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I noticed that on the LM108 schematic shown below (also available from https://www.analog.com/media/en/technical-documentation/data-sheets/lt0108.pdf, page 8):

enter image description here

NPN transistors are drawn in 2 different ways -- some with two lines and some with one line as per:

enter image description here

Does that indicate some difference between the transistors, or is it just inconsistent drawing? If the transistors are different in some ways, then what are the differences?

Originally, I was thinking that one of the symbols might be for the LM108's input superbeta transistors, but I couldn't reconcile that notion with the rest of the circuit. For one, using superbeta transistors anywhere else don't make much sense given their fragility; for two, the LM108's output is driven by Q18 and Q19, and I find it simply inconceivable to use a fragile superbeta in any of those positions.

Some more, 'insider' info on the LM108: https://www.righto.com/2016/12/inside-lm108-op-amp-superbeta.html

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  • \$\begingroup\$ Bob Widlar must have had a lot to do with that design \$\endgroup\$ Commented Dec 27, 2024 at 14:57
  • \$\begingroup\$ @D.A.S. indeed that linked document names him. \$\endgroup\$ Commented Dec 28, 2024 at 13:05
  • \$\begingroup\$ TLDR yea Bob... @JasenСлаваУкраїні \$\endgroup\$ Commented Dec 28, 2024 at 20:21

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Analog Devices's copy of the LM108 retains the same schematic as National Semiconductor's original.

Texas Instruments (which now owns National Semiconductor) has the following to say:

Figure 4 shows a simplified schematic of the LM108. Two kinds of NPN transistors are used on the IC chip: super gain (primary) transistors which have a current gain of 5000 with a breakdown voltage of 4V and conventional (secondary) transistors which have a current gain of 200 with an 80V breakdown. These are differentiated on the schematic by drawing the secondaries with a wider base.

https://www.ti.com/lit/an/snoa624b/snoa624b.pdf

The superbeta transistors are protected by the circuitry that surrounds them.

All the PNP transistors are "Laterals" and they have a breakdown voltage of 80 volts.

The second stage is a differential amplifier using high gain lateral PNPs [...] These devices have current gains of 150 and a breakdown voltage of 80V

So it's only the thin-base NPNs that are the fragile super-beta parts.

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