Low Gain JFET MosFET?

[soggybag]

Are there low gain versions of JFET and MosFET devices? Seems the BJTs come in a wide variety of gains. I'm not so clear on the gain of JFETs.

[petemoore]

 MPF 102 for Jfets
  Not sure about Mosfets.

[brett]

Hi
the lower the Vgs(off) (ie more -ve), the lower the gain.  The J201 (typically -1 V) can give lots of gain, but the MPF102 (typically -6V) can't give too much, even when flat out.
cheers

[R.G.]

Are there low gain versions of JFET and MosFET devices? Seems the BJTs come in a wide variety of gains. I'm not so clear on the gain of JFETs.

You have to be careful what you're calling gain.

Bipolars have a clearly defined current gain. For FETs, this is infinite, because the input resistance is nearly infinite, so you can't put ANY current into the gate. The output changes its full range for zero change in input current.

Both bipolars and FETs have a transconductance. The transconductance (g_m) is the change in output current divided by the change in input voltage. The raw voltage gain of a bipolar, JFET, MOSFET, etc. is g_m times the load resistor: change in input voltage produces a change in current times a resistance, which is a voltage.

You can compare JFETs by looking for the g_m or Y_fs as it's sometimes listed, on the datasheet.

Bipolars have a much, much higher g_m than FETs in general. This is because all of the usable change in input voltage is right on the knee of the Vbe diode curve. A bipolar has an input *voltage* range of about 0.05 to 0.1V, end to end, for 100% of its output current range. JFETs have a much bigger input voltage range; generally from Vgsoff to Vgs=0. So Vgsoff tells you the range of input voltages. This can be as little as 0.1V (notably the J201)  to 10-20V. The MPS102 can be as much as -9V. And as we know from our collective miseries with JFET matching for phasers, Vgsoff varies from FET to FET.

By now you're thinking "butbutbut I'm out standing in the weeds. None of this is what I needed to know, even if it's true, which I can't tell. I just asked a simple question." You did, indeed. But the answer to what you asked is very complex.

Yes, FETs come in low and high gain. In general, JFETs are much lower *transconductance* than bipolars, and infinitely higher current gain than bipolars. The ultimate voltage gain you can get from JFETs in simple circuits is lower than the ultimate voltage gain you can get from bipolars in simple circuits, if everything else is equal. But everything else can't be equal because JFETs and bipolars are so different. And JFETs vary widely in transconductance, from type to type and from JFET to JFET within a type. MOSFETs are more consistent than JFETs, but still vary.

As Brett says, JFETs with small Vgsoff have a smaller input voltage range, so they have a potentially bigger g_m. However, the Idss of the JFET limits how much current you can get through them. JFETs have an interrelationship of g_m, rds, and idss something like the relationship of mu, rp, and gain of a triode.

[soggybag]

Thanks for the reply. It's all good information. I'll have to read it a few more times before I can absorb it all.

I had this idea to build a Fuzz Face with FETs. Not sure if this would work. But I figured it might be best to try devices with lower gain.

[petemoore]

  EBC is the 'voltage order' of Bipolars, Base above emitter by a diode drop, collector above base by...a lot more [biased toward 1/2v].
  Jfets are different, the order of voltage is GSD, source above gate by a certain amount [see Fetzer valve for tips, GEO Foolin' with Jfets I think it's called]. Drain somewhere around 1/2v also.
  I've seen and tried 'Jfet hybrid FF', but went back to bipolar after some bit of fiddling with the fets.

[PRR]

> You have to be careful what you're calling gain.

Thanks for typing ALL this.

I would have started with "Current gain?? Voltage gain?? Power gain??... but my fingers did not feel like typing "Basic Amplifiers 101 week 7".

Footnotes:

FET _current_ gain is "infinite" for small audio purposes. (We feel the gate capacitance above the audio band; or when using BIG FETs for loudspeaker amps.) OTOH a BJT's current gain is finite and very variable one part to another (and same part in different conditions).

A BJT's transconductance is so VERY predictable that they "never" put it on the datasheet. For Silicon, Gm = Ie/0.026 with corrections for extreme temperature. OTOH the transconductance of a practical FET is much smaller than the Ie/0.026 limit and very variable one part to another (and same part in different conditions).

