# Comparator Circuit

Discussion in 'Homework Help' started by ac_dc_1, Jan 28, 2013.

1. ### ac_dc_1 Thread Starter Member

Jan 27, 2013
74
0
Hi

I am trying to draw the circuit of a comparator that produces in the output 5.4V and -4V when the input voltage is inferior or superior to 1.5V.

Should i use and histerisis comparator like the one in the figure?

I know that for this montage

V0=VCC

V+=Vcc(R2/(R2+1))+Vi(R1/(R1+R2))

Vi=-Vcc(R2/R1)

Vo=-VCC

Vi=Vcc(R2/R1)

Then chosing that resistor i could get the reference point for comuting,the problem is that the output would be equal to Vcc or -Vcc and not to 5.4V or -4V,right?

Thanks

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2. ### WBahn Moderator

Mar 31, 2012
23,093
6,946
Your problem statement doesn't indicate any hysteresis at all, so why not just use a straight comparator circuit. When you say that the output should be +5.4V, that implies that +5.3V and +5.5V are not acceptable. Is that the case? What are the tolerances you can allow on the two output levels?

3. ### ac_dc_1 Thread Starter Member

Jan 27, 2013
74
0
The problem does not indicate what levels of tolerance should i consider.So you would say to use a comparator without hystersis with some reference voltages v1=5.4v and V2=-4V.For V2 i have to invert the polarity of the voltage source right?

Thanks

4. ### WBahn Moderator

Mar 31, 2012
23,093
6,946
It's hard to tell what they are expecting you to do because we don't have the necessary context in which the problem is given (such as what topics are being presented as part of the material that this problem is a part of). What you are needing to do is clamp the output at your two desired levels. There are a number of ways of doing this. Some better, some worse. Some simpler, some more complex. It all depends on what your constraints are.

5. ### ac_dc_1 Thread Starter Member

Jan 27, 2013
74
0
But the solution i proposed is a valid one,when looking at the question that is made which only states:

draw the circuit of a comparator that produces in the output 5.4V and -4V when the input voltage is inferior or superior to 1.5V.

6. ### WBahn Moderator

Mar 31, 2012
23,093
6,946
What solution that you propose? I can't tell from your description what circuit you have in mind. You talk about using two references and inverting one of them. How do you intend to do that. Show a circuit of what you are proposing or, short of that, describe it in clear and unambiguous terms.

7. ### ac_dc_1 Thread Starter Member

Jan 27, 2013
74
0
I was thinking is using a circuit of this type(see attach),to define the transition point of 1.5V.Then to produce the outputs maybe i could use a non-inverting amplifier,and a inverting amplifier.

In the non-inverting Av=1+(R2/R1) with R2=5k and R1=926 Ohm to produce the 5.4V output and then use a inverting amplifier Av=-R2/R1 with R2=10k Ohm and R1=2.5 k Ohm.

I am not sure how to link this all together

Thanks

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8. ### WBahn Moderator

Mar 31, 2012
23,093
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Av=1+(R2/R1) with R2=5k and R1=926 Ohm results in Av=6.4.

Okay, aside from wondering where you are going to get a 926Ω resistor from, that means that you need to feed an input voltage of 5.4V/Av = 844mV into that amp. In the inverting side, you have a gain of Av=-4, so you would need to feed an input voltage of 1000mV into it.

How are you going to get those two voltages?

Your first stage circuit, the basic comparator, is not going to produce well defined output levels, they will simply be "as high as the comparator can output" and "as low as the comparator can output". In general, these voltages will not be the supply voltages, but rather something on the order of one to two volts inside of the them.

Think of how you might use zener diodes to create well-defined voltage levels when the comparator outputs are anywhere near the rails. Then think of how to condition those well-defined levels to be the particular levels you want.

Last edited: Jan 29, 2013
9. ### Ron H AAC Fanatic!

Apr 14, 2005
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674
Is there any reason why you can't use vcc=5.4V and Vee=-4V?

10. ### ac_dc_1 Thread Starter Member

Jan 27, 2013
74
0

Throug tension divisers(using two resistors and voltage fixed voltage source
Even if the comparator outputed exactly the VCC and VEE i would still need the zeners,but i have to consider VCC and VEE minus 2 V in the calculations thats why you mention it.
Using ,the zeners and a transistor at the output of the comparator ,we could create a mesh to produce the voltage drops that we want .The transitor would be to switch between the meshes (but maybe we do not need that..

11. ### ac_dc_1 Thread Starter Member

Jan 27, 2013
74
0
I thought of it using a Hysteris comparator and some voltage divisors,but can we feed the integrated with such tensions?Are there any hysteris comparator that work with -4 V and 5.4V?

12. ### WBahn Moderator

Mar 31, 2012
23,093
6,946
Probably. I'd have to check data sheets. If you go that route, then unless you have good "rail-to-rail" comparators (or opamps, if you go that route) you would need to adjust the supply voltages to whatever they needed to be to get the voltages you wanted. And then tomorrow you might need to readjust them because the temperature in the room is a bit different.

13. ### ac_dc_1 Thread Starter Member

Jan 27, 2013
74
0
But if i used the zeners plus the comparator plus the amps inverter and non-inverter i would not have that problem?

14. ### WBahn Moderator

Mar 31, 2012
23,093
6,946
You'd still have that problem, but not as much and quite possibly the problem would be small enough that you could neglect it.

Another approach is to use what is known as an open-collector comparator, which are pretty common, but I don't know if this is fair game for this problem.