Op-amp voltage followers/buffer problem.

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JingleJoe

Joined Jul 23, 2011
186
I'm having alot of trouble making the standard op-amp voltage follower/ buffer work.
I've had them working before absolutely fine, but a few weeks ago they just stopped working for me :confused: Do they hibernate in the winter or something? I suspected I had a bad chip so changed it, but to no avail.
I really have no idea what could be wrong with them. My wiring is fine as far as I can tell.

I'll elaborate a bit more so that one may be able to help me figure out whats up with my op-amps;

I'm using this circuit


Observed problem: The output goes to about half Vs and stays there.

Tech info
Vs: 10V
chip: LM324 (I know it's not very good but it's all I've got and should work for this)
input: sawtooth wave, about 1.5V P-P, biased to about half Vs.
 
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SgtWookie

Joined Jul 17, 2007
22,230
Well, if you are using those pin numbers with the LM324, that's your problem.

Here's the pin numbers and functions:



The pins for 4-channel opamps are fairly standard, but always check the datasheet to make certain; you can get bitten if you make assumptions.
 

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Thread Starter

JingleJoe

Joined Jul 23, 2011
186
Oh, not at all! I didn't even see those tiny numbers :rolleyes:
I go by the schematics where I can, not numbers, my circuit is wired up correctly going by the schematic.

I changed the image to avoid any further confusion.
 

Yako

Joined Nov 24, 2011
245
chip: LM324 (I know it's not very good but it's all I've got and should work for this)
I don't agree with that. LM324 is pretty good.

Much like the LM339, the production of them was HUGE, so they are cheap because there is an abundance of them. They fall into armature hands and earn a bad name.
 

SgtWookie

Joined Jul 17, 2007
22,230
This is why we need to have complete and accurate schematics of the circuit in question.

At this point, without further evidence, I'll have to suspect wiring (such as solder balls or whiskers, stray strands of wires, etc) or excess heat applied when soldering, PCB not etched all the way, too much heat used when soldering resulting in damage to the IC, or something similar.
 

SgtWookie

Joined Jul 17, 2007
22,230
The LM324 beats the heck out of a 741. It's old and slow, but still - it can sense and output down to the negative rail, so it's very handy for single supply use. If you understand its' limitations, it is still a useful opamp.

You don't have to use a Ferrari to pick up the groceries.
 

Thread Starter

JingleJoe

Joined Jul 23, 2011
186
I don't agree with that. LM324 is pretty good.

Much like the LM339, the production of them was HUGE, so they are cheap because there is an abundance of them. They fall into armature hands and earn a bad name.
Armature? Do you mean amateur?
The LM324 chip has been fine for my circuits in the past, I'd recommend it, however I hear that it's not very good with high frequencies among other things.


This is why we need to have complete and accurate schematics of the circuit in question.

At this point, without further evidence, I'll have to suspect wiring (such as solder balls or whiskers, stray strands of wires, etc) or excess heat applied when soldering, PCB not etched all the way, too much heat used when soldering resulting in damage to the IC, or something similar.
It's on breadboard and I didn't even draw a schemtic because I know that simple voltage follower off by heart ;) I thought perhaps there may be some common culprit for non-functioning op amp buffers or something obvious to someone more experienced than I.
 

Thread Starter

JingleJoe

Joined Jul 23, 2011
186
What sort of projects have you made using the LM324 man?
Oh all kinds of things, mostly simple amplifiers, buffers and comparators though.
I've used them in everything from a small robot to a sound machine which used a 555 timer too.

... Just wish I knew why they've suddenly stopped working when used as a buffer :confused:
 

Thread Starter

JingleJoe

Joined Jul 23, 2011
186
Can you be specific about your complaints with this IC?
I don't have any (apart from this voltage follower thing) but on half the websites I read people only have bad things to say about it; like how it has bad frequency response above 5kHz or that it's output is very distorted. I haven't used it for anything that required critical accuracy or low distortion so I have not had any of these problems.
 

Yako

Joined Nov 24, 2011
245
Don't believe everything that you read. Why don't you hook up a signal generator to a simple CRO and find out for yourself.
 

Audioguru

Joined Dec 20, 2007
11,248
Please post the complete schematic showing how it is biased.
Show the input coupling capacitor value.
Show the frequency of the triangle wave.
What is the load on the LM324 opamp?
 

SgtWookie

Joined Jul 17, 2007
22,230
That's all well and good, but the voltage follower is still not working.

Until we get a full schematic - and most preferably a photo of the breadboard - it's going to be difficult to make any progress on resolving the issue.

No matter how poor the frequency response of the LMx24 series is, in a voltage follower configuration the output should still resemble the input.
 

Thread Starter

JingleJoe

Joined Jul 23, 2011
186
Please post the complete schematic showing how it is biased. see below
Show the input coupling capacitor value. None. I want the DC offset.
Show the frequency of the triangle wave. it's about 400 Hz to 1KHz
What is the load on the LM324 opamp? nothing yet, so aproximately infinity ohms, I added a 1meg ohm resistor to my oscope input when testing it
I had input a saw/ramp wave acctually, but have and will use this circuit for triangle waves too.

The wave in question is the charge and discharge of a capacitor in a schmitt trigger oscillator utilizing a 4093 NAND gate with built in schmitt triggers on the inputs (Sgt wookie told me about this one ages ago acctually!;)) This circuit has an added transistor to allow for voltage control of the frequency.
See attachment for circuit diagram.
The simulation doesn't work quite right (oops just noticed that's because my transistor is backwards!) but it works in practice, I probed around with my o-scope and it functioned correctly after the voltage follower was added, but I get no output from the voltage follower.

P.S. the supply in the diagram reads 5 volts but I used 10.
 

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Audioguru

Joined Dec 20, 2007
11,248
It is a horrible circuit:
1) Its transistor is upside-down.
2) The 4093 quickly charges the capacitor through the diode but nothing discharges the capacitor except the backwards transistor acting like a diode.

Maybe you should make the voltage-controlled oscillator shown in the LM324 datasheet. It has square and triangle outputs. Adding a diode and resistor can convert the triangle into a sawtooth waveform if you want.
 

Thread Starter

JingleJoe

Joined Jul 23, 2011
186
It is a horrible circuit:
1) Its transistor is upside-down.
I did point that out, I only made the schematic because you asked for it, otherwise I knew what I was doing and put everything together correctly.
2) The 4093 quickly charges the capacitor through the diode but nothing discharges the capacitor except the backwards transistor acting like a diode.
Yup, thats about right.
Maybe you should make the voltage-controlled oscillator shown in the LM324 datasheet. It has square and triangle outputs. Adding a diode and resistor can convert the triangle into a sawtooth waveform if you want.
Looking at that circuit and many similar, they seem to impliment a schmitt trigger made from an op amp, which this "horrible" circuit does but with a logic gate with a built in schmitt trigger, ordinarily people seem to like to use the 40106 for this kind of oscillator but I only had some 4093's to hand.
I don't see whats so horrible, it's simple and effective if you ask me :p

The LM324 can take inputs down to the negative rail, but has a restriction that the common-mode voltage has to be below V+ -2V. Could that have something to do with your problem? See datasheet: http://www.national.com/ds/LM/LM124.pdf
Yes I'm pretty certain it was within the limits; my wave being around half Vs (supply of 10V, half = 5) with a peak of about 1.5, makes a maximum of 5.75 a lower end of 4.25 (correct me if I'm wrong)
 
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