Equivalence of FET based and IC circuits ?

[Vivek]

Is it possible to build an IC and diode based distortion circuit that is an exact equivalent of another FET design ?

and vice versa ?


Or is there something specific about FETs and IC stages that makes it impossible to built circuits that have exact same clipping, amplitude, frequency and phase response at each stage using the other class of active devices ?

[iainpunk]

you can make FET based op amps quite easily just like a lot of BOSS pedals, making them equivalent, but things like frequency response and clipping knee will be off


but its a question with multiple answers as long as you don't specify more exactly what's allowed.
its entirely possible to get the same response from a IC/opamp as a Mos/J FET, but its hard and clumsy to make such a large circuit. its easier to get a FET to sound like an Opamp than to get an Opamp to sound like a Fet,

cheers, Iain

[ElectricDruid]

In theory, you can build the "exact same circuit" lots of different ways. That is to say, we can tweak the parameters and design of our active circuit to achieve some specific end, and we can be as fussy as we like.

In practice, it'd rapidly get so complicated as to not be worth it. As Iain said, hard and clumsy. That's why the Boss "discrete op-amp" distortions use a "op-amp" design that is only 4 or 5 transistors. Even a 741 has a lot more than that!

[11-90-an]

If any of you have a simple-ish idea for a really convincing JFET-circuit-sound emulation with op-amps... I'm all ears...

[teemuk]

The first problem is defining what is a "FET design".

A FET opamp? Source follower? Common source? What's the bias point? Is there negative feedback? Source bypass? Etc.

[garcho]

^ Exactly, and if anyone can *actually* hear the difference they speak of in a blind a/b test, then please post audio evidence (my ears aren't impressed with charts or sims).

[Vivek]

The first problem is defining what is a "FET design".

A FET opamp? Source follower? Common source? What's the bias point? Is there negative feedback? Source bypass? Etc.

I was interested to know

All these FET emulations of Guitar Amps

Like Baja and ROG

Could they have also been designed with Opamps instead ?

[garcho]

^ there is no magic mojo in a FET device. They can be a cheap and cheerful solution to guitar pedal design objectives, so they're popular. But the whole tube-in-a-transistor thing is folly.

[iainpunk]

If any of you have a simple-ish idea for a really convincing JFET-circuit-sound emulation with op-amps... I'm all ears...

well, there is a non-simple-ish is way to emulate triodes, that's kinda close to FET...
a few steps:
1) decrease signal level
2) opamp and diode log amp
3) bias the signal just right for non linearity
4) add clean gain
5) add one sided clipping (which side you clip depends on your specific log amp)

this is a gross simplification of the concept, the original one i designed uses 5 opamps, and curves just like a E80CC run at 9V. the frequency response isn't taken in to account, but that could be thrown in with the first decresing of the signal.
a single Jfet with source diode bias curves kind of similar.

cheers, Iain

[idy]

People do design "amp simulator" circuits with either/or; FET's the hobby horse of many, most followed being ROG. Opamps will also produce "reasonable facsimiles" of frequency response, clipping. Baja has used both, sometimes in same circuit. Maybe you should search and compare AC30 "sims" and see that some use FET, some OPAMPs. Same with Marshall etc.

[FiveseveN]

All these FET emulations of Guitar Amps
Like Baja and ROG
Could they have also been designed with Opamps instead ?

I feel like the popularity of discrete FET amps-in-a-box had peaked a while back.

Have you seen these?


Looking at the Tech 21 Character series, Friedman BE-OD, Wampler Triple Wreck, Mesa Throttle Box, REVV stuff and so on, I'd say op amps are currently the norm.
I for one wish there was more love for inverters. Fertile ground for a new-ish fad, I suppose.

[amptramp]

Whatever solution you come up with, an op amp has a specification for PSRR (power supply rejection ratio).  Moat FET's have a high output impedance which means the ripple on the drain resistor output is almost as high as the power supply ripple whereas it would be typically 60 db down for an op amp circuit.  You can use differential amplifiers biased by current sources but if you start doing that, you start getting more complication.

One part of a FET design is you are working with a device that has a square law transconductance curve that provides considerable second harmonic.  It is difficult to get an equivalent to that in an op amp circuit unless you have a FET somewhere in the feedback loop or you have a squaring circuit built from op amps and log-antilog bipolar transistor elements designed to give a square-law output.

[Rob Strand]

IMHO, the question was more along the lines whether a diode + opamp circuit can *emulate* a JFET circuit - say something specific like a gain stage.     That's not going to be a simple task in general - perhaps making trouble for yourself.     The closest toa  JFET solution is probably along the lines of Peavey's transtube idea.   Maybe an opamp version of that.