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While trying to understand the internals of op amps, I tried to implement a 741 circuit from Wikipedia in LTspice like this: (hooked up as 4x non-inverting amplifier, 0 and 24V supplies, some light output loading)

741 circuit in LTspice

DC sweep from 0 to 12 looks like this:

DC sweep

Well, the gain of 4 is visible at least. But while I certainly did not expect rail-to-rail performance from this circuit, the output bottoms out at 7.12V, and that just seems unreasonable to me. Just to be sure, I tried increasing R14 and R15, but that's not it. Since the output is apparently not really being pulled down, I tried removing Q20, and to my surprise that made no difference. I'd post the DC sweep again, but it's indistinguishable from the one above.

But surely Q20 is there for a reason. Perhaps I wired this up wrong or made some other mistake?

Just to make sure, I independently rebuilt the 741 circuit in a different simulator, not that I'm blaming LTspice of course, but to minimize the chance of me making making the same mistake twice, perhaps due to not understanding the tools. Apologies for the layout and numbering differences.

741 in EasyEDA

The exact numbers come out a little bit different (well I didn't put the resistors between the output and the rails this time), but qualitatively the result is similar (orange/brown line is output)

enter image description here

So, what's going on here? Did I implement the circuit incorrectly both times? Is this actually the right result after all, and the circuit is just that bad? Did I get bitten by my naive assumption that I can just arbitrarily put 2N2222 and 2N2907 as a kind of "default" BJTs?

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  • \$\begingroup\$ my guess is that in the actual 741, there are some transistors with larger or smaller cross sections than others. In particular, I would guess that current sources from current mirrors do not all have identical currents. But this is just a guess. \$\endgroup\$ Mar 22, 2022 at 3:39
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    \$\begingroup\$ Here's the 741. Note the circled area? That's not the only difference, either. Carefully look over the real circuit vs your circuit. If interested in more detail, look here. Be very, very careful about what you imagine as the real IC circuit vs what you are simulating! \$\endgroup\$
    – jonk
    Mar 22, 2022 at 3:43
  • \$\begingroup\$ @jonk apparently the schematic I followed was based on the one shown in the LM741 manual which is apparently a bit different, but it's still supposed to be an op amp circuit \$\endgroup\$
    – user136131
    Mar 22, 2022 at 3:49
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    \$\begingroup\$ @harold That's because the datasheets provide behavioral diagrams. They get the point across. But they do NOT disclose all of the details. \$\endgroup\$
    – jonk
    Mar 22, 2022 at 3:50
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    \$\begingroup\$ @harold There already is a transistor level example in the default installation: open My Documents/LTspiceXVII/examples/Educational/LM741.asc and see for yourself. \$\endgroup\$ Mar 22, 2022 at 8:39

3 Answers 3

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For your information,

From microcap12 files, 741 modeling ...
DC Analysis, dual supply +15 V/-15 V, open loop.

Added models used.

.MODEL DD D (CJO=2P RS=1.5)
.MODEL QPL PNP (BF=10 CJC=4P CJE=6P RB=20 VAF=50 TF=1N TR=20N )
.MODEL QNL NPN (BF=80 CJC=2P CJE=3P RB=100 VAF=50 TF=300P TR=6N CJS=2P)

enter image description here

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    \$\begingroup\$ +1 Those lateral PNP transistors are real dogs. \$\endgroup\$ Mar 22, 2022 at 14:02
  • \$\begingroup\$ Yes. There are :-). Should I add also AC analysis for reference? \$\endgroup\$
    – Antonio51
    Mar 22, 2022 at 14:10
  • \$\begingroup\$ could you improve your answer and explain how the terrible PNPs help to extend output swing? \$\endgroup\$
    – tobalt
    Mar 22, 2022 at 15:31
  • \$\begingroup\$ Will try first change BF ... But don't know how the "results" would be (for now). \$\endgroup\$
    – Antonio51
    Mar 22, 2022 at 15:46
  • \$\begingroup\$ No change ... Just added a little offset voltage (about -200uV) \$\endgroup\$
    – Antonio51
    Mar 22, 2022 at 15:55
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First of all, the 741 needs dual supply. With +/- 15 V power, the output range is +/- 14 V according to its datasheet, or -13.3 V to 14.3 V according to simulation.

Also, in your first LTspice schematic, R3 should be 4.5k, not 4.5R.

There are two common variants of the 741 schematic:

  • The 20-transistor variant (that you simulated) is less common but can be built with discrete transistors, like XL741/741SE kits.
  • The 24-transistor variant has slightly improved performance and is analyzed in details in textbooks.
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  1. As Shuo Chen pointed out in his answer, R3 in your first schematic should be 4.5 k\$\Omega\$, not 4.5 \$\Omega\$.

  2. A simple non-inverting amplifier has two resistors in the feedback network. These are R12 and R13 in your first schematic. R14 and R15 are "odd" and probably don't belong.

  3. Because the 741 is not rail to rail, you should not connect the negative power rail to the inverting input. The input terminals should be at least 2 volts or more above the negative supply rail.

When these three items are corrected, the circuit seems to simulate correctly in CircuitLab

schematic

simulate this circuit – Schematic created using CircuitLab

enter image description here

If, however, the amplifier's negative power rail is ground, and the inverting input is tied to ground, the circuit has the following behavior

schematic

simulate this circuit

enter image description here

Why is the minimum output over 7 V above the negative power rail?

To find out why, we will first observe that the floor voltage depends upon the gain set by R12 and R13, and that the voltage at the inverting input has a floor.

schematic

simulate this circuit

enter image description here

One can't tell from this static image of the DC sweep, but the inverting input voltage is the same for all values of R13. The floor voltage of the inverting input is 1.790 V. We will switch our configuration to a voltage follower to simplify our problem. The floor of the inverting input changes slightly to about 1.813 V.

schematic

simulate this circuit

enter image description here

enter image description here

Below a non-inverting input voltage of about 1.5 V, the non-inverting input bias current is essentially 0. At about 1.5 V, the input bias current begins to rise to a plateau value of about 22 nA. The rising input bias current indicates that the transistors on the non-inverting side of the input stage of the op-amp are starting to conduct. However, the output of the op-amp does not begin to rise until the input bias current reaches 12-15 nA. Before that point, the op-amp does not track the non-inverting input voltage, but behaves as if it were tracking a voltage of about 1.820 V.

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