The textbooks are mute on the subject. But from a quick google, there appears to be conflicting advice concerning tying off the unused portion of a dual opamp.

For an LM393, the advice was
IN1- to IN2+,
IN2- to GND, and
let OUT2 float.

For an MC1558, the advice was to "configure as a voltage follower":
IN2+ to GND, and
IN2- to OUT2.

In one case, we were told to
leave IN2+, IN2-, and OUT2 all floating, and "accept the power dissipation".

So, the questions.
Are there various classes of dual opamp, each of which requires a different method of tying off? Is there a small set of rules, that I (as a beginner) can apply for each class of opamp?

Or maybe it all boils down to a single rule?

Thanks very much for your time and help.

Alan K

 

I suggest you try all variations and see what difference (if any) it makes - it may make none whatsoever - all these 'pin tying' ideas basically originate from CMOS gates.

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I think some devices draw more current based on whether inputs are being pulled high or low. Data sheets usually provide info for unused amps or gates. If overall current consumption is not an issue then it is a don't care.

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It depends on the op-amp.

Most of the time you can connect the + and - pin to different voltages and leave the output to float at either high or low. I'd strongly recommend this for CMOS op-amps with a class A output stage which will use more power if the output sits between the rails as both output transistors will be on simultaniously.

I've heard that some op-amps don't like this and prefer to be configured as a voltage follower with the input connected to 0V.

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The datasheets for the LM393 or MC1458, two opamps I have now used, do not directly mention "CMOS" or "class A output". Should I be able to tell by looking for some specific transistor in the opamp's schematic diagram?

It looks, though, like tying down is almost optional.

Thanks.

 

They are both bipolar op-amps, CMOS output op-amps always have a n-channel and p-channel MOSFETs on the output.

Take the CA3160 for example, see page 4 of the datasheet, note how Q8 and Q12 are connected.

http://www.intersil.com/data/fn/fn976.pdf

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I f you don't tie down the input of unused opamps then they might oscillate and make a mess of the active opamps.

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I see the MOSFETs, so now I know how to look a little longer at the schematic diagrams.

And, for all unused 1/2 opamps, I'll use the method where the IN1- to IN2+, IN2- to GND, and let OUT2 float.

Thanks for removing some uncertainty.

 

The LM393 is a dual comparator with an open collector output. It is not an opamp. If you want a single comparator then use an LM311.

The MC1458 is a dual opamp. If you want a single opamp then use an MC33171 (low power) or MC34071 (normal power) instead.

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How did you know the question I needed to ask, but couldn't frame? Of course! I'll use the single comaprator, and the single opamp.

Thanks very much for your help.

 

Or use the LM392 which is both.

You can use an op-amp as a poor man's comparator and a comparator as a poor man's linear amplifier.

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I haven't even thought about the lousy old LM392 for about 35 years.
I have never used one nor seen one.

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I've got a few lying around in my junkbox, they come in useful from time to time.

I like to keep my designs as small as possible. If I have a quad comparator like the LM339 and have a spare comparator on the IC and I need a low frequency amplifier. I'll use the spare comparator as an op-amp, rather than using another IC.

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A comparator does not have internal frequency compensation like an opamp (rolls off the gain above a few Hz) so it will oscillate if negative feedback is applied like an opamp usually has.

So you cannot use a comparator as an amplifier.

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Its a low spec opa, but it can been done, its OK for low frequencies.

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