Transistors as amplifiers

[barfkoswil]

I have been working through the projects in the book Make:Electronics by Charles Platt. Everything was going well until we used a PUT and a capacitor to make an LED flash and this to was ok. Then we are asked to repeat the circuit using a speaker instead of an LED but the sound is quiet so a 2N2222 transistor is used for amplification. Unlike the PUT where it's output goes directly through the speaker it now goes to the base of the transistor and the current drawn through the speaker is now direct from the battery via 100 ohm resistor. please can someone explain how the amplification happens?

 

[MikeMl]

Step one: understand what a transistor does.
Step two: how to make an amplifier using a transistor.

Which do you need help with?

 

[barfkoswil]

Thanks for a quick reply but it's the latter of the two.
Thanks

 

[cowboybob]

Welcome to ETO, barfkoswil!

**broken link removed**
you can use a 2n2222 (NPN type) for Q1. Speaker between C3 and Gnd.

There are tons of examples out there.

 

[audioguru]

You forgot to post the schematic of your transistor amplifier so we cannot comment on its errors. You also forgot to post the schematic of your PUT oscillator driving a speaker.
BUT why did you connect the speaker to a 100 ohm resistor? The resistor will attenuate the signal if the transistor circuit actually works.

The single transistor amplifier posted by cowboy Bob cannot drive a low impedance speaker.

Here is a PUT circuit that will click the speaker:

 

[cowboybob]

AG, this is what the OP asked for:

... please can someone explain how the amplification happens?

A straight forward query, thus my post, i.e., how does a generic transistor amp circuit work?. I was hoping the OP might investigate the concept a little further to help him understand the amplification process.

... You forgot to post the schematic of your transistor amplifier so we cannot comment on its errors. ...

He made no mention of any errors.

... The single transistor amplifier posted by cowboy Bob cannot drive a low impedance speaker...

Of course it can, if C3 (for that matter, ALL the components) is appropriately selected and sized. To be sure, a speaker's low impedance will adversely effect the gain of the circuit but we have not yet been given any indication of the OP's desired result, other than some additional volume, which I understand to have been already achieved.

 

[audioguru]

"We (the entire class?) are asked to repeat the circuit using a speaker instead of an LED but the sound is quiet so a 2N2222 transistor is used for amplification. .... Please can someone explain how the amplification happens?"
Why doesn't the teacher explain how amplification happens? And show that an audio power amplifier usually has a very low output impedance.