# How to limit a opamp's output?

Discussion in 'Homework Help' started by pedroca1, Oct 9, 2016.

1. ### pedroca1 Thread Starter New Member

Oct 9, 2016
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0
Hi everyone,

I have a signal (called Y) that is a sawtooth wave and goes to 5V to -5V. I also have a temperature sensor (called T) that spits two values: 6V or 0V. When T = 6V, that signal Y must be 0 and when T = 0V, Y should remain unchanged!
My professor gave us this challenge and we can only use opamps and resistors... I have thought to use a comparator opamp and use Vcc+ = GND and Vcc- = Y, but I can't do this because Y isn't a constant voltage. That is the only thing I could think to solve this challenge...
I want to limit Y's wave using T, how can I do this?

2. ### Alec_t AAC Fanatic!

Sep 17, 2013
7,095
1,478
Welcome to AAC!
The moderators will probably move this thread to the 'Homework help' forum where it belongs.
Do you have dual-polarity supplies available for the opamp/comparator?

3. ### pedroca1 Thread Starter New Member

Oct 9, 2016
2
0
Oh, that's ok. Sorry for that, I'm new here :x
Yes, I do!

4. ### shteii01 AAC Fanatic!

Feb 19, 2010
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616
I think you have to build a logic gate.

For example. If you use good rail to rail op amp. You use output of sensor T as power source for the op amp that is configured as buffer.

For example.
Put NOT gate between T and positive power supply of the buffer op amp. When T is 6 volts (I assume Logic 1), the output of NOT gate will be 0 volts (Logic 0). Then you feed the output of NOT gate to the positive power supply of buffer op amp, right now this is 0 volts. Since the positive power supply of the buffer op amp is 0 volts, the positive portion of sawtooth wave will be destroyed and you will see just the 0 volt flat line, in other words a positive portion of Y will be 0 like you want. The negative portion of swatooth wave will still be present because we did not modify the negative supply of the op amp.

Bottom line. The way I see this working is that you pass signal Y though buffer op amp, and you switch the buffer op amp On and Off based on the output of sensor T.

Last edited: Oct 11, 2016
atferrari likes this.