Forums » CircuitLab Feature Requests

It seems that the library FETs are limited to discrete transistors, whose substrate, even when it is brought out on a separate pin, is by custom tied to the source.

It will be very useful if the library offers FETs with a separate substrate terminal, and that the device equations include the modulation effect of the substrate. This will enable CircuitLab to simulate integrated CMOS (analog) circuits, where series stacked devices operating at different source voltages are built on a common substrate.

You could in theory just adjust the thermal voltage to compensate for the back body effect. However a 4 terminal device would be much nicer

@wsender,

Which MOSFET model parameter is the thermal voltage?

Whch levels of MOSFET model supports it?

Do the simplified CL models support it?

Also bear in mind that CL does not currently support direct temperature variation in the way the TEMP parameter does in spice:

https://www.circuitlab.com/docs/circuit-elements/#basics

CL also does not yet support parameters in the way spice does.

In CL you have to use a voltage or current source to represent the variable you wish to parameterise. For instance:

@signality,

Sorry I meant Threshold Voltage. It's a pretty straightforward adjustment that I'm sure most are familiar with.

Like I said though, a 4 pin package would me much easier, but users can compensate for it now.

@wsender,

Ah, think I understand now.

Could you post an example to illustrate your suggestion?

:)

I would like to see a 4 pin MOSFET where substrate is not custom tied to source terminal. This is needed to simulate IC design where body effect parameter comes into play.

I'm trying to use a N-ch MOSFET as a pass gate for a sample and hold circuit. Would like to have a MOSFET model that doesn't have the body diode from source to drain.

Have you thought about using an N channel jfet instead?

I used that approach very successfully, many years ago for a sample and hold for an analogue music synthesiser. Although the on resistance is higher than can be achieved with MOSFETs, the gate to source and drain capacitances ca be nuch smaller so reducing feedthrough.

Another approach is to do this:

The gate drive voltage swing must be greater than that of the input.

:)