The Phantom said:
Just to make sure we're talking about the same thing, is the circuit
under discussion here?
+9V
|
.-.
| |
33k| |
'-'
|
.----------------------------------------------o----o Vout
| |
| o-------o |
.-. | | |
| | | 4.3M .-. |
| | 330k === | | +------o V out
'-' GND | | |
| '-' |
| +|| | |-
o---/\/\---o---/\/\--o--/\/\----o---||--o--->| JFET MPF102
| | | | || |-
| 330k | 330k | 330k | 10u |
| | | | o----.
| | | | | |
| | | | .-. |
--- 1.0u --- 1.0u --- 1.0u --- 1.0u | | | +
--- --- --- --- 1.0k | | --- 2 caps
| | | | See '-' --- 3200uF
| | | | note | | total
=== === === === below === ===
GND GND GND GNF GND GND
The circuit is essentially the same, except that I've changed the source
bias to three Si diodes in series, and a 1k resistor. I think I'll
lower the 1k even lower, to get the bias point to where the sine wave
swings are more symmetrical.
I suggested using a cap because I think you do in fact have some DC
bias at the gate. It is possible that you are getting some slight
rectification of the AC on the gate (since it's a junction FET), and
With the attenuation of the RC netwrok being 18, I don't see how the
signal swing could be more than 9V / 18 or 1/2V peak-to-peak.
the 10 uF capacitor looks to be an electrolytic (I infer this from the
+ sign on the left plate), and probably has enough leakage to give
some gate bias.
I used a larger cap before, and it was leaky, which upset the bias. I
changed it to 10uF, and the leakage problem went away. I'm not saying
that the 10 uF has none, just that it seems that it's much, much less
than the larger cap. As a precaution, I could change it to a different
cap. However I don't have a 10 uF in a non-'lytic that's anything
reasonable in size. I can parallel a few 1 uFs instead. The reactance
of the 10 uF is about 24k at .67Hz. If I go to a 1 uF the reactance
will be 240k, which is more than 5 percent of the 4.3M gate resistor,
and adds appreciably to the attenuation. In this circuit, with its
damped osc problem, that can make the difference between sustained
oscillation and damped.
I wanted you to measure the gain in the circuit as it
is. I suspect leakage from this electrolytic may be causing some of
your instability problems.
Instability? Inability - to sustain oscillations. Before with the
larger leaky cap, I couldn't get the drain more than a half volt above
the source, the leakage was causing the gate to be too 'open' and let
too much current thru the FET. Now, after a half hour, the oscs damp
out and the drain voltage is in the 6 or 7V area, which means that the
FET's gate is doing a good job of 'closing', IOW the neg voltage
(measured between source and gate) is sufficient. This leads me to
conclude that there isn't appreciable leakage into the gate.
(I just grabbed several 10 uF electrolytic caps, with voltage
ratings from 15 to 63 volts. With 5 volts applied I get around 1 uA
of leakage current. You have a DC path to the 10 uF cap through your
330K resistors, and 1 uA of leakage could raise the voltage across the
4.3 Meg resistor to several volts if it weren't for its tendency to
forward bias the gate of the (junction) FET. And, of course, the
leakage changes slowly with time as you apply voltage to the 10 uF
cap. Then, when the circuit is powered down, the capacitor de-forms,
as it were, and the leakage starts out higher and then decreases again
the next time you power it up.)
I put the DMM on .2VDC range across one of the 330k resistors. It
measured under 100mV when I powered on, and after maybe 4 or 5 minutes
it measured under 30mV, when the readings started to fluctuate as it
began oscillating. So choosing the 33mV point, .033V / 330,000 gives
1/10 uA, which when multiplied by 4.3Megs gives .43V, which isn't enough
to upset the bias and is more than the actual value, when the 10uF
finally gets charged up.
This is another disadvantage of the lead form of the phase shift
network that hadn't occurred to me until now. With the lag form, you
would have several smaller caps in series from the drain, with a
resistor at each stage to bleed off the leakage.
I think you got those reversed, above. Lead is CR, lag is RC as in the
schematic above.
For the schematic above, it's a disadvantage because the JFET requires
negative bias, hence DC isolation from the lag network. If it were a
MOPSFET, the above schematic could be an advantage because it would be
self biasing, and the four 330k resistors would be part of the bias
network. IOW it would be simpler than the lead network, requiring less
resistors. But yeah, the lead network would be simpler in this case
with the JFET, and less prone to leakage. I started out with the CR
lead network, but didn't have success maintaining oiscillations. So I
switched, probably on recommendation of others here.
The one thing I like about the CR lead netork as far as BJTs go is that
the waveform is less distorted because it's a low pass filter. Higher
harmonics at the collector get fed back to the base where they are
cancelled.
I think it's been on long enough to stabilize and the oscs are damped
out. BRB. Well, the oscs are almost damped out, and I measure across
one of the 330ks about 1 to 2 mV, which is about .005 uA leakage.
That's less than .02V across the 4.3M bias resistor. But now that it's
damped out and stable, I'll measure the DCV across the 4.3M. BRB.
Well, the meter is still jumping around zero, but as best as I can tell
it's swinging from neg 10mV to pos 11 or 12mV, which seems to agree
somewhat with the leakage estimate. Of course I didn't include the
DMM's resistance, but I think it's 100M on the 200mV range.
I'm convinced that the leakage isn't a prob, and changing the cap isn't
gonna help. What do you say?? Maybe I should try a 10uF tantalum, but
is that gonna be a help?
If you have a 10 uF film cap handy, you might try putting that in
place of the electrolytic you have there now (I'm assuming it is an
electrolytic now).
I would have to use a handful of 1 uFs, but I think I will have a huge
problem with picking up a lot of extraneous noise and hum, since this
huge glob of caps will all be at very high impedance and acting like a
large antenna. :-(