Source resistor on a JFET boost or overdrive stage?

[lenwood]

Changing the source resistor on a JFET design booster.

Does lowering it increase or decrease the gain/distortion?

I have searched and I just wanted some explanation on how this part affects this.

Thanks

[lenwood]

Bump

Ok I have read about this but can someone explain it in common terms?

[JimRayden]

Well, highering it makes the gain go down. And in my experience, soundwise, lowering doesn't make one damn ding to it, perhaps mess with the bias a bit.

------------
Jimbo

[The Red Rooster]

When designing circuits using transistors in general (Jfet, MOS and BJT) designers talk about 2 different situations:

a) DC

DC deals with how the transistor works without any AC input signal. This operating point is often called the Quesent point or Q-point, and deals with the current and voltages related to a standby situation of the specific circuit. Looking at a JFET in a self bias circuit, the designer would choose a drain current and a drain resistor utilizing a voltage at one half of the used supply voltage. This would insure the biggest possible symmetric voltage swing when working.

Inserting a source resistor results in a certain source voltage depending on the choosen Drain current and the specific JFET. Since we're looking at a self bias circuit the Gate voltage is 0V. This means that the Q-point will be determinated by the choosen Source resistor - the GS voltage is 0V - VRS.

Since all JFET's have af big variation in specs the choice of Source resistor is quiet importent. Choosing the right Source resistor makes it possible to control the Q-point in an acceptable range of allowable drain current variation.

b) AC

AC deals with how the transistor works with an AC input signal at a specific Q-point. 

A JFET has a transfer function describing its drain current as a function of the Gate-Source voltage. Drawing a tangent to this transfer function at any point gives you gm (Mutual Conductans). gm is an importent value used for calculating the AC amplification in a given Q-point. Please note that the gm is highly depending of the actual Q-point.

So to answer you question: "Does lowering it increase or decrease the gain/distortion?"

Yes, changing the value of Source resistor does change the gain of the circuit. The gain changes because you move the circuit Q-point at the transfer function. Doing this you also affects the available symmetric voltage swing at the Drain pin, this because a change in Q-point from half-supply will decrease the available unclipped output signal.

How to deal with this bias problem? I recommend doing a google search for application note AN102 by Siliconix.

Regards

Kurt

[MartyMart]

It depends on the specific circuit and how the bias is set up, if you take a typical
boost/drive circuit such as the ROG fetzer valve, the source resistor is a 1k5 with
a 22uf bypass cap too. ( which effects gain )
The drain is set to 1/2 V at 4.5 volts, lowering the 1k5 to say 820 ohms AND increasing
the bypass cap to between 33uf and 47uf will give you a decent gain increase.
If this is followed by one or two further gain stages, with perhaps a "gain" pot of 500k or
1M between, you'll drive the next gain "stage" harder, causing distortion.
See some more ROG ( www.runoffgroove.com ) projects, or some of my own in my
layout gallery above, for great examples of multiple gain stage OD's .
My "London 2 step" is a great sounding 2 stage example of this very thing ...

Marty.

[lenwood]

Thanks guys for the replies.

So, say in a fetzer valve for example,  increasing the source resistor and re-biasing to 4.5volts would make it cleaner, right?

If so, would it have more or less headroom?

Marty, I'll also check out the your London 2-step OD.

[MartyMart]

Thanks guys for the replies.

So, say in a fetzer valve for example,  increasing the source resistor and re-biasing to 4.5volts would make it cleaner, right?

If so, would it have more or less headroom?

Marty, I'll also check out the your London 2-step OD.

Yes, if you ran the circuit at 12 or 18v you'd have more "headroom" ( clean signal ) with a little re-bias.
You can "clean up" a two stage 9v version by having your 1/2 v drains at around 5 - 6 volts.
I'd suggest that you build a little circuit and use some sockets to swop out the relavent parts
for some experimentation.
Often, a two/three stage circuit can get a little "quiet" with gain ( between stage drive pot ) decreased, even when
you crank the output vol pot.
The "Classic 30" amp sim that I put together, has a "clean" path from 1st to last stage, which is
nice, it's always available along with the "driven" signal, this gets around that problem of overall
level drop in these designs.

MM.