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Hi,

I'm sure this has been asked before but in searching I couldn't find anything that addressed my issue, so apologies if this is a duplicate.

As an exercise I'm modeling the 741 op-amp internal structure (from this ) but I can't get the lateral PNP to behave as expected (Q13A/B). I get the expected 730uA through the reference current mirror, and the expected ~20uA through the Q20 mirror. But through Q13B I should be sourcing 0.75*Iref and I'm only getting ~12uA; similarly through Q13A i should be sourcing 0.25*Iref and I'm only getting 75pA.

Model statements used for all parts are in the schematic. The only difference between Q13 and the standard PNP used is the saturation current Is changed from 10fA (standard) to 7.5fA (Q13B) and 2.5fA (Q13A). Apparently I'm missing something else about how this stage is supposed to function, seemingly with Q13. Any ideas to get this working?

Edit: Schematic is uploaded in files/temp, "741.asc"

Manauo,

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Have you looked at this schematic, that comes with LTspice?

I don't know how it compares to your efforts, but it is another one to consider.? I'm guessing that one just copied the "equivalent" schematic from the old 741 datasheet, without much more effort.

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I do not recall seeing anyone attempt to accurately model the transistors in a 741 before.? I think your question is unique.? Be aware that the 741 is ancient history.? Personally, I might question the validity of anything that talks about its design, even though it may be OK.? ?How well do you trust its source?

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Did you know that LTspice has both a different PNP symbol and a different PNP model for lateral PNP transistors?? The symbol is "LPNP" and has the base-to-substrate diode and the connection to the P-type substrate (negative supply).? Don't be fooled by the PNP4 symbol which is not for a lateral PNP - but anyway that is just a symbol, not the model.

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The lateral PNP's model name is also "LPNP" so I think you would write something like this:

to get that lateral PNP model that comes with SPICE.? I do not often see it used.? (FYI, there is even a lateral NPN model, probably never used, but it is there in case someone were to make ICs from an N substrate.)? If I remember correctly, these LPNP and LNPN models are not unique to LTspice and they come with other SPICE programs too, perhaps even with Berkeley SPICE, but documentation about them is scarce.? I don't know what sets them apart from normal PNP and NPN SPICE models.? I would expect that they work best when they have the fourth pin (to substrate) in each of those LPNP transistors in your circuit.

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I do not know if changing those models from PNP to LPNP makes much difference.? I did some experiments many years ago but I didn't finish them.

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Do you suppose that a forward Beta of 50 is reasonable for a lateral PNP?? I would have thought that it is much lower.? But don't take my word on that.? Back in the early days of analog ICs, a Beta of 50 would have been good for a normal NPN and impossible for a lateral PNP.? Again, don't take my word on that.

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Andy

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Hi Andy,

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Thanks for the suggestion - I'll take a look at that example and the built-in LPNP model.

Your point about the antiquity of the device is well-taken. I'm working on learning some more in-depth analog design and the 741 seemed like a good foundational block given how widely documented it is, and then able to use that to move into more advanced/modern circuits.

off topic

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subst741.zip @ Fil/g/LTspice/files/Temp?p=created,,,20,2,0,0es

is my best guess of the device . . . so far