# New OpAmp circuit to analyse

Discussion in 'Homework Help' started by PsySc0rpi0n, Nov 16, 2015.

1. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
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Hi...

I have a new OpAmp circuit to analyse and part of the analysis is already done!
It is a "Widow Detector" and we have the circuit split in 3 separate blocks. Block 1 which is a Schimtt Trigger, Block 2 which is an Integrator and the 3rd one I'm still not sure what it is!

I think it is at least an Half-Wave rectifier but it has probably some other function that I don't know!
I need some help to identify it and give an explanation of the role of this block on the whole circuit!

I have tried to simulate Block 3 on LTSpice, first using a sine wave and I tried with a triangular wave. But for the triangular wave I was not able to setup the input voltage source the way I wanted as it is in the attached file!
I have also tried to use discrete values (see picture below) on the right voltage source and tried to use the directive ".param" to make things a bit more simple to read but I couldn't! I can' figure out what's wrong with the parameters in the simulation!

http://i.imgur.com/kDKuvLz.png

• ###### 2.1.c.asc
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Last edited: Nov 16, 2015
2. ### MrAl Distinguished Member

Jun 17, 2014
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Hello,

The 3rd block looks like a comparator with adjusted output impedance: low impedance source and higher impedance (10k) sink.

3. ### KeepItSimpleStupid Well-Known Member

Mar 4, 2014
1,298
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(3) is a comparator that can only source current.

4. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,191
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Ok, and what about not being able to set the voltage supply for that block like I pretend???

5. ### KeepItSimpleStupid Well-Known Member

Mar 4, 2014
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Usually, you have to provide A03, inverting input with a low Z source. i.e. Buffer the reference.

6. ### GopherT AAC Fanatic!

Nov 23, 2012
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Blocks 1 and 2 are integrator and comparator. Together, they make a triangle wave oscillator. hint: if there is no input terminal, it is usually an oscillator). The block 3 is a rectifier (half wave as you said) that can clip as little or as much of the mountain base off of each triangle as you want (pot control) and that op amp acts as a comparitor to make a square wave - PWM.

This circuit needs a +/- 15 V power supply (not a single supply).

Last edited: Nov 16, 2015

Feb 17, 2009
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8. ### GopherT AAC Fanatic!

Nov 23, 2012
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I wish I would have thought of that.

Any reference to what a PXX circuit is?

9. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
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I'm using a split voltage supply with +12V/-12V. If it's not enough, I can use the 15V level, no problem, but I'm using 2 voltage sources!

I also can't find anything about PXX circuits!

Last edited: Nov 16, 2015
10. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Have look at my attached file? And do you analyze the results ??
And do you ever herd about Pulse Width Modulation ??

11. ### GopherT AAC Fanatic!

Nov 23, 2012
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I have heard of PWM but I've never heard it called PXX. Kind of strange to make a three letter acronym for a three letter acronym.

12. ### crutschow Expert

Mar 14, 2008
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Yes, that's a PWM circuit with the output pulse width determined by the potentiometer setting.
But I don't understand the purpose of the output diode and resistor in that circuit.

13. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Maybe they want only a positive PWM at the output.

14. ### crutschow Expert

Mar 14, 2008
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Most IC comparators have only a positive output but that certainly makes sense if the comparator has a bipolar output.

15. ### AnalogKid Distinguished Member

Aug 1, 2013
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While that certainly is a very common approach, there is nothing in the original problem that states or implies that. A03 (note that the A might mean amplifier as opposed to a C for comparator) could be 1/4 of the same quad opamp used for A01 and A02.

ak

16. ### AnalogKid Distinguished Member

Aug 1, 2013
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I agree. I think the output is a converter for a CMOS logic input, but another possibility is that it is a part of the problem included to cover something else being taught.

ak

17. ### marcf Member

Dec 29, 2014
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The title of this thread refers to A0, A1 and A3 as op amps, not comparators. I think the difference is that comparators are open collector devices that can only sink current and op-amps have the ability to both sink and source current.

I understand that while you can use a op amp as a comparator is not necessarily a good idea to do so.

If A0 - A3 are comparators, I would expect to see at least a pull up resistor on A3.

18. ### crutschow Expert

Mar 14, 2008
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A02 must be an op amp since it is being used in the linear mode as an integrator.
A comparator would oscillate in that application since they are not compensated for linear operation.

19. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
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Yes I have looked into your attached file. I also looked at the results. I just didn't know that PWM was also known as PXX.

I'm not sure if your intent with your .asc file was just the results or if you were trying to tell me something about the way I tried to set the vin voltage source.

I spoke to my teacher and sometimes I think you might be a psychic. That diode is in fact to get a positive only output voltage.

And is the resistor next to the diode, a pull-down resistor or what is it doing there?
What about my attempt to set the Vin voltage as I tried? Why doesn't it work?

20. ### JoeJester AAC Fanatic!

Apr 26, 2005
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I call the output of block 2 OSC and the output of block 3, at D1/R4 junction, Vo. I started the transient analysis with zero initial values.