How to choose the right op amp band for triangle generator?

Thread Starter

alpheratz

Joined Nov 10, 2019
17
Hello everyone! Please help me to understand a thing: if I want to use my op amp to generate a triangle wave with max 20kHz (classical design, op amp + schmitt trigger comparator) what is the right op amp bandwidth?
I would say 1MHz is enough (this is what I want to use), but my friend says you need two other decades so at least 2 decades (therefore 2Mhz).

What would you chose and why?
 

MrAl

Joined Jun 17, 2014
7,592
Hello everyone! Please help me to understand a thing: if I want to use my op amp to generate a triangle wave with max 20kHz (classical design, op amp + schmitt trigger comparator) what is the right op amp bandwidth?
I would say 1MHz is enough (this is what I want to use), but my friend says you need two other decades so at least 2 decades (therefore 2Mhz).

What would you chose and why?
Hi,

Do you know what slew rate is? That is a very important specification that you need to know in order to solve this. The output depends highly on this spec not just the bandwidth.
 

Thread Starter

alpheratz

Joined Nov 10, 2019
17
Yes, thanks for your answer. I know what it is. My triangle wave needs 0.2V/us during rise/fall times while my op amp (opa277) gives me 0.8V/us. Is it ok or I'm too close?
 

MrAl

Joined Jun 17, 2014
7,592
Yes, thanks for your answer. I know what it is. My triangle wave needs 0.2V/us during rise/fall times while my op amp (opa277) gives me 0.8V/us. Is it ok or I'm too close?
Hi,

That sounds good, but how did you get that 0.2v/us? is your supply voltage plus and minus 2.5v or near that or single supply 5v or near that, or is your triangle plus and minus 2.5v or just 0v to 5v?
 

Thread Starter

alpheratz

Joined Nov 10, 2019
17
It changes between 5V and 10V. Let me explain:
my voltage suppy is + - 15V but I have a second NE555 configured as schmitt trigger comparator that is powered to ground and 15V so it has 5V and 10V as thresholds. Therefore my triangular wave changes between these two thresholds (5 and 10) so it has 5V peak to peak and at 20kHz I have 0.2v/us. (So it has a 7.5V DC component).

Actually it's not a generator but a VCO. This is the schematic:
 

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MrAl

Joined Jun 17, 2014
7,592
It changes between 5V and 10V. Let me explain:
my voltage suppy is + - 15V but I have a second NE555 configured as schmitt trigger comparator that is powered to ground and 15V so it has 5V and 10V as thresholds. Therefore my triangular wave changes between these two thresholds (5 and 10) so it has 5V peak to peak and at 20kHz I have 0.2v/us. (So it has a 7.5V DC component).

Actually it's not a generator but a VCO. This is the schematic:
Yes i was guessing 5v peak to peak because of your 0.2v/us so that must be right.
So you are going from 5v to 10v ok.
I did not analyze the circuit itself but from what you said so far it sounds right.
0.2v/us isnt that fast so it should work ok with something around 0.8v/us as long as it really is that.
Not sure how much rounding of the top and bottom peaks you will get though. If i remember right, a triangle wave has harmonics 1/n^2 of the fundamental for odd n, so you could come up with an estimate of the maximum frequency you would like to pass. You could also simulate to make sure it looks ok.
 
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