Simplest amplifier design with resistor and transistor

[circuit man]

Hi guys,
I need to build a REALLY simple amp for a friend. I only want to use a resistor and a transistor plus the normal stuff(battery and speaker). Please help.

Thanks in advance,
Circuit Man

 

[ranger]

What are your power requirements? What sort of signal are you amplifying? The term "simple" is a little subjective. How about distortions on the output?

All of these factors will affect the amp design in some way. The design will require more that just a transistor and resistor.

 

[circuit man]

I want to amplify a computer speaker (iPod) to a mono speaker not very loud.

 

[dlgoff]

Well here you go. You can't get much simpler than this 3 Transistor Audio Amp.

 

[f95toli]

It sounds like you are planning to build a simple class A amp.
What is the purpose? Is it just a demonstration?

Practical transitor amplifiers tend to be fairly complicated, class A amps are simple in principle but in reality you need more than a single transistor and a resistor to make something that sounds decent.
Also, class A amps are very inefficient so a 9V battery won't very long.

If you want to build something very simple just use an operational amplifer such as LM3875 or something similar. Then you can actually build something that works using a few resistors and a capacitor or two.

 

[ranger]

For your audio purposes, a simple transistor-resistor setup won't cut it. You will at least need a push-pull output stage (needs two transistors). This is a good idea because as f95toli mentions - the single transistor will simply be too wasteful. But the push-pull as some distortion of its own. You'll need to eliminate this crossover distortion; usually by biasing the push-pull with diodes (see dlgoff's link).
Here is a two transistor amp:
http://www.geocities.com/tomzi.geo/2tr_amp/2tr_amp.htm
But this suffers from inefficiency.

 

[circuit man]

I only want to amplify an iPod so a couple people could hear it. I was hoping to do something like this. http://kitsrus.com/projects/sg10m.pdf

 

[ranger]

Okay, here is a 1 watt amp using a darlington pair:
http://sci-toys.com/scitoys/scitoys/computers/solderless/amplifier.html

 

[circuit man]

cool thanks

 

[circuit man]

How much louder would a 1 watt amplifier make an iPod? It is probably obvious but I don't know

 

[ranger]

Are you familiar with the decibel scale?

The rule is as follows: to reproduce a sound twice as loud as the original, the amplifier needs to supply 10 times more power at the output.
Assuming that your ipod has a 30mW output, you get a gain of about 15dB.

 

[circuit man]

yes, thank you