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Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Just soldered this up.... More to come in a 'Poject' thread soon, but my layouts appear to be improving almost to the form of art.... couple of 'errors', maybe, but getting there.....
I bet no-one can guess what it's designed to drive?..... not tested it yet, it might require a bit of tuning.....
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
I managed to fry the first version because I'd believed the textbooks and was trying to measure voltage instead of current..... Ohm's law isn't easy to apply to the output of this circuit...... The Mk 2 version is complete overkill, and I exceeded the output requirements as soon as I started tweaking.... Adding extra parallel resistors to the source..... I need to do a bit more tweaking to learn the true nature of this creature, but it has so far exceeded all expectations, and I feel I've barely begun yet..... More to follow.....
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
Is it a bi-phasic jfet terminator ?
I do not know the application but it is clearly not for a true sinewave output, and the component values seem 'odd' for 1 kHz operation, what is it for ?
It looks to me that you have wired j105's in parallel, If your fets get inexplicably hot it may be due to parasitic oscillations, probably at such high frequency that all your 'scope will 'show' is a thick fuzzy trace instead of a nice thin line, if so then ferrite beads on jfet leads will help.
Generally, using UHF transistors in low frequency circuits can cause parasitic oscillations - if the designer is not familiar with r.f. design. The common symptoms are . unexpectedly high current draw or component heating . at certain levels in waveforms the 'scope trace looks 'fuzzy' . 'scoping at different points gives inconsistent waveforms due to the 'scope lead aiding or loading a self-oscillating circuit.
No obvious sign of parasitic oscillation in the 'photo above but something worth keeping an eye open for.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Shrad wrote ...
I bet those would make some nice voltage followers which sound as good as one could expect... what is the range of gate capacitance?
35-160pF
Interesting article you've linked to. While these are only rated for 25Vdg, I'm running them from a 38V supply, as at least 15V is across the drain resistor, so the device 'sees' less than 25V.
(EDIT: I've just realised what you mean by voltage follower.... Yes, it seems to be an 'ideal' voltage follower )
Sulaiman wrote ...
Is it a bi-phasic jfet terminator ?
I do not know the application but it is clearly not for a true sinewave output, and the component values seem 'odd' for 1 kHz operation, what is it for ?
It looks to me that you have wired j105's in parallel, If your fets get inexplicably hot it may be due to parasitic oscillations, probably at such high frequency that all your 'scope will 'show' is a thick fuzzy trace instead of a nice thin line, if so then ferrite beads on jfet leads will help.
Generally, using UHF transistors in low frequency circuits can cause parasitic oscillations - if the designer is not familiar with r.f. design. The common symptoms are . unexpectedly high current draw or component heating . at certain levels in waveforms the 'scope trace looks 'fuzzy' . 'scoping at different points gives inconsistent waveforms due to the 'scope lead aiding or loading a self-oscillating circuit.
No obvious sign of parasitic oscillation in the 'photo above but something worth keeping an eye open for.
It'gives a perfect sinewave output, Suliaman.
They seem to work in parallel exactly the same as individually. No 'fuzzy' trace, just perfect sine wave.
It's actually for driving a spring reverb tank with input impedence of 8 Ohms. As you're aware this is the impedence at 1kHz, at ten kHz impedence is 80 Ohms. It requires at least 28mA, with the series resistor of 246 Ohm, at 10kHz, total impedence is 326 Ohms, and it is driving 36mA @ 1kHz and 30mA @ 10kHz, with a perfect sine wave.
These are nothing like the JFET's from 50 years ago, things have moved on a bit since then....
(EDIT: regarding your point re. parasitic oscillations when using fast switches..... Would that be when using solid state voltage controllers?..... I wanted to avoid them because I could foresee that kind of problem, I'm only using copper and iron for voltage regulation with this circuit. 'Old Skool' technology.....
Plasma wrote ...
If you want ac voltage emitter ,a capacitor in series on the top right leg,to remove the DC bias
the capacitor on the bottom leg blocks any DC. The resistor on the top leg is to reduce the difference in current at 1kHz and 10kHz by giving a total impedence at 1kHz of 254 Ohms and a total at 10kFz of 326 Ohms.
connecting the output in this way isolates the output. It also provides a very low impedence output.
This is the latest incarnation, I'm driving it with a more conventional JFET topology stage which gives a gain of 25 because this circuit needs to be driven with a signal of 8V P-P. I still need to add some filtering to the gain stage, though.
Registered Member #61406
Joined: Thu Jan 05 2017, 11:31PM
Location:
Posts: 268
I see, would the voltage and current be in phase, voltage leg would get less affected by a capacitor, thand the lower leg current leg, Does this sound right?
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Function and form....
