HELP: BJT input op-amp's input bias current compensation

frickecello · 17th June 2007, 04:38 PM

Hi, Im working in a microphone preamplifier using the classic instrumentation amplifier topology, but modified for linear gain control, I am really happy with the results as the circuit is very quiet and rejects RF and power line noise (at least for my application).

I am very confused regarding NPN BJT op-amp's (like the NE5532) input bias current compensation, I've read many datasheets and some books, but I still dont get it.

I am planning to use any of these op-amps NE5532 or LM833, (currently using LM833's with DC input bias current path to ground through resistors) the reason is that Im currently living in a country where most recent op-amps are unavailable (like OPA2134 or INA217, etc).

My main questions are:

If I use the NE5532 as the main op-amp in place of LM833 what is the most simple input bias current compensation?

What op-amp do you think is more suitable for this application NE5532 or LM833? (those are the only audio op-amps I can find here)

I apologize for my bad english, thanks in advance.

- Marc Fricke

Juergen Knoop · 17th June 2007, 04:58 PM

bias induced offset voltage should not exceed the input offset voltage
so I can't see the need for a special bias compensation
regards

unclejed613 · 17th June 2007, 08:53 PM

usually, input bias compensation is accomplished by using the same input resistor values for both inverting and noninverting inputs...... in other words, if you have a 1k resistor to ground from your inverting input, you place a 1k resistor in series with your noninverting input. with most modern opamps and low to medium gain (2 to 500) it's not really an issue anyway. input bias currents really become a problem at high gain.
since the gain of this preamp is so low, and it's also ac coupled through a cap, it's nothing to worry about.

frickecello · 18th June 2007, 07:42 PM

Thanks for your kind replies. This mic preamp's gain is aprox. 1000 or 60dB, the first stage gain is 10 times 100 of the diff. amplifier so it equals 1000.

Is bias current nothing to worry about in this case?

What op-amp do you consider better for this application NE5532 or LM833?

Juergen Knoop · 18th June 2007, 08:53 PM

did you already build the amp?
then just swap the op-amp and see what happens...
ac and dc-specs don't differ very much between the two and there is always something like production variation
regards

frickecello · 18th June 2007, 11:52 PM

Yes I have built 8 channels mounted on a single pcb, but some channels are dead quiet and others have some kind of hum, the other funny thing that happens is that I am not able to run it from a power supply because it is extremely noisy, just from batteries.

frickecello · 19th June 2007, 12:00 AM

Then I realized that it was impossible it was not working with the power supply if it already worked with batteries, so I went to a lab where I used a expensive bulky PSU unit to feed the circuit and it worked really well, with the exception of some channels that had some low freq hum.

That is my main problem right now, what is causing that?

17th June 2007, 08:53 PM	#3
unclejed613 diyAudio Member Join Date: Dec 2006	usually, input bias compensation is accomplished by using the same input resistor values for both inverting and noninverting inputs...... in other words, if you have a 1k resistor to ground from your inverting input, you place a 1k resistor in series with your noninverting input. with most modern opamps and low to medium gain (2 to 500) it's not really an issue anyway. input bias currents really become a problem at high gain. since the gain of this preamp is so low, and it's also ac coupled through a cap, it's nothing to worry about. __________________ Vintage Audio and Pro-Audio repair ampz(removethis)@sohonet.net spammer trap: http://www1284177414881.v-dc.net/

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17th June 2007, 04:58 PM	#2
Juergen Knoop diyAudio Member Join Date: Jan 2007 Blog Entries: 3	bias induced offset voltage should not exceed the input offset voltage so I can't see the need for a special bias compensation regards

18th June 2007, 07:42 PM	#4
frickecello diyAudio Member Join Date: Aug 2006	Thanks for your kind replies. This mic preamp's gain is aprox. 1000 or 60dB, the first stage gain is 10 times 100 of the diff. amplifier so it equals 1000. Is bias current nothing to worry about in this case? What op-amp do you consider better for this application NE5532 or LM833?

18th June 2007, 08:53 PM	#5
Juergen Knoop diyAudio Member Join Date: Jan 2007 Blog Entries: 3	did you already build the amp? then just swap the op-amp and see what happens... ac and dc-specs don't differ very much between the two and there is always something like production variation regards

18th June 2007, 11:52 PM	#6
frickecello diyAudio Member Join Date: Aug 2006	Yes I have built 8 channels mounted on a single pcb, but some channels are dead quiet and others have some kind of hum, the other funny thing that happens is that I am not able to run it from a power supply because it is extremely noisy, just from batteries.

19th June 2007, 12:00 AM	#7
frickecello diyAudio Member Join Date: Aug 2006	Then I realized that it was impossible it was not working with the power supply if it already worked with batteries, so I went to a lab where I used a expensive bulky PSU unit to feed the circuit and it worked really well, with the exception of some channels that had some low freq hum. That is my main problem right now, what is causing that?

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