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Old 7th May 2011, 11:32 AM   #1
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Default Mic pre amp Problem

Hi folks,
I need some help in the mic amplifier where I have to find a suitable op amp. My specs are an input of max amplitude of 150mV from mic and an output of 3v max amplitude from op amp. The output bandwidth from microphone is stated to be from 200Hz to 20kHz. The op amp will be supplied from +12V and -12V (dual supply). I have a good idea on parameter i have to consider like bandwidth, slew rate etc in fact I decided to work with OPA2134.

There were some hints in class regarding the addition of rc filters but the thing is that in the problem it doesn't specify any dc offset in the input and so I can see the need for a coupling capacitor at the input. And when it comes to low pass do you think there is a need?

the problem is attached.

thanks a lot
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Old 7th May 2011, 02:55 PM   #2
SY is offline SY  United States
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Since this is clearly a homework assignment, and you clearly wouldn't want to cheat by passing off others' answers as your own, let's just ask a few questions. First: Given the max input voltage and the max output voltage, what does the gain need to be?
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Old 7th May 2011, 03:47 PM   #3
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10dB
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Old 7th May 2011, 03:48 PM   #4
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Default Gain

26dB sry
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Old 7th May 2011, 04:47 PM   #5
SY is offline SY  United States
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OK, what is it in plain old linear (not dB)?
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Old 7th May 2011, 04:56 PM   #6
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Default gain

Av=20 (linear)
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Old 7th May 2011, 05:09 PM   #7
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Good! Now what does the resistor ratio have to be for a gain of 20?
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Old 7th May 2011, 05:12 PM   #8
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If we decide to put R2 = 2k , then Rf would be 38k.. we'll use 36k for a standard value which will give a gain of 19 which is quite close.. what do you think?
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Old 7th May 2011, 05:22 PM   #9
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Have you thought about the input to the opamp, DC conditions etc ?
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Old 7th May 2011, 05:25 PM   #10
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Yes first I thought of creating a voltage divider (two 47K) from +12V vcc to -12V vcc for the input to swing on 0v and have plenty of headroom..
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