Why Can't I get This FET To Oscillate

"Watt

snipped-for-privacy@corp.supernews.com>) about 'Why Can't I get This FET To Oscillate', on

Yeah. And then that causes the freq to change a bit compensate, etc, etc.

But you also have in series with every component, the parasitic inductance of the leads, especially with a bread board type setup. I can usually tell when there are VHF oscillations because of the "antenna" effect. You put your hand within an inch of the circuit and get major changes in behavior. I think you would have noticed if this were happening. Anyway, you're not using the lead form of the network now.

Again, the leads of the 47 pF cap have inductance, especially when breadboarding where the leads are left long, and there is a frequency where the cap and its leads are series resonant. This can *cause* oscillations, rather than suppress them, if the transistor has a high enough cutoff frequency. I used to think it would be a good idea to have a transistor with a very high cutoff frequency to use for everything. That way you would never have to worry if the transistor was fast enough. So I tried using a

I have gotten the impression that your 9V supply is a "transistor radio" type

Anyway, I think the variable bias supply and no capacitor would be a good experiment to try.

With Vgs(off) in this range, your FETs should have transconductances much higher than the 2000 minimum. I just can't understand the behavior you're getting (oscillations dying out). I may have to see if I can find an MPF102 of my own and breadboard this thing.

I hope you can try the fixed source bias idea; I think we could get some good info from that.

Dark Remover\""

have

upset

inductance of the leads,

there are VHF

within an inch of the

noticed if this were

pF

breadboarding where

its leads are series

if the transistor has

idea to have a

That way you would

using a 2N3866 for

will oscillate if

Yeah, been there, done that, got the T-shirt! I worked with UHF and microwave (radars) so I'm wary and wise enough to know when and what to do to keep things behaving, RF-wise.

damped

across

agree

the

tantalum,

of

impedance

radio" type 9V

substantially longer life

Either a HP 6214 or a Power Designs 2005 precision regulated PS, so I set it and the DMM reads 9.000. :-)

But at the currents that this osc is using it would be just fine, a 9V battery should last months at a hundred microamps!

a good experiment to

Well, I still have to bypass the bias supply, so it'll have almost no degeneration on the oscillations. I changed from a zener to TL430, and some pot that will handle a bit of current. I have to get the TL430 in there somewhere. I'm working on it.

You bypassed it with 3200uF before; at .67Hz that's an impedance of

Dark Remover\""

Below

the

at

them

transconductances much higher than

(oscillations dying

breadboard this thing.

some good info from

Yeah, I set that up last night. I used a TL431 with a 270 ohm resistor to 9V, so there's about 25 mA current thru the resistor, and 2.5V across the '431. Then I put a 200 ohm pot across the '431 and connected the wiper to the fet's source and the 1000 uF byp cap, and I removed the

I now can adjust the source to a somewhat fixed voltage. When I turned it up to about 1.5V Vs, it started oscillating, then the voltage kept creeping up to 2V, and the oscs died out. I had to fiddle with it, and wait a bit for the caps to charge up. When the Vs is at 1.55 to 1.6V, it seems to stay fairly stable and it's been oscillating all night long. Yeah, finally! Not fully rail to rail, but stable oscillations. My guess is that the square law characteristic of JFETs might be the reason it's limited.

I had three diodes in seires that were giving about the same Vs, along with the 1k resistor. I didn't reduce the resistor lower, but I think that if I had used a 470 or maybe lower, it might have stabilized and not damped out. I may try that later, since this is just an experiment.

-->| o--------o-----/\/\/\---o 9V |- | | 270 S| | 2.5V o----+ FET | | | | | / ----/ | o---->\ / / \---+ / ---- 200 \ | TL431 | | o--------o | === GND

be

and

in

I forgot to include the byp cap in the followup I just posted, so here's a corrected schem. I reduced it from 3200 to 1000 uF. That's enough as you say above. It's there, and doesn't do much except help prevent noise from the rest of the circuit from getting into the source.

