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I have a 500 Hz sine wave with amplitude 1V, and I need to amplify this signal 3 times, i.e. the required amplitude is 3V.

I tried to connect the circuit two times as shown below, using gain 3/1, which is 3 as required, and 3k/1k, which is also 3 as required. However, the output is not as required, the signal is just doubled in amplitude not tripled.

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The output is: 2

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    \$\begingroup\$ Not sure if it causes your problem, but you should definitely have ground connected in the middle of the op-amp power supply \$\endgroup\$
    – user253751
    May 27, 2022 at 15:44
  • \$\begingroup\$ 6V between ground and pin 7 and -6V between ground and pin 4. The simulator probably doesn't know whether it's 12V and 0V, or 0V and -12V, or 1000V and 1012V. A common simulation mistake. \$\endgroup\$
    – user253751
    May 27, 2022 at 15:52
  • \$\begingroup\$ look up "split-rail power supply" \$\endgroup\$
    – user253751
    May 27, 2022 at 15:54
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    \$\begingroup\$ It's not your problem here, but I have to point it out: the 741 is extremely obsolete and should never be used. There are much better op amps out there, even for the same price. \$\endgroup\$
    – Hearth
    May 27, 2022 at 16:11
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    \$\begingroup\$ I'm actually confused as to how you're getting a gain out of this at all, given that you have positive feedback. It should just be outputting a square wave--you might be driving the 741 into phase reversal or something, I haven't looked into what that would do here. \$\endgroup\$
    – Hearth
    May 27, 2022 at 16:19

3 Answers 3

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Your opamps are wired up as comparators with hysteresis (positive feedback), not as finite gain amplifiers (negative feedback). And your resistors are way too small for the op amp to drive properly. You should be getting square waves and not what you are currently seeing.

So your fundamental circuit isn't correct for your intention.

If you are trying to amplify without inversion then you need the non-inverting amplifier circuit. If you just take an inverting amp circuit and swap the opamp inputs it doesn't produce the non-inverting amp. It just produces a comparator with hysteresis.

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    \$\begingroup\$ Oh, yeah. Well, doesn't change what the problem is. \$\endgroup\$
    – DKNguyen
    May 27, 2022 at 15:49
  • \$\begingroup\$ Oh, yeah also right. I wasn't paying enough attention. But then you have a comparator. I don't think your circuit is doign what you think it is. The resistances are way too small. \$\endgroup\$
    – DKNguyen
    May 27, 2022 at 15:50
  • \$\begingroup\$ @M-125 Not the same thing because you are referencing your input to 0V but you have set the negative supply to 0V. So your signal is not referenced mid supply but it's trying to go negative and has nowhere left to go. For example, you can reference your input signal to 0V and give the opamp +12V and -12V supply so that an output below the reference can be produed if the output needs to go below it.. Or you can give it a +12V and 0V supply but then you must reference your signal at 6V so the output can below the reference. \$\endgroup\$
    – DKNguyen
    May 27, 2022 at 15:52
  • \$\begingroup\$ @M-125 changing inverting to non-inverting is not as simple as swapping + and - \$\endgroup\$
    – user253751
    May 27, 2022 at 15:52
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    \$\begingroup\$ This has gone on too far off track. If you have additional questions make a separate question, multiple if necessary for the different proper circuit configurations with the specific modification you have in mind. Because as is, the circuit in your question doesn't even match your intent and doesn't even work properly for what it is wired as. \$\endgroup\$
    – DKNguyen
    May 27, 2022 at 16:24
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Here is a correct way to connect the parts. I've used +/-15V supplies. It will work down to +/-6V but output swing will be limited.

schematic

simulate this circuit – Schematic created using CircuitLab

As you can see, the gain is -3.

enter image description here

To do a gain of +3 you will need a different configuration and a different resistor ratio.

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    \$\begingroup\$ @M-125 Notice that the feedback is connected to the inverting input, not the non-inverting input as you have it. If the feedback is on the non-inverting input, that's (to gloss over some unusual cases that aren't relevant here) positive feedback, which turns the amplifier into a comparator with hysteresis instead of an amplifier. Although I'm pretty sure the 741 isn't that good to use as a comparator. \$\endgroup\$
    – Hearth
    May 27, 2022 at 16:14
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    \$\begingroup\$ @M-125 But the way you've connected this is as a comparator, not as a non-inverting amplifier. \$\endgroup\$
    – Hearth
    May 27, 2022 at 16:20
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If the output of the opamp is supposed to have the same polarity as the input signal, the input signal needs to be connected to the positive input. The feedback has to be on the negative input either way. If you want an amplification by a factor of 3, the feedback needs to be divided by 3, so you'd use a voltage divider towards ground (2k2 and 1k1, for example).

schematic

simulate this circuit – Schematic created using CircuitLab

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