Discrete Opamp Open Design

[fas42]

Just to show how close one can go to matching the JFET curves, below is a grab from the BF862 datasheet, and LTspice's version thereof with modified parameters. There is still a problem unresolved, because the constant Vgs curves shown have been fudged, LTspice keeps insisting on equal spacing of curves between equal stepped values -- those shown represent other than Vgs of -0.1 through to -0.5. Which means that the curve of drain current as a function of gate source voltage doesn't match the datasheet.

This can be circumvented by adding a behavioural voltage at the gate, but maybe there's some fine tuning elsewhere that can be done ...

Frank

[scott wurcer]

This is looking better, but there also still seems to be a kink at the transition. The problem for me was simulating the null of seconds for a simple phase splitter. It always walks down to the kink, where a real device has a sweet spot with a very deep null.

The level 3 model has an ALPHASP parameter that fits the entire curve smoothly with a tanh function which has a continuous derivative.

What did you end up using for lambda and B, and did B start doing something?

You might ask why something as fundamental as lambda is so far off in the extracted model.

[RNMarsh]

IMO curve fitting as used here is often a sub for an incomplete model. but often necessary and useful.
what model is being used and is it complete?

[grhughes]

I would like to see the schematic of a simple circuit to match jfets BF862 at the voltage they are gona work most linearly in the circuit. I have one for BJTs. Thanks, Ray

[grhughes]

Both p channel and n channel? Thanks, Ray

[qusp]

they dont come in a Pchannel

[grhughes]

But obviously if I'm gona built a circuit to match jfets then it would be nice to match p channel as well. All I'll probably have to do is reverse voltage polarity??? Thanks, Ray

[grhughes]

It would be nice to measure pinch off voltage on a p channel jfet. I've just never done it. Thanks, Ray

[rsavas]

I did a google search on "Testing jfets", lots of information!!

[grhughes]

Been there! Done that! I'm interested in people who have DONE IT and interested in avoiding the pittfalls and interested in the circuit parameters of the SWOPA that I should be looking for; what optimum voltage, what optimum Idss of the BF862. They are probably all over the map out of a sample of 100. Thanks, Ray