beginner: the unused half of a dual opamp

nyoo · Oct 6, 2008

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

Nigel Goodwin · Oct 6, 2008

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.

Mikebits · Oct 6, 2008

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.

Hero999 · Oct 6, 2008

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.

nyoo · Oct 7, 2008

Hero999 said:
I'd strongly recommend this for CMOS op-amps with a class A output stage....

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.

Hero999 · Oct 7, 2008

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.

https://www.electro-tech-online.com/custompdfs/2008/10/fn976.pdf

audioguru · Oct 7, 2008

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

nyoo · Oct 9, 2008

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.

audioguru · Oct 9, 2008

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.

nyoo · Oct 10, 2008

audioguru said:
The LM393....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.

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.

Hero999 · Oct 10, 2008

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.

audioguru · Oct 10, 2008

Hero999 said:
Or use the LM392 which is both.

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

Hero999 · Oct 11, 2008

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.

audioguru · Oct 11, 2008

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.

ericgibbs · Oct 11, 2008

audioguru said:
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.

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

Hero999 · Oct 11, 2008

audioguru said:
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.

Yea I know, that's exactly why you need to add a compensation capacitor.

So you cannot use a comparator as an amplifier.

Of course you can but it won't be very good.

Comparators don't make very good linear amplifiers and op-amps don't make very good comparators - they're very slow and often have a high saturation voltage.

Some op-amps can't even be used as comparators because they vecome unstable when either input is at too high or lower voltage or the output is saturated to either rail.

Attached as a paper by Texas Instruments that highlights the potential problems using an op-amp as a comparator can cause.

I frequently use op-amps as comparators and comparators as linear amplifiers but I am aware of the risks and limitations of not using the device for what it's designed to do. As a general rule, only use op-amps as linear ampliers and comparators as comparators.

ericgibbs · Oct 11, 2008

The first half dual LM393 makes a dc/LF opa that can drive the second half as a comparator.

These are ideal for light/dark, hot/cold 'amp/comp' applications where the signal is varying slowly.

audioguru · Oct 11, 2008

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

I have never seen a comparator used an an amplifier with negative feedback like that. Its frequency response is down 3dB at 21Hz. Its gain is 1 at 2kHz.

ericgibbs · Oct 11, 2008

audioguru said:
I have never seen a comparator used an an amplifier with negative feedback like that. Its frequency response is down 3dB at 21Hz. Its gain is 1 at 2kHz.

Well you live and learn.

As I clearly said its a LOW frequency amp about 20Hz at -3dB.

I have used them in this way before, also I have built one on a project board this afternoon for a test rig I am designing.

Why have you considered it necessary to quote a 2kHz point for a 20Hz amp.????

They are best suited for [repeated]

These are ideal for light/dark, hot/cold 'amp/comp' applications where the signal is varying slowly.

also battery monitoring etc..........

Hero999 · Oct 11, 2008

Because he's an audioguru and as far as he's concerned the only use for an amplifier is for audio.

He obviously doesn't know anything about other uses of amplifiers such as battery voltage/current monitoring or light meters.

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