I know, i know not a very good title...

But that is exactly my question,is there any issues to using the ancient quad op amp for multiple purposes.

I'll explain, i want to use 1 op amp as a AC coupled non inverting amplifier,it 's ouput fed to input of another op amp as a precision rectifier, it's output fed to the last 2 op amps configured as a VCO.

Why you ask, well it's related to a project i want to do using a audio signal to alter the counting speed of a CD4017.

I was using a 555 w/ the 5 pin coupled to a speaker output through a e- cap, it works buuut not exactly the way i wanted.

One problem i was having was wanting to drive it w/ a much weaker signal hence the first stage amplifier.

The second issue was it was pretty glitchy,i assume it's because of the nature of the AC signal hence rectifier section.

Lastly using the two final amps as a VCO just makes sense to save me from using a 555 along w/ the rest.

I mocked the thing up based on the data sheet on the falstad simulator and it does work.
Buut i know the falstad simulator is crude and uses generic values etc.(still love its simplicity)

My question comes in as i noticed some random glitching on the falstad scope, the generic counter didn't seam to notice them but i think a 4017 would.

Would there be a issue w/ crosstalk or other when using the lm324 in this manner?

If you guys think this will work ill scetch it out and come back to talk about values to set base frequency of the VCO and gain on the amplifier.

All input is appreciated even from Audioguru

Thanks,

-luvv-

 

There is cross talk on all monolithic quad op amps, but really at a very low level, so I suspect if you circuit doesn't work it wont be crosstalk on the chip. More likely crosstalk in your pcb design. Keep hi impedance input tracks to minimum length.
crosstalk usually 130dB down. Thats like 10,000,000,000,000 so nothing to worry about really.

 

Mr. Chips is compiling a Components Selection Guide, with some input from other forum members. Post #4 of this thread is about amplifiers and comparators, including some general purpose op amps. Perhaps you can find some ideas there ?

http://forum.allaboutcircuits.com/showpost.php?p=448160&postcount=4

Maybe the LF411 (in the Op Amp J-FET box) is suitable for your application ? Horowitz and Hill had selected this IC as their "Op Amp to rule them all" in The Art of Electronics. Only that they didn't call it rule-them-all but "jellybean" which sounds a bit more modest

 

Are you using the VCO circuit shown in Natrional Semi's datasheet of the LM324? It uses only two opamps so you could use an LM358 dual opamp instead of the LM324 quad.
Its highest frequency is not high because those lousy old opamps are very slow.

I would use an MC33172 higher speed dual opamp for low power and use an MC34072 for much higher speed and normal opamp power. They aso have inputs that work as low as the negative supply which can be ground.

 

@ Blofeld: I've looked around here and other sites at the caparisons of wide range of op amps.
Reason i wanted to attempt it w/ the 324 is two fold, one and most importantly i have 50 of things, secondly because it's quad and it just so happens i need four amps.

I'm not married to it, but would be nice if i could make it work smoothly.

@ Audoguru: Really wanted to use four stages 1. AC coupled non inverting 2.Precision rectifier 3. and 4. VCO.
Yes i was planing to use the data sheet for all but the rectifier it's from Wikipedia lol

I don't know that frequency is a issue from what i see its range needs to be 20hrz- 2 khrz max.
While i was looking around i seen a few post by you here and there and seen your disdain for this lousy chip and the people that wrote the datasheet.
That's why i mentioned you in the 1st post,couldn't wait to see what you had to say about loading it up.
Have to say, was much tamer then i imagined =P
I'll throw it together in LTspice and come back for some component values from you guys .
Thanks for the responses guys!

-luvv-

 

I very rarely use a spice program. Why not breadboard your circuit and give the LM324 a go? As old as it may be, it is not a bad choice for a lot of simple applications. Post your circuit and we'll suggest some component values.