# Need Help - Comparator and Summing Amplifier Lab

#### gbtbtbt

Joined Mar 18, 2020
72
I need help understanding the difference in waveforms when connecting resistor 1 in parallel with the negative voltage in a 741 op-amp comparator circuit. In addition what effect does changing values on either resistor 1 or resistor fraction (Rf) on a two-input summing amplifier have? If more information is needed please let me know.

Edit
Would both figure 41.1 a and b have the same output sine waves and if so why?

Figure 41.3 has a voltage peak to peak of 8 volts when all resistors are 10k. R1 as 22k, voltage peak to peak is 5.80 V and 17.6 when Rf is 22k ohms, while R1 is 10k ohms.

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#### dl324

Joined Mar 30, 2015
17,153
Edit
Would both figure 41.1 a and b have the same output sine waves and if so why?
We don't answer school problems. You need to show your best efforts and we'll try to guide you in the areas you don't understand.

Circuits in question:

#### gbtbtbt

Joined Mar 18, 2020
72
Both measurements for output voltage peak to peak came out as 22 volts. The pictures are snipped and voltage divisions are different, but the output sine wave is a square wave for both.

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#### dl324

Joined Mar 30, 2015
17,153
but the output sine wave is a square wave for both
Do you think the output for 41.1a is wrong?

It would be helpful if you displayed the sine wave at 1V/division.

Do the same for the 41.1b image and place 0 for both waveforms in the center of the screen.

#### gbtbtbt

Joined Mar 18, 2020
72
I made channel B 5v/div as it would be off-screen if lowered.

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#### dl324

Joined Mar 30, 2015
17,153
I made channel B 5v/div as it would be off-screen if lowered.
Why are your schematics so messy compared to the ones you were give?

Focusing on part A, what is the voltage at the non-inverting terminal? Do you think that's correct?

EDIT: change the coupling to DC on all signals.

#### gbtbtbt

Joined Mar 18, 2020
72
The voltage at the non-inverting terminal is 9.20mV which makes sense as it's going to ground so it should be close to zero. However, do I want it to be zero volts is another thing. I coupled the oscilloscope to read DC, but values still remain constant.

Should I be expecting these results or is there something wrong with the output voltages for both 41.1 a and b?

I know this much, for a non-inverting amplifier both input and output are going to be in the same phase.

#### dl324

Joined Mar 30, 2015
17,153
Should I be expecting these results or is there something wrong with the output voltages for both 41.1 a and b?
You have mistakes in the connections to the the inverting inputs on both opamps. If you drew your circuits more neatly, it would be more apparent.
I know this much, for a non-inverting amplifier both input and output are going to be in the same phase
Technically, the opamps are being used as comparators because there's no feedback so they're operating open loop.

Last edited:

#### gbtbtbt

Joined Mar 18, 2020
72
This is a cleaner version. Still getting the same for figure 41.1a. Hope this visual can give better insight.

+v and -v are 12 volt

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#### BobTPH

Joined Jun 5, 2013
9,296
Look at your scematics again. They are not the same circuit you are supposed to be analyzing.

Correct the mistakes, then see if the output makes more sense.

Bob

#### dl324

Joined Mar 30, 2015
17,153
This is a cleaner version. Still getting the same for figure 41.1a. Hope this visual can give better insight.
You changed the opamp, the amplitude of the input voltage, and you still have the wrong voltage on the non-inverting input. What is this supposed to accomplish?

The first thing you need to do is be able to enter the circuit correctly in the simulator. I can't understand how you can make so many mistakes in such a simple circuit.

#### gbtbtbt

Joined Mar 18, 2020
72
I encourage anyone to redraw the circuit as I don't know how to prevent the positive input wire from touching a wire connected to the negative input if that is the problem. I don't know how to exactly replicate the circuit on Multisim.

#### dl324

Joined Mar 30, 2015
17,153
I encourage anyone to redraw the circuit [snip] Multisim.
Multisim isn't a free program and MultisimBlue (Mouser's free version), which I've used, is no longer available.

Don't accept Multisim's circuitous routing. You can use the shift key when routing wires to make them straighter. When the program insists on giving you a bad route, you can move the wires to clean them up.

In some of your previous schematics, you've crossed wires without connecting them. So you do know how to cross wires without connecting them.