LM3900 substitute?

Neil Groves · 8th November 2011, 12:52 AM

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?

cheers

Neil.

abraxalito · 8th November 2011, 01:14 AM

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.

Neil Groves · 8th November 2011, 04:07 PM

Thanks abraxalito, i'll start again with a different op amp.

Neil.

8th November 2011, 12:52 AM	#1
Neil Groves diyAudio Member Join Date: Sep 2011 Location: Near San Francisco	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? cheers Neil.

8th November 2011, 01:14 AM	#2
abraxalito diyAudio Member Join Date: Sep 2007 Location: Hangzhou - Marco Polo's 'most beautiful city'. 700yrs is a long time though... Blog Entries: 62	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. __________________ When a measure becomes a target, it ceases to be a good measure. C.A.E. Goodhart

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8th November 2011, 04:07 PM	#3
Neil Groves diyAudio Member Join Date: Sep 2011 Location: Near San Francisco	Thanks abraxalito, i'll start again with a different op amp. Neil.