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I saw this tutorial on a website.

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From that, I designed these circuits:

enter image description here

In my circuits the voltage source is 12 volts.

The tutorial says the output gain should be 2. In my second circuit the gain is 2 but in the first one that is designed with transistors it does not give me 24 volts like the second one.

Why is the output voltage in the first circuit not the same as from the second circuit?

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3 Answers 3

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When making expectations of what an op-amp can or cannot do, one of the first things you learn is that the output voltage of the op-amp cannot be beyond the power rails applied to the circuit or device.

So you have an op-amp (made from transistors) that has a gain of 2 and then you apply an input of 12 volts (RV1 at maximum) and your expectation is that the op-amp output should be 24 volts. Well, this cannot happen.

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  • \$\begingroup\$ Are using feedback resistors in reality useless? \$\endgroup\$
    – kand
    Apr 28, 2022 at 13:38
  • \$\begingroup\$ Feedback resistors are very important in many, many circuits. In your situation they are still useful but, it's other limitations that are causing you problems. In other words; you can't use your circuit to magnify the voltage of your power source. \$\endgroup\$
    – Andy aka
    Apr 28, 2022 at 13:40
  • \$\begingroup\$ @kand In other words, what Andy is telling you is that an opamp behaves (approximately) like the ideal opamp of basic tutorials only if they remain in their linear region, i.e. if the output doesn't try to go beyond the rails. In this latter case you end up saturating the opamp and feedback is no longer able to keep the circuit linear, so the very notion of gain is meaningless in this condition. \$\endgroup\$ Apr 28, 2022 at 21:40
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in my circuits the voltage source is 12 volts. tutorial says the output gain should be 2.and it is happening in my second circuit but in first one that is designed with transistors it does not give me 24 volts like the second one.

The output voltage of an op-amp will never exceed the supply voltage. So, if your supply voltage is 12 volts, the output cannot be 24 volts.

However, many simulators will ignore this fact with op-amps show a completely unrealistic output that is not constrained by supply voltages. That is a short-coming of the simulation, and what you see in real life will not match.

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Note that the Vcc and Vee pins of the U1:A (pins 8 and 4) are unconnected.

Whatever Proteus does when these power pins are disconnected, is resulting in an output of 24V, which of course is impossible for most op-amps (most won't output >16V because they can't be supplied more than +/-16V or 32V total.)

Try supplying it with +12V on top and 0V on bottom to be more alike your discrete op-amp.

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  • \$\begingroup\$ I can't imagine it would be a problem for the average op amp to have 0/+30 applied to it, if it's rated for -15/+15 (which quite a lot of them are), so it shouldn't be hard to get an op amp to output 24 V. Can't say I've tried, though. \$\endgroup\$
    – Hearth
    Apr 28, 2022 at 13:44
  • \$\begingroup\$ Could do that, but it is unusual. Typically op-amps are (single-ended) powered from 5/0V, 10/0V, 12/0V, 15/0V, or dual-ended +/-5V, +/-10V, +/-12V, or +/-15V. Can't say I've seen one powered from 24V/0V in the field. Yet another reason to be careful with simulators. \$\endgroup\$
    – rdtsc
    Apr 28, 2022 at 14:11

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