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I've been recently experimenting with converting transistor-level SPICE models of ICs into subcircuits. One thing I try to keep consistent with my subcircuits is that they remain floating; i.e. no internal connections to node 0 (aka GND). This represents the real world more accurately, and also allows for inserting noise sources or series resistances into the ground pin on the subcircuit. Any internal connections to node 0 would bypass these external effects. I've successfully made behavioral opamp subcircuits which easily satisfy this.

Then, I moved over to trying my hand at an LM741 since its internal transistor-level schematic is available in the datasheet (page 7). When I parsed the netlist into a text file to make the subcircuit, I was reminded that all BJT substrate nodes in SPICE are by default connected to node 0. If I was making a logic IC or regulator IC subcircuit, I would simply remap the node 0 connections to whatever the ground pin on the subcircuit is. However, this is an opamp and has no ground pin. Just VCC and VEE supplies.

So this begs the question...what are the BJT substrates in a bipolar-based opamp IC connected to? Are they connected to VEE? Are they left floating? Are they shorted to each individual emitter? I know next to nothing about IC design, and this situation piqued my curiosity.

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Substrate is normally connected to the most negative voltage in the circuit (Vee in the case of the 741) since it is p-type. Since the emitter and base are built on top of the collector diffusion in a vertical NPN, the collector-substrate junction characteristics are of interest.

Lateral PNP transistors as you'd find in the 741 are going to be different, I assume one would have a particular model for such transistors built in the process in question and not use the standard PNP model.

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    \$\begingroup\$ Even though LTspice forces a lateral PNP by default, I'm specifically calling out LPNP for compatibility with other SPICE programs. The LPNP models a base-substrate junction instead of a standard collector-substrate junction. If the p-type substrate for the IC is connected to VEE, then it only makes sense for the PNPs to be connected to the substrate via a base junction. If I'm understanding this correctly, this lines up exactly with what you laid out above. Thank you for the answer! \$\endgroup\$ – Ste Kulov Aug 6 '20 at 21:15
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    \$\begingroup\$ Yes, the base-substrate junction would be the important one for a monolithic lateral PNP. \$\endgroup\$ – Spehro Pefhany Aug 6 '20 at 21:42
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    \$\begingroup\$ It would be interesting if you could duplicate the typical behavior of a 741 from the schematic. \$\endgroup\$ – Spehro Pefhany Aug 6 '20 at 22:24
  • \$\begingroup\$ There is an example circuit in LTspice's "Educational" examples folder called LM741.asc which is the same thing I'm doing. They even call out LPNP explicitly. I don't know enough about IC design to determine if it exhibits "typical behavior", but it seems to work with basic opamp circuits. \$\endgroup\$ – Ste Kulov Aug 7 '20 at 23:16
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    \$\begingroup\$ You can make a new question- I don’t know the answer. \$\endgroup\$ – Spehro Pefhany Aug 8 '20 at 0:23

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