Negative peak detector

Thread Starter

cboot

Joined Nov 27, 2009
7
Hi,

I am trying to make a negative peak detector using a LM324 for a low frequency (e.g., 440Hz), low voltage (500mV max) signal. I'm kinda lost because this op amp is biased at 0V so I think I need some kind of virtual ground, but I'm not very experienced... could someone give me a hand?

Thanks!
 

SgtWookie

Joined Jul 17, 2007
22,230
Hiya Cboot,
Welcome to AAC! We're generally a pretty friendly and helpful bunch. Hang around and learn stuff!
I am trying to make a negative peak detector using a LM324 for a low frequency (e.g., 440Hz), low voltage (500mV max) signal.
OK, the LM324 is a really ancient, Really SUPER slow operational amplifier. Not good for your application.

It would help a great deal if you posted your circuit, so that we can see what you're really trying to do.
I'm kinda lost because this op amp is biased at 0V so I think I need some kind of virtual ground, but I'm not very experienced... could someone give me a hand?
We'll give it a good go - but you have to give us a bit more information.

Circuit diagrams (schematics) are the language that we speak around here.
They eliminate almost any ambiguities (uncertanties), and almost everyone can understand them, even if English is not their primary language.

.png format is what I prefer. The images load fast, and you can create them quickly using MS Paint.

I look forward to helping you with your situation - please post your schematic.

(Use the "Go Advanced" button below the text box, then "Manage Attachments" button to attach your images.
 
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Thread Starter

cboot

Joined Nov 27, 2009
7
Hiya Cboot,
Welcome to AAC! We're generally a pretty friendly and helpful bunch. Hang around and learn stuff!
Thank you :)

OK, the LM324 is a really ancient, Really SUPER slow operational amplifier. Not good for your application.
Hmm I've heard about that. Nevertheless, I wanted to learn how to make a negative peak detector using such a 0V biased op amp, so I could learn how to deal with virtual ground and etc

It would help a great deal if you posted your circuit, so that we can see what you're really trying to do.
Here you go... basically what I have now is a positive peak detector... and what I want to make is a negative peak detector parallel to that.

What I am trying to do is measure the period of a signal which is polluted with harmonics. I'm using the technique of peak detection to accomplish that. Currently, the output of the positive peak detection is being fed to a voltage divider set to 90% of the peak and then it goes to a comparator which compares the original signal (non-inverting input) with 90% of the peak (inverting input). So the comparator outputs 1 when the original signal is at its peak and 0 otherwise. Now I want to add hysteresis so the output will drop to zero only when the signal reaches 90% of its negative peak... thats why I want a negative peak detector. The hysteresis circuit is no problem for me.
 

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Audioguru

Joined Dec 20, 2007
11,248
I don't know why your schematic is a negative. I fixed it.

The MC3317x and MC3407x opamps have many of the single supply features of the old LM324 but have a much wider bandwidth and no crossover distortion.

I added an inverting opamp and diode to give the negative peak detection.
Don't allow its input voltage to go too much negative then the input of the opamp will be negative which might destroy it.
 

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Thread Starter

cboot

Joined Nov 27, 2009
7
I don't know why your schematic is a negative. I fixed it.
Was my schematic wrong?

The MC3317x and MC3407x opamps have many of the single supply features of the old LM324 but have a much wider bandwidth and no crossover distortion.
Thanks, I'll go buy it when I have a spare time!

I added an inverting opamp and diode to give the negative peak detection.
Don't allow its input voltage to go too much negative then the input of the opamp will be negative which might destroy it.
Thanks for your schematic!
I just didn't get, though... could you explain to me how it works?
 

Thread Starter

cboot

Joined Nov 27, 2009
7
About LM324's bandwidth... why would I want a wider bandwidth if LM324 has 1MHz, which should be more than enough for audio applications?

And about crossover distortion... can you tell me more about it? How does it happen? What's its noise like? High frequencies? If so, How high? I am a computer science student diving into the world of electronics, and I have to say analog circuits are a pain in the a** to learn!
 

Audioguru

Joined Dec 20, 2007
11,248
About LM324's bandwidth... why would I want a wider bandwidth if LM324 has 1MHz, which should be more than enough for audio applications?
The LM324 opamp has full output level up to only 2kHz because its output cannot slew fast enough. The 741 opamp has full output level up to 9kHz. The MC3317x is also low power like the LM324 but has full output level up to 35kHz. The MC3407x and many other opamps have full output level up to 100kHz.

And about crossover distortion... can you tell me more about it? How does it happen? What's its noise like? High frequencies? If so, How high?
Look at crossover distortion in google. It is caused when the output transistors in an amplifier do not have enough bias current. It sounds like a buzz especially at certain fairly low levels. It is 3% in the LM324 and LM358.
TL07x audio opamps have distortion at only 0.003%.
 

Thread Starter

cboot

Joined Nov 27, 2009
7
@Bill: yes I've heard of it, but I'm not quite familiar with it yet... the first time I heard of it was a month ago or so, when I looked at the positive peak detector schematic.

The LM324 opamp has full output level up to only 2kHz because its output cannot slew fast enough. The 741 opamp has full output level up to 9kHz. The MC3317x is also low power like the LM324 but has full output level up to 35kHz. The MC3407x and many other opamps have full output level up to 100kHz.
I think I saw 1MHz in a Motorola datasheet... must have read it wrong anyway. I understand that the LM324 is a bad solution for audio... I'll try to find the MC opamp as you said... but for now I'll continue to test using LM324 because I think only wednesday I'll be able to go downtown look for it.

Look at crossover distortion in google. It is caused when the output transistors in an amplifier do not have enough bias current. It sounds like a buzz especially at certain fairly low levels. It is 3% in the LM324 and LM358.
TL07x audio opamps have distortion at only 0.003%.
Thanks! I looked at it in google... but thats a lot of information to process... I'll try to digest it slowly.

For now, I'm really curious about how to implement the negative peak detector... I'll be glag if you could explain to me briefly the schematic you posted!
 

Thread Starter

cboot

Joined Nov 27, 2009
7
The LM324 barely gets to 1.8kHz at full output level. A low cost TL07x audio opamp and many more get to over 100kHz. Most people who are not deaf can hear to 20khz.
Hmm I see... Thanks for clarifying it. I definitely agree with you. I'm still kinda lost when I look at some datasheet graphs hehe

But what about the negative peak detector? Could you explain to me the schematic you posted?
 

Audioguru

Joined Dec 20, 2007
11,248
The schematic I posted uses an inverting opamp to invert the negative input and add it to the opamp that has the positive input.
 
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