Single transistor amplifier project

[CarlS]

I want to do a project for end of school year - a single ( power ) bipolar transistor amplifier with source from a CD player and driving a small speaker. I do not understand and maybe you can explain :

1. How come the very small input signal ( can be milivolts ) will "beat" the ground to base voltage ? How does that work ?

2. If my speaker is 32 Ohms how do I select the voltage and current going through the transistor ? I see in the datasheet some curves - how are they used ?

[rstofer]

1) You bias the base with a pair of resistors between power supply and ground.  This causes the transistor to conduct even with no signal.  For a class A amplifier, you will adjust the bias such that the collector voltage (given the current flowing through the collector resistor (load) is about 1/2 of the power supply.

This allows the collector voltage to swing from Vcc to Gnd.

You capacitively couple the signal into the base.

2) I have never tried to drive a low value load like 32 Ohms.  If the power supply was 12V and it was all dropped across the speaker, the current flow would be 375 mA and that seems like a lot.  I'd have to think about that...


Watch the videos by W2AEW - he does a magnificent job explaining how these things work.  For simplicity, not energy efficiency, you want to look at Class A operation.  You can Google for circuits as well.

[rstofer]

You might find this interesting.  One way to resolve the low load impedance is to use a transformer - maybe 20:1 as mentioned in the comments.  So, the collector will see 20 * 32 ohms or 640 Ohms, just like w2aew's video.



Expect a bunch of heat.

You can also check around for capacitively coupled speakers.  That eliminates the transformer but usually requires quite a high value capacitor.