adjusting microphone bias voltage

[monkeysolder]

I am looking for the simplest way to solve a problem I am having with a microphone circuit. I am powering a microphone (not the one attached to the handsfree earpiece) using a cellphone headset jack. My problem is I am getting a very faint microphone audio signal. The microphone needs a bias voltage different than what the cellphone provides, so I think this may be the issue.

Mic (150 Ohm): needs ~5V bias
Cellphone headset jack: provides 1.8V bias

This is a two line system, so signal and the bias voltage both share the same line. My hunch is that this may need some kind of transformer, but I am not familiar enough with audio engineering to design something from scratch. Could somebody give me a hint in the direction I should go with this? Thanks,

 

[ScottyUK]

I think you're probably looking at an electret microphone, which is the commonest type of small microphone. They usually incorporate a small amplifier to buffer the signal and bring it up to a useable level.

Try typing "electret microphone amplifier" without the "" into Google and sift through the results. There are some good pages with a lot of info. You should be able to design the FET amplifier to operate reasonably well with 1.8V available, but not without some changes to the circuit designed for 5V.

Hope this helps a little.


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I don't suffer from insanity. I enjoy it...

 

[monkeysolder]

Thanks, that search was helpful. I am concerned about the extra voltage that would be applied on the cell phone mic jack though.

Let's say I biased the microphone at the proper voltage. The output of that mic now sits at 5V + audio signal. How do I mix that back onto a line that is held at 1.8V by the cell phone?

Not sure if this is clear or not. I will try to explain my concern better if not.

 

[ScottyUK]

Two ways:

Design the amplifier on the mic to work properly at 1.8V and use the existing voltage (or buy a mic which is designed for it - probably easier)

You need to work out how the cellphone is set up internally. I guess there is another capacitor internally to decouple the audio from the 1.8V bias signal, so you might be able to use a capacitor externally to couple between the 5.0V section and the 1.8V section. Not 100% sure - you may well suffer some HF rolloff because the two caps will form a T filter with whatever the 1.8V supply components are.


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I don't suffer from insanity. I enjoy it...

 

[monkeysolder]

Thanks! Coupling the two lines through a capacitor sounds reasonable to me.

 

[monkeysolder]

Any recommendations for a coupling capacitor size/type?
Thanks.

 

[monkeysolder]

I've found the real problem - the microphone impedance in the mic I want to use is way too low. The symptom of this is that when the microphone is plugged in, it drags the bias voltage down to almost nothing. When I swap it out with a 1k Ohm mic, the voltage does not get dragged down as far and it functions fine. Is there a way to change the source impedance after the cell phone headset jack, so the the cell phone mic input effectively becomes low impedance?

Resistor in series with the mic? A transformer of some sort? I'm not sure how to implement a transformer impedance matching solution, as the line is DC biased and I don't want to overdraw the source or melt the transformer.