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I have a school excercise which I don't fully understand. The excercise is that you build an opamp circuit with a negative gain, -40 and a bandwidth of 100KHz. The gain-bandwidth product is 1000 000.

If I calculate the GBP -40 * 100Khz = -4000 000, so I have to split my opamp up into two opamps. One opamp with a gain of 10 and one of -4 to have a gain of -40. If I enter the circuit in Multisim I don't get an attenuated signal, the signal is just amplified.

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Am I doing something wrong here or is it correct? I just don't really understand it. The gains for each amplifier should be correct.

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    \$\begingroup\$ Your second opamp is non-inverting then its gain is 1 + (4k/1k)= 5 times, not 4 times. \$\endgroup\$
    – Audioguru
    Apr 27 at 19:24
  • \$\begingroup\$ Yeah I was already thinking that there was something wrong with my amplification because when I "feed" in an ac signal of 0.25v I got 12+V back ... Thanks for clearing it out! \$\endgroup\$
    – John
    Apr 27 at 19:28

1 Answer 1

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The term "negative gain" can be ambiguous. It could mean "attenuation", and it could also mean amplification but with reversal of polarity.

If a gain is given the units of decibels, then, almost certainly, a negative gain, in this case means attenuation.

If the gain is given as a simple muliplicative factor, then a negative gain is most probably an amplification with a reversal of polarity.

When you describe the gain as a combination of a gain of 10 (note: no decibels mentioned!) combined with a gain of -4 (again no mention of decibels), then what is most certainly meant is an amplification of the signal with a reversal of polarity. That is why your circuit gives an amplified and not an attenuated signal as output.

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  • \$\begingroup\$ Yes they didn't mention anaything about dB. So if I understand correctly in this case it is just an amplification of the signal BUT the polarities will change. So basically an amplification and an invertion? \$\endgroup\$
    – John
    Apr 27 at 19:23
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    \$\begingroup\$ @John ----- yes. \$\endgroup\$ Apr 27 at 19:26

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