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Old 31st Mar 2014, 8:59 am   #1
arjoll
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Default Quick dumb op-amp question

This is probably a stupid question, but with a quick google I can't track down the answer anywhere.

The background - I'm knocking up a basic -10/+4 converter for a guy who is planning to co-site at one of our transmitter sites. He's on a tight budget so is just picking up his existing LPFM station and re-broadcasting it on a newly licensed frequency - basically his LPFM is a temporary link. Rather than using a relay receiver he's got a domestic tuner.

The circuit is simple - a TL074, wired at two channels, each channel having an inverting amplifier with gain, and an inverting amplifier with unity gain.

I've been using op-amps since I was a teenager, but it's been a few years since I've built anything so thought I'd just double-check on one thing. When I learned about them Dad had some old books on ICs and op-amps and they always showed a resistor from the non-inverting input to ground - I've usually thrown in 100k. Virtually every example of an inverting op-amp just shows the non-inverting input to ground though.

I have borrowed the books and cannot find any information in the text about the function of this resistor. They just always seem to show one, usually either 10k or 47k, so I'm not sure where I got 100k from!

So my question is - is this resistor required? And, if so, what is its function and the rule of thumb for value?
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Old 31st Mar 2014, 9:07 am   #2
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Default Re: Quick dumb op-amp question

It is there to balance the input offset voltage due to the input current. Never needed in an AC coupled thing and hardly needed at all now with modern op-amps.
 
Old 31st Mar 2014, 9:22 am   #3
Aub
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Default Re: Quick dumb op-amp question

I think i always used to make it the same value as the resistor going to the inverting input, often 10k.

Like you, it's been a long time ago!

Cheers

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Old 31st Mar 2014, 10:11 am   #4
Karen O
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Default Re: Quick dumb op-amp question

Yes, in older opamps you needed to ensure that both inputs saw the same impedance so that the voltage offsets caused by the small but significant input bias currents cancelled. With modern FET input opamps the input bias currents are so tiny you don't have to worry about such measures.
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Old 1st Apr 2014, 9:14 am   #5
lesmw0sec
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Default Re: Quick dumb op-amp question

A very reasonable question - not dumb at all, which has been answered correctly. Life for the audio designer is a little easier these days!
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Old 4th Apr 2014, 10:17 am   #6
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Default Re: Quick dumb op-amp question

Built, tested, working and going onsite tomorrow - thanks for the info

And the obligatory pic. Not the neatest thing in the world, but it works. The power supply is on a separate board because I already had it lying around from another project that I never got around to finishing
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