# Another electret Q

P

#### phil

Jan 1, 1970
0
What about a configuration such as this one here
http://www.uoguelph.ca/~antoon/circ/fmt2.htm

We know that the base of the transistor settles on 0.7V.
Does this mean that the microphone will only have 0.7V,
they usually need more than that, don't they?

G

#### grahamk

Jan 1, 1970
0
phil said:
What about a configuration such as this one here
http://www.uoguelph.ca/~antoon/circ/fmt2.htm

We know that the base of the transistor settles on 0.7V.
Does this mean that the microphone will only have 0.7V,
they usually need more than that, don't they?

The base voltage is about 0.7 volts higher than the emitter volts.
If the emitter is 2 volts then the base is 2.7 volts.

P

#### phil

Jan 1, 1970
0
This is true, the problem is that electrets vary considerably
in dc current. For example, some would use 0.5mA, others
would use 5mA. There does not seem to be any way to
determine what voltage is going to be at the emitter.

phil said:
What about a configuration such as this one here
http://www.uoguelph.ca/~antoon/circ/fmt2.htm

We know that the base of the transistor settles on 0.7V.
Does this mean that the microphone will only have 0.7V,
they usually need more than that, don't they?

The base voltage is about 0.7 volts higher than the emitter volts.
If the emitter is 2 volts then the base is 2.7 volts.

I

#### Ian Bell

Jan 1, 1970
0
phil said:
This is true, the problem is that electrets vary considerably
in dc current. For example, some would use 0.5mA, others
would use 5mA. There does not seem to be any way to
determine what voltage is going to be at the emitter.

Quite right. Now you know why this is a poor design.

Ian

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