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Topic: How can I use get a negative voltage for this schematic? (Read 2269 times) previous topic - next topic

TECH GEEK

How can I use get a negative voltage for this schematic? I need to amplify a electret mic's signal 100x...

As always... Thanks for posting!!!

anachrocomputer

Wire up another battery, with the positive terminal connected to ground?

dc42

You could use a +5v to -5v dc-dc converter, if +/- 5v is enough for the chip. Or, if the current required from the negative supply is low enough (say 5mA or less), configure an Arduino PWM pin to output an approximate square wave, then use 2 diodes, 2 capacitors and a current limiting resistor to generate -5v from that pin. This will give you about -3.5v.
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MarkT

Why use an instrumentation amplifier for a microphone?  Instrumentation amps are designed for precision and exceptional common-mode-rejection-ratio.  Amplifying an electret microphone doesn't require this - it can be done with any rail-to-rail opamp using just the single 5V supply.   Also many instrumentation amps don't cover the full audio bandwidth...
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TECH GEEK


Why use an instrumentation amplifier for a microphone?  Instrumentation amps are designed for precision and exceptional common-mode-rejection-ratio.  Amplifying an electret microphone doesn't require this - it can be done with any rail-to-rail opamp using just the single 5V supply.   Also many instrumentation amps don't cover the full audio bandwidth...


that is all i got
As always... Thanks for posting!!!

retrolefty

Quote
Why use an instrumentation amplifier for a microphone?


Also that kind of mic requires a small positive voltage bias applied it to, making kind of a waste the differential input circuitry on the instrumentation amp as it would have to be wired as a single-ended input.
Do you require a AC audio output signal from this amp's output? There are many simple single FET mic pre-amp circuits that work well feeding standard op-amps.

All in all using a instrumentation op-amp for a electret mic is kind of a mismatch and wasted costs. Perhaps if you would explain better what you are going to use the mic for we can suggest a simpler and cheaper solution?

Lefty

Edsoncan

it is not necessary a positive and negative power supply.
It is necessary a half voltage in ref pin and microphone reference.
The only problem is the sound will have a dc voltage togheter with ac voltage.

Edson

MarkT

Its unusual to get an instrumentation amp optimized for low/single rail supply though - check the datasheet, it might fly if you use +2.5V as 'ground'.  You probably want the output above 0V anyway so the analog input can cope with it.
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TECH GEEK

I just need to measure volume, up to the maximum level of "talking loud but not yelling" to "yelling loud like your computer just exploded and caused every electronic gadget in a 23mile radius to fry"
As always... Thanks for posting!!!

retrolefty

#9
Aug 26, 2011, 09:32 pm Last Edit: Aug 26, 2011, 09:35 pm by retrolefty Reason: 1

I just need to measure volume, up to the maximum level of "talking loud but not yelling" to "yelling loud like your computer just exploded and caused every electronic gadget in a 23mile radius to fry"


Then you really need to rectify and filter the audio output so that you get a average DC voltage level proportional to the audio sound level being received by the mic, a so called 'envelope detector'. This can be done before the op-amp, but more commonly done after the op-amp. Again a instrumentation op-amp is kind of overkill for such a circuit.

Also keep in mind that 'volume' as detected by human hearing is on a logrithmic scale, and any simple envolope detector based on a linear amp/detector circuit will not give a good range of wisper to explosion sound range. A log amp would be required if a large dynamic range is required or desired.

Lefty

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