-->| o--------o-----/\/\/\---o 9V |- | | 270 S| | 2.5V o----+ FET | | | | | / ----/ | o---->\ / / \---+ | / ---- +| 200 \ | TL431

That capacitor *has* to be got rid of. There are other problems as well though. The low voltage across S-D is running the jfet in a place where it has a low output impedance, and lots of distortion. The even-harmonic components in the distortion is probably accumulating on the capacitor and shifting the bias point, away from oscillation.... a sort of leisurely squegging.

If you have to stay with a 9v supply, then a folded cascode circuit could take away a lot of the fundamental problems. As below, LTSpice says it works ok.

Come on, Tony, you done gone and ruined this whole thread by introducing some actual engineering thought into the process ;-)

...Jim Thompson

Sorry Jim.......

The folded cascode (with dc feedback bias) was my favorite circuit when using a single jfet, it evades all sorts of problems. By ratio'ing the currents it is even possible to do the self-bias for constant gm, as per the Siliconix app note TA70-2.

In this case, getting 29x off a 9Vdc rail is possible, but still a little too marginal for comfort.

"but for Wales", uh, LED flashing ?:-)

...Jim Thompson

Chuck Harris said something like, "get it oscillating at a normal frequency first, then fine-tune it" - at first, I thought, "Well, yeah, if you divide all the caps by, say, 100 - but would that mess with the impedances?" And the part with the brains said, "No, the impedances will be the same, but at the higher frequency".

At least you'd be able to see it go through a cycle or two before the coffee gets cold. ;-)

Good Luck! Rich

Dark

yeah, if

will

Well, I don't drink coffee. But I took their advice and put a fixed

But it's kind og a waste to have to go to all that trouble to get the FET to the right bias point. I'll have to again try to get it to oscillate with the three diodes and resistor in series with the source. Now that I know where the right bias point is. I'll have to play around some more.

Hey, Rich. You live in Whittier? Have the rains filled up Whittier Narrows and turned it and the recreation area into a giant mudflat? :-P

This is that stuff that them edjamacaited people talk about - designing out part variations, or some such. ;-)

I do live in Whittier, but I don't get out much; in fact this is the first I've heard of "The Whitter Narrows" - I don't even know where it is.

I was kind of surprised the other day when I saw that fault map - there's a "Whittier Fault", and it appears from the map that I'm sitting right on top of it. On a slab, in an RV, so It's not that big of a deal, I guess. Unless the Earth opens up and swallows me whole. ;-)

Cheers! Rich

:-P

first

Hard to miss, since it's as big as the whole city. Here's a map. The green area at the top is the recreation area, golf course, lakes, etc. Roughly the 60 Fwy and Rosemead. The bottom or south end of the green is the big dam.

sitting

deal, I

Everyone in L.A. lives on a fault. :-P

On Tue, 18 Jan 2005 23:08:09 -0800, Watson A.Name - "Watt Sun, the Dark

Ah. Gotcha. I haven't been through there since before the raining started. And by now, it's been clear for almost a week, so it's more or less moot.

I live between downtown Los Neitos and the five corners between there, Whittier, and South Whittier.

Eek! That's a long one! ;-)

And now what's-his-face can find me. ;-)

Thanks! Rich

Tony Williams' JFET phase-shift oscillator. Very nice.

. ,---------------------------------------+--- 9V . | | . \ \ . /2k /1k5 . \ \ . _|_ 0.15mA approx, | . \_/ +--

TA70-2 ==> Siliconix / Vishay AN102 From an old post by Tony, "AN102 is obviously drawn from the original TA70-2 and seems to be a 1997 update+rewrite in electronic form. The guts of the information given in TA70-2 is still there, relatively unchanged."

It's not a very nice oscillator really Win, more an exercise in trying to get around the very serious problems a jfet has when Vd-s is too low.

LTspice gives roughly 4.8V pk-pk across R1, so there would be about 1/29th of that at the gate of J1. The only practical place for a take off looks like across R1, perhaps with a jfet (or pnp) follower.

There's another smidgeon of Gain available, by lifting the 3x C's off 0V and connecting them directly to the Source of J1 (reversing their polarity of course). However the way the distortion increases shows how cruddy asymmetrical clipping is, as the amplitude control for a phase shift oscillator.