Help me design peak detector with LM324 for AC signal with frequency less than 1MHz

[Mathias]

Hi!

I have one problem with my school project. I have to design peak detector with LM324 for AC signal with frequency less than 1MHz (attachment). My problem is that circuit doesn't work correctly with frequency more than cca. 10kHz - but GBW of LM324 is 1MHz. Which are other important parameters of op amp on which I need to be careful If I wish to design such circuit ?

Thank you very much!

Mathias

 

[Davood Amerion]

Re: Peak detector

nothing
you must choose op-amp at least 10-20 times greater band width 10-20 MHz min
in such circuits.

Regards
Davood Amerion

 

[Kral]

Re: Peak detector

Mathias,
GBW product is a small-signal parameter. The important parameter for your circuit is slew rate. This is the guaranteed minimum rate of change of output voltage with respect to time that the device can produce. For the LM324, the slew rate is only 0.5V/microsecond. You need an op amp with a higher guaranteed slew rate. Example:
~
A 10 volt peak 1MHz sine wave has a slew rate that is 10 times the slew rate of a 1 volt peak sine wave of the same frequency.
Regards,
Kral

 

[flatulent]

Re: Peak detector

There is also the saturation problem. Put a diode between pins 1 and 2 so that the output can not go lower than 0.6 V below the input. This will keep the op amp out of saturation and help the speed problem.

Can you have a negative voltage supply for the amplifier?

On the input use a diode in series (not shunt) and a megohm resistor from the amp input to ground. This will keep the input from going below the power supply range.

 

[IanP]

Re: Peak detector

On top of what has been already said (additonal diode, symmetric power supply ..) you should consider adding a buffer .. see drawing below ..
Regards,
IanP

 

[Mathias]

Re: Peak detector

Thank you all for you help. Now I understand this thinks much better. But I still don't understand something. I tried to use OP238 with slew rate 15V/us. Now if I have wrong please correct me:
I have single supply +15V. And sinus wave input signal with magnitude 5V. Now... maximum frequency of my peak detector is determinated by slew rate of op amp. So It take around 0.3us that soutput signal from op amp reach from zero to 5V, and 0.3us that signal reach zero again. That is 0.6us together. Frequency is then a little more that 1,5MHz. For me, if i understand , this is my maximum frequency limit. But my problem is - i consider also all mentioned solution above - that my peak detector doesn't work to this frequency. My peak detector wotks just to around 20KHz. I am newbie and I do not know how to solve this problem, but I assume that I have problem with saturation.

Thanks for your help!


Mathias

Added after 1 minutes:

I forget to say that I haven't try peak detector with buffer yet, because I wish to understand first circuit for the beggining.

Thank you again.
Mathias