# Single Transistor Amplifier Circuit Help

Discussion in 'Electronic Basics' started by trevor, Mar 8, 2006.

1. ### trevorGuest

Hi,

I am working on a High school Physics project, and we are trying to set
up the second circuit found at:
http://www.pbs.org/transistor/teach/teacherguide_html/lesson4.html

I am having trouble with this circuit--when we set it up, it just makes
a "static clicking" noise when we rub the leads together. Does anyone
have any experience with single transistor amplifier circuits--or any
suggestions on where I can find another one, or have any ideas about
this circuit. I've been searching Google for alternatives and found a
few, but I cant get those to work either....maybe I'm doing something
trivial wrong.

Thanks
Trevor

2. ### Pooh BearGuest

No surprise. It's not the cleverest experiment I've ever seen.

Graham

3. ### John PopelishGuest

I take it you are talking about the second circuit, with an output
transformer.

Are you using a transformer that has worked before, or did you
purchase one new, for this experiment?

Are you sure you have the leads on the transistor correctly sorted out?

Have you checked that you don't have the two resistors switched?

Same question but about the capacitors.

Have you seen the note about what the dots mean where two lines cross?

4. ### trevorGuest

Thanks for your suggestions...I bought new transformers.

I actually have a question about the transformer. There are three leads
going in and three going out...but on the circuit diagram, there are
only two output gates. Is this normal...if so, which outputs should i
use...I've been using the top and the bottom one, disregarding the
middle one. Should I tie that to the ground....or am i doing something
wrong.

I'll recheck the capicators and resistors....

but does this circuit look like it should work?

thanks,

Trevor

5. ### Jonathan KirwanGuest

This worries me. Since the transformer should have been designed for
a speaker on the output side, I'm wondering if this transformer is the
right one. I gather from the schematic that they want a 1k primary
and an 8 ohm secondary for audio frequencies:

I looked at the picture there and, looking closely, it appears to only
have 5 wires. The red and white should be your audio 8 ohm side and
the green, black, and blue should be your audio 1k ohm side. Further,

Frequency response: 300Hz to 10kHz
1,000-ohm center-tapped primary
8-ohm secondary

Note that they don't mention the phrase "center-tapped" in the case of
the secondary.

I think you got the wrong part. The page you mentioned lists the
Radio Shack part #273-1380. Are you sure you got that one?

Jon

6. ### John PopelishGuest

Transformers can be made in almost infinite variety, with the windings
turn count the main variation. The circuit you are building needs a
transformer with at least one center tapped winding, that is made to
connect to a 1k source, in order to make the positive feedback work in
the audio range. The second winding has to have many fewer turns, to
oscillation stalls out. If your transformer has 3 leads at both ends,
I am doubtful that it has one end designed to drive a speaker. Do you
have any specs on the transformers you have, so far?
It is very similar to the very first transistor circuit I built about
45 years ago. I learned to produce Morse code with it.

It is a fairly forgiving circuit, if you have a transformer that even
comes close to what is specified. Back in the days when 5 transistor
radios were in everyone's shirt pocket, it was easy to pull the output
transformer out of one and build this circuit. The center tapped
primary (for a 2 transistor push pull stage) to speaker output
transformers are harder to find, today. Where are you getting yours?

7. ### trevorGuest

I got mine at RSR electronics...thanks for the suggestions--I'm going
to go and buy that transformer and try it in the circuit.

8. ### GERMANIUM CRAINIUMGuest

Check the website at http://www.scitoys.com

He has some fantastic simple amplifier circuits that are tested and
will work great. Also,

http://www.techlib.com

has some fun stuff for you to build (and also all circuits tested and
work fine)