0
\$\begingroup\$

I'm trying to implement a linear inverting circuit using NI Multisim. I'm using a 1 Hz sine wave with a 1 V amplitude as input.

enter image description here

According to my circuit, the gain should be:

$$A_v = \frac{-R_2}{R_1}=-10$$

My input is:

enter image description here

My output is a bit concerning though. The positive half wave amplifies properly, but the negative half wave doesn't. The inverting property is working, but the gain in the negative half wave isn't proper. Here is my output (red is the output and cyan is the input signal):

enter image description here

I have taken the scale for the input signal to be 1 V/Div and for the output signal to be 5 V/Div, so it should take up 2 divisions to be 10 V. The positive half cycle is fine, but the negative half cycle isn't properly showing the amplification. Why is this? Even if I switch R2 to 5 kΩ, I still get a similar kind of behaviour of the negative half cycle. Here it is:

enter image description here

Why is only the negative cycle affected?

\$\endgroup\$
5
  • 3
    \$\begingroup\$ I have not used Multisim recently so not sure what it expects: is the op amp supposed to fall back to some kind of implicit power supply when the supply pins aren't connected? I don't see you actually connecting the op amp's supply pins to any sort of power supply, so I'd expect it to be malfunctioning. \$\endgroup\$
    – nanofarad
    Nov 1, 2022 at 17:30
  • \$\begingroup\$ @nanofarad Our lab assistant told that Multisim Opamp automatically takes power? He said that connecting a power supply isnt always necessary? \$\endgroup\$
    – Techie5879
    Nov 1, 2022 at 17:31
  • \$\begingroup\$ @nanofarad But if the supply was the problem, why is it giving such behavior only in negative half cycle? Also, if the Gain were -0.1 instead of -10, it seems to be working \$\endgroup\$
    – Techie5879
    Nov 1, 2022 at 17:32
  • 1
    \$\begingroup\$ "Multisim Opamp automatically takes power" - may apply to a different device model, such as a generic op amp macro. e.g. cadence has an ideal op amp for quick simulations that doesn't require power, but if I wanted my PDK's op amp or another "real" op amp I'd have to be sure to power it. "why is it giving such behavior only in negative half cycle?" - Like many circuits, the LM741 has an internally asymmetric construction. If it is indeed missing its power supply (meaning its internal transistors are not properly biased), it may be doing wacky things internally only during one half cycle. \$\endgroup\$
    – nanofarad
    Nov 1, 2022 at 17:38
  • 1
    \$\begingroup\$ Since you didn't connect the power rails it appears the model is assuming a positive rail but ground for the negative rail. Perhaps it felt it would be presumptuous to assume a bipolar supply. \$\endgroup\$
    – td127
    Nov 1, 2022 at 18:09

1 Answer 1

3
\$\begingroup\$

It seems that connecting the power supply explicitly solved the problem. Otherwise the op amp was malfunctioning somehow.

enter image description here

Now it does show a proper amplification of the input signal. enter image description here

\$\endgroup\$
3
  • \$\begingroup\$ Great! By the way, you sent me on a short trip to see if I could even find a 7-pin Spice model for the 741. No luck, so far. I even went and dug into my old ORCAD files (circa 1990) and all they have are 5-pin macro-models for it. Glad to see that it was "the obvious" and easily fixed and not something more complicated. (First thing most of us do is perform little 'sanity checks' like this when we encounter a problem. It can be a misplaced wire that looks right but doesn't actually go anywhere, for example. If something odd happens, just do a once-over with your eyes to double-check.) +1. \$\endgroup\$
    – jonk
    Nov 1, 2022 at 19:23
  • 1
    \$\begingroup\$ Oh, and be sure to select your own answer. That will close this out so others don't waste further time on it. \$\endgroup\$
    – jonk
    Nov 1, 2022 at 19:25
  • 1
    \$\begingroup\$ @jonk Sure I will! But I can only do it in 2 days' time \$\endgroup\$
    – Techie5879
    Nov 1, 2022 at 19:35

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.