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Electret microphone biasing question.

M

MRW

Jan 1, 1970
0
Hi again!

Image link:
http://img50.imageshack.us/img50/1584/micpreampac2.jpg

In regards to the circuit above, can someone explain to me the part of
the circuit circle in red? I understand that for most electret mics I
need to bias it and that the resistor connected to my bias source is
normally my source resistance. In the red circle, why is the bias
network setup that way?

Also, for the circuits circle in blue, I just need to verify that I'm
looking at it the right way. For the 10k and 1uF, the corner frequency
is around 15Hz and for the other network is around 16kHz. So are the
following the right assumptions:

- if my input signal is 20Hz, then my closed-loop gain is around 11?
- at 17kHz, the opamp would just be a voltage follower?
- at 17kHz, the 10kohm op amp is there to match the input bias
currents?

Thanks!
 
J

James Beck

Jan 1, 1970
0
Hi again!

Image link:
http://img50.imageshack.us/img50/1584/micpreampac2.jpg

In regards to the circuit above, can someone explain to me the part of
the circuit circle in red? I understand that for most electret mics I
need to bias it and that the resistor connected to my bias source is
normally my source resistance. In the red circle, why is the bias
network setup that way?

Also, for the circuits circle in blue, I just need to verify that I'm
looking at it the right way. For the 10k and 1uF, the corner frequency
is around 15Hz and for the other network is around 16kHz. So are the
following the right assumptions:

- if my input signal is 20Hz, then my closed-loop gain is around 11?
- at 17kHz, the opamp would just be a voltage follower?
- at 17kHz, the 10kohm op amp is there to match the input bias
currents?

Thanks!
Looks like a power filter to keep noise from the +5 from inducing noise
in the audio.
 
J

john jardine

Jan 1, 1970
0
MRW said:
Hi again!

Image link:
http://img50.imageshack.us/img50/1584/micpreampac2.jpg

In regards to the circuit above, can someone explain to me the part of
the circuit circle in red? I understand that for most electret mics I
need to bias it and that the resistor connected to my bias source is
normally my source resistance. In the red circle, why is the bias
network setup that way?

Also, for the circuits circle in blue, I just need to verify that I'm
looking at it the right way. For the 10k and 1uF, the corner frequency
is around 15Hz and for the other network is around 16kHz. So are the
following the right assumptions:

- if my input signal is 20Hz, then my closed-loop gain is around 11?
- at 17kHz, the opamp would just be a voltage follower?
- at 17kHz, the 10kohm op amp is there to match the input bias
currents?

Thanks!
Yes. At 20Hz, gain is 11. At 17kHz you still have lots of gain though and at
this 'corner' frequency, the voltage gain will '3db down' or 71% of it's low
frequency value.
I.e 11* 0.7 = x7.7
In this particular circuit, the bias currents are irrelevant and the 2
resistors are there entirely for gain setting.
john
 
D

default

Jan 1, 1970
0
red part

One 2K resistor is to drop some voltage and is decoupled with a cap
the other is to develop signal with. Think in terms of an open
collector inside the microphone cartridge.

The electret is really a capacitor microphone - in the old days
condenser mikes had high voltage supplies to bias them - today they
use electrets. Electrets are bits of plastic insulator that have been
impressed with an electrostatic field as they solidify (you can make
one yourself with some curing epoxy and a high voltage source). The
electret holds a static charge for a very long time and eliminates the
need for a HV bias supply.

Inside the cartridge is the capacitor. Sound causes one of the plates
to change its distance from the other plate. Changing distance
changes bias voltage. The problem is that the impedance is near
infinite - two bits of foil with an insulator. There's a junction FET
inside the mike with its gate connected to the capacitor plate its
source at ground and drain to the output pin - you supply the external
drain load resistor - on a two wire electret mike - three wire ones
contain the resistor inside them. Two wire is more common
 
J

John O'Flaherty

Jan 1, 1970
0
MRW said:
Hi again!

Image link:
http://img50.imageshack.us/img50/1584/micpreampac2.jpg

In regards to the circuit above, can someone explain to me the part of
the circuit circle in red? I understand that for most electret mics I
need to bias it and that the resistor connected to my bias source is
normally my source resistance. In the red circle, why is the bias
network setup that way?

Also, for the circuits circle in blue, I just need to verify that I'm
looking at it the right way. For the 10k and 1uF, the corner frequency
is around 15Hz and for the other network is around 16kHz. So are the
following the right assumptions:

- if my input signal is 20Hz, then my closed-loop gain is around 11?
- at 17kHz, the opamp would just be a voltage follower?
- at 17kHz, the 10kohm op amp is there to match the input bias
currents?

On the low frequency end, you have another pole from the .01 uF and the
two 1 Mohm resistors in parallel, at 31.83 Hz, so at 20 Hz the gain
will be less than the full 11.
 
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