Confusion about open-loop op-amp comparator operation

In summary, the conversation discusses determining the output level for each comparator in Figure 4-52. The equation A_{ol}=\frac{V_{out}}{V_{in}} is mentioned and the participant is unsure about the correct approach. They are told to compare the voltages in the noninverting input to the voltages in the inverting input and decide the output based on that. The participant also mentions the possibility of the output being pinned at the supply rails and asks for clarification on the internal design of op-amps. The expert suggests looking at comparator datasheets for more information. Overall, the conversation concludes with the participant confirming their answers and expressing surprise at the simplicity of the internal design of op-amps.
  • #1

JJBladester

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2

Homework Statement



Determine the output level (maximum positive or maximum negative) for each comparator in Figure 4-52.

figure-4-52.jpg


Homework Equations



[tex]A_{ol}=\frac{V_{out}}{V_{in}}[/tex]

The Attempt at a Solution



I am new to comparators but I thought that since all of these op-amps are open-loop configurations, the output voltages would be pinged at ±Vmax where ±Vmax is the maximum output voltage of the op-amp according to its datasheet.

So, for (a), my answer was:

[tex]V_{out(max)}=(A_{v(ol)})(V_{in(max)})=(A_{v(ol)})(-1V)[/tex]

However, I was told "you don't need the open-loop gain of the amplifier. You need to compare the voltages in the noninverting input to the voltages in the inverting input, then decide the output of the comparator."

Still... I know that even a small difference between the two inputs of an op-amp without feedback can drive the device into saturation and thus the output voltage to ±Vmax.

How do I rectify what I believe to be true about comparators with what I was told by my professor?
 
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  • #2
Hey JJ!

Yeah, if the inputs are different by hardly anything, the output will be pinned at (or near) the supply rails. To know how close, you would need specs on the comparators themselves. In fact many comparators have an open collector output with respect to ground, which doesn't seem to be reflected in the diagram that you are given.

So my guess is that you are just supposed to say which rail the output is pinned at for each of the 3 configurations...?
 
  • #3
berkeman said:
...many comparators have an open collector output with respect to ground, which doesn't seem to be reflected in the diagram that you are given.

So my guess is that you are just supposed to say which rail the output is pinned at for each of the 3 configurations...?

Hi berkeman. Thanks again for your input. I'm nearing the end of my B.S. and I sort of took this op-amps course out of order. Going from control systems to this is a backwards transition to say the least.

I think what the professor is looking for in terms of the "correct" answer is (a) -Vout(max), (b) +Vout(max) and (c) -Vout(max).

Too bad I can't see inside the black box that is the op-amp. Is internal op-amp design a Master's level course or can I find it somewhere at the B.S. level? The other thing is that we linearize everything. Heaven forbid we would have messy integrals to deal with!
 
  • #4
I think your answers are correct.

On your question -- the best first step is to spend some time with comparator datasheets. They generally will have an equivalent circuit in them, and will address things like input offset voltages and currents (just like opamps), and maximum output voltage swing for various output load currents.
 
  • #5
I just found http://www.ti.com/lit/ds/symlink/lm741.pdf and was surprised that the 741 only has 20 transistors and an internal RC network inside it. I was led to believe there was magic inside. :)

Thanks for the advice berkeman.
 

1. What is an open-loop op-amp comparator?

An open-loop op-amp comparator is a type of operational amplifier (op-amp) circuit that compares two input voltages and produces an output voltage based on which input is greater. It operates in an open-loop configuration, meaning that there is no feedback from the output to the input, and the output voltage can swing to its maximum or minimum values.

2. How does an open-loop op-amp comparator work?

The op-amp comparator has two inputs, a non-inverting input (+) and an inverting input (-). When the voltage at the non-inverting input is higher than the voltage at the inverting input, the output voltage will be at its maximum value. When the voltage at the inverting input is higher, the output voltage will be at its minimum value. This allows the comparator to compare the two input voltages and produce a digital output signal.

3. What are the applications of an open-loop op-amp comparator?

The open-loop op-amp comparator is commonly used in electronic circuits for tasks such as voltage level detection, square wave generation, and digital signal processing. It is also used in systems where precise and fast switching between two voltage levels is required, such as in motor control and power electronics.

4. What are some common sources of confusion about open-loop op-amp comparator operation?

Some common sources of confusion about open-loop op-amp comparator operation include understanding the difference between open-loop and closed-loop configurations, the effects of positive and negative feedback, and the limitations of the comparator's output voltage swing. It is important to have a clear understanding of these concepts in order to properly design and use an op-amp comparator circuit.

5. How can I troubleshoot issues with an open-loop op-amp comparator circuit?

If you are experiencing issues with your open-loop op-amp comparator circuit, there are a few steps you can take to troubleshoot the problem. First, make sure that all connections are secure and there are no loose or damaged components. Next, check the power supply and ensure that it is providing the correct voltage. You may also want to check the input voltages and make sure they are within the comparator's input range. If the problem persists, it may be helpful to consult a circuit diagram or seek assistance from a knowledgeable individual.

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