simple amplifier circuit.

[chrono10]

after much hassle and headache, i've finally turned for help to get this thing working.

I have a simple minijack microphone (unamplified) that i want to use with my PC, but my PC requires the microphone to be amplified, so I had the bright idea of making a simple amplifier.

I thought it would be simple, just hook the output (minijack out) on the collector side of a NPN transistor and hook the input (mic) at the base end.

I managed to get it working, but it didn't look like that was the right way. I've attached the diagram of what I did.

I wanted to get it working like a common emitter amplifier shown here:

But it would not work that way.

Could anyone explain why it the common emitter setup does not work for me, and show me the optimal way i can set up a minijack microphone amplifier. I would very mcuh appreciate it, thanks!
-rian

View attachment 36274

 

[audioguru2]

Hi Chrono,
In your 1st circuit, the transistor is turned-off all the time because it doesn't have base bias current. When you managed to get it working, its output must have been extremely distorted because the transistor would have conducted only during the very loudest positive-going peaks from the microphone.

In your 2nd circuit, the transistor is either conducting all the time if the microphone has a low resistance (dynamic coil and magnet type), or the transistor doesn't have enough base bias current because the microphone is an electret type.

In both circuits, if the transistor does conduct, a very high DC current would flow through the speaker and transistor, possibly damaging them.

Why not just determine what type of microphone it is and use a suitable mic preamp/speaker power amp circuit?
A mic preamp circuit can directly feed the audio line level input of your PC.

 

[MP1]

rian,
Check out this site for more info:
http://www.hut.fi/Misc/Electronics/circuits/microphone_powering.html#electretintro

MP