In principle the _Voltage_ gain of either a BJT or a JFET is very high. Higher than we can possibly get with simple circuits at sane voltages. Using NOT-reasonable assumptions the BJT would have higher voltage gain; using Reasonable (for guitar-stuff) assumptions the voltages gains may be remarkably similar.

A given circuit may want, need, or be sensitive to device voltage gain, current gain, none of the above (within reason), and/or some other parameter we have not touched here.

> FETs come in low and high gain

There's some pattern. The true figure of merit is transconductance at the same current or change of transconductance with current. If a JFET took huge current yet gave low transconductance, it would not sell well. OTOH, a "really good" JFET may not have advantage in many high-demand FET circuits (mostly not audio). Those non-audio apps favor different things: adequate gain with large AGC range, huge Vto for switching, or low Cgs for UHF work. Actually I think neither equipment designers nor JFET manufacturers really understand the product, and the mess-o-parts we see on today's market could be rationalized better.

> JFETs have an interrelationship of gm, rds, and idss something like the relationship of mu, rp, and gain of a triode.

This could be stated more clearly. I don't think that would help. The Gm/Ik of a triode does vary sorta-like Gm/Is of a JFET, but triodes have Mu of 10 to 100 fairly consistent with Ik, while JFET Mu is sometimes close to 500 and when it is less it is declining rapidly (Vds gets near Vto). We could take the vacuum pentode for analogy... but even real tube heads don't intuit pentodes so well.

And to dig deep into the origin of the V/I graph: at VERY small currents, FETs solid or vacuum become asymptotic to the BJT's Gm=Ie/0.026 line or one parallel to it. Current in an FET flows "past" the control electrode, and control gets poor. OTOH in a BJT the current flows _through_ the control electrode, and the control gets intimate influence over every electron/hole. (Thermionic FETs have the additional problem of a working temperature three times higher than the temp we can work Silicon FETs at; I/0.026V actually has "T" in it.)

> a Fuzz Face with FETs. Not sure if this would work.

A direct port: no. First, the FuzzFace biasing is set up for BJT voltage relations and needs a major re-think to even "come alive" with JFETs. Then you hit what R.G. mentioned: a BJT can be slammed with 50mV, a JFET needs roughly 20 times more input to slam it. Quite gentle guitar playing will push a BJT into gross overload; guitar straight to a JFET can barely stress it even with strong-arm playing. Of course you can put a 1:20 booster ahead of the JFET.... but now it isn't a "a Fuzz Face with FETs" but getting into a totally new design.  

Also a FuzzFace "needs" to load the guitar as a serendipitous way to slant-off the highs for more euphonious grossness. The raw FET does not load the input at all.

[soggybag]

Thanks for the replies. I had been looking at Tim Escobedo's "Many Faces of Fuzz" page. He had set up some interesting fuzz face like circuits using MosFET, JEFTs and BJTs. It got me to thinking that the "sweet spot" in gain for Ge Fuzz Faces is quite low. With transistors around 100hfe. The MosFET and JFET's used in Tim's examples were what I had considered higher gain 2n7000 and 2n3819.

So I started to wonder if there was some interesting territory to explore with low gain FET devices.

I had this other idea, which I'll throw out there for the fun of it. I may be wrong but, it seems positive feed back is a defining feature of the Fuzz Face. I was thinking about playing around with variable positive negative feedback. I noticed someone had posted a phase shifter that had a variable feedback made up from a single transistor and a few other parts.

[PRR]

> it seems positive feed back is a defining feature of the Fuzz Face.

There's Positive Feedback in a FuzzFace? Where?

[soggybag]

May be I am misinterpreting the resistor from Q1's base to the emitter of Q2.

[brett]

Hi
re: feedback in a FF
It's a voltage feedback thingy.  There's some signal feedback as well as setting the DC bias on Q1 (notice that there's no other bias control other than the 0.3V drop from base to ground, which only works after the feedback network turns Q1 "on")  Some people like to reduce the feedback resistor to something like 33k or 47k (for more signal feedback and more DC current).
cheers

[cpm]

May be I am misinterpreting the resistor from Q1's base to the emitter of Q2.

i'd say thats negative feedback. Signal gets inverted at Q1 colletcor.
Theres a trick indeed to add a cap in parallel to that feedback resistor to get a fixed DC voltage bias through a big resistor, and then vary the AC feedback through the cap as a method to reduce gain, cleaner that the usual pot at Q2 emitter.