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Old 8th November 2011, 12:52 AM   #1
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Default LM3900 substitute?

Is an LM324 a good replacement for an LM3900? I couldn't locate one of the latter so relayed my circuit to use an LM324 instead, well it doesn't work, i checked my output from the first stage and it sits at 10V although the input is a 4V p-p sine wave from my generator, i am building the circuit here:

3 Channel Spectrum Analyzer

I understand the LM3900 is no longer available, so what would be a good substitute?


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Old 8th November 2011, 01:14 AM   #2
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The LM3900 had a unique current differencing input stage - the circuit you link could be made to work with a normal opamp with a similar GBW (2.5MHz) to the LM3900 but does require modifications to account for the change to the input stage. For example the resistors biassing the +ve inputs (R3,R14,R16,R18) need to go to +6V with a normal opamp, not +12V as shown here for the LM3900.

The LM324 is only 1MHz so may not have enough bandwidth.
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Old 8th November 2011, 04:07 PM   #3
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Thanks abraxalito, i'll start again with a different op amp.

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