you have? –  stevenvh May 26 '12 at 16:33
As basic as it can be. A little quality is enough. I'dont have a power supply. –  Marilyn Robinson May 26 '12 at 16:57
Amplifier Basics - How Amps Work (Intro) by Rod Elliott covers a good percentage of the basics of using transistors as an amplifier. It is not a construction article, but focuses on explaining the basics. –  mctylr Jul 26 '12 at 18:24

There is an easy schematic for a class A audio amplifier with a darlington pair. A darlington pair is just two transistors that square the gain of the first transistor(if you are using identical transistors). So if the first transistor's gain was 20, then the gain of the darlington pair would be 400. This website uses a darlington pair in the form of one transistor:

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I don't recall whether or not it uses transistors, but I'm positive Randy Sloan's excellent Tab Guide to Understanding Electricity and Electronics contains an entry-level amp. Regardless, it's audio-centric so you will find many tasty things!

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Yes, all the examples in the book are discrete (not based on ICs) and include fairly common generic transistor audio amplifiers with 1 to 4 transistors, as well as some specific audio amplifier projects in the 15 to 60W range. –  mctylr Jul 26 '12 at 18:13

There is nice example of transistor amplifier in popular book "Art of Electronics"

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Like I also commented to Bitrex I have the idea that Marilyn isn't acquainted with transistors yet, so this may require some explanation, esp. the difference amplifier. –  stevenvh May 27 '12 at 15:50
Does the author explain thoroughly, in detail how this circuit works? –  abdullah kahraman May 27 '12 at 16:15
I think "yes". The whole book is the best in class. (curcuit though might be from 1st edition, not from 2nd ?) Still waiting for 3rd edition! –  user924 May 27 '12 at 16:57
It's been a long time, but IIRC some chapters end with a list of Bad Circuits, though there was nowhere an explanation about what was wrong with them. –  stevenvh May 27 '12 at 17:00

If you want something simple to get started you could build this class A amplifier:

(from this excellent site). $R_E$ stabilizes the DC setting, the bypass capacitor $C_E$ is needed to allow an AC signal to change the output, otherwise $R_E$ would try to keep the output constant.
A class A amplifier is not very efficient. There will always flow a current through it to keep the output at Vcc/2.

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What power does this circuit outputs? –  Marilyn Robinson May 26 '12 at 16:50
Thanks for you answer. This circuit is hard to understand for me. Is there a page that explains it or is it your own design? –  Marilyn Robinson May 26 '12 at 17:05
@Marilyn - I added a simpler schematic which you may prefer to get started with. –  stevenvh May 26 '12 at 17:36
The first circuit is junk. The bias voltage at the output coupling capacitor, gain, and input impedance are completely dependent upon the parameters of the 2N3904. It has way too much gain, and no negative feedback. The output stage has a thermal runaway hazard. There's DC in the voice coil. Blech! –  Bitrex May 27 '12 at 14:18
@Bitrex - Yes, I know, I had my doubts too. I should have given it a better look. I'll delete it. –  stevenvh May 27 '12 at 14:38

If you just want to build a transistor amp, start with the TAB books, or with Forrest Mims experimenters books (usually available at Radio Shack).

Learning how to DESIGN one for a specific application, and do it right, is a different matter. That takes SERIOUS study.

For fun, Radio Shack's datasheet for the TI SN76477 sound chip includes a very simple complementary symmetry transistor push-pull amplifier. They call out Radio Shack part numbers for the transistors, but you can substitute 2N3904 (NPN) and 2N3906 (PNP) with no other component changes.

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@JohnRStrohm, Can you bring some of that data over to our site? –  Kortuk May 27 '12 at 4:50

## protected by Dave Tweed♦Sep 9 at 13:58

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