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Old 16th August 2009, 03:10 AM   #1
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Default "It won't work"

I suggested to someone that they connect the offset trim pot in this classic JFET buffer as shown in the schematic below, the reason being that it keeps the current source's current from flowing through the trimpot's wiper.

I was told "It won't work."

Now, my memory isn't what it used to be, but I'm pretty damn sure I'd used this connection method before and it worked just fine. Nor can I think of any reason why it wouldn't work.

I don't have any JFETs on hand at the moment or I'd just stuff up a breadboard and check for myself. So I thought I'd put it before y'all.

Is my memory failing me or will this method indeed work?

Thanks.

se
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Old 16th August 2009, 04:13 AM   #2
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The circuit you have drawn will work nicely. When biased correctly FET buffers like this are nice as they have no DC offset between the input and output.

Tektronix used to use matched J-FETs in a setup like that with fixed resistors for inputs.
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Old 16th August 2009, 04:32 AM   #3
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Thanks, Gary.

Yes, I'm already familiar with that basic classic JFET buffer.

The issue at hand concerned how the trimpot was used. In other words, how I have it drawn in the schematic where the wiper goes directly to the JFET's gate versus having both the gate and the trimpot's wiper going to the negative rail as is also commonly done.

The way I see it, both will work just fine, but as I drew it, it has the advantage of not having any current flowing through the trimpot's wiper.

se
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Old 16th August 2009, 07:35 AM   #4
SY is offline SY  United States
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That's absolutely the way to do it. Only thing I'd add is a safety resistor between gate and negative in case the wiper poops out.
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Old 16th August 2009, 03:01 PM   #5
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Tek always had protection on the input J-FET gate. This usually consisted of a low leakage, low capacitance diode connected between the gate and the negative supply, cathode to gate, anode to the negative supply. On the vertical inputs it was common to have an RC combo consisting 470K resistor and .001uf capacitor between the input and gate of the upper J-JET. The resistor provided current limiting in the event of overdrive. In the positive overdrive condition the J-FET forward biases. In the negative direction the protection diode forward biases limiting the voltage on the J-FET gate.

To see the many variations of this circuit look in old Tek scope manuals. Look at the vertical input buffer or the external trigger input. To down load old Tek manuals go to bama.sbc.edu/. There is an amazing amount of information to be found perusing through the old manuals, both solid state and tubes (provided that the scans have the schematics included as it seems that some don't).

Gary
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Old 16th August 2009, 03:17 PM   #6
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Quote:
Originally posted by SY
That's absolutely the way to do it. Only thing I'd add is a safety resistor between gate and negative in case the wiper poops out.
Aye. Good idea. Thanks!

And thanks, Gary for the additional info and the link to the BAMA site!

se
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Old 18th August 2009, 02:00 AM   #7
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Hi,
what are the JFETs supposed to be proactively protected against by low capacitance diodes, trimpots, capacitors and reverse biasing? Swine flu? They have an outstanding ability to protect themselves when just being biased suitably and cleanly.
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Old 18th August 2009, 03:16 AM   #8
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Quote:
Originally posted by Gary P
Tek always had protection on the input J-FET gate. This usually consisted of a low leakage, low capacitance diode connected between the gate and the negative supply, cathode to gate, anode to the negative supply. On the vertical inputs it was common to have an RC combo consisting 470K resistor and .001uf capacitor between the input and gate of the upper J-JET. The resistor provided current limiting in the event of overdrive. In the positive overdrive condition the J-FET forward biases. In the negative direction the protection diode forward biases limiting the voltage on the J-FET gate.

To see the many variations of this circuit look in old Tek scope manuals. Look at the vertical input buffer or the external trigger input. To down load old Tek manuals go to bama.sbc.edu/. There is an amazing amount of information to be found perusing through the old manuals, both solid state and tubes (provided that the scans have the schematics included as it seems that some don't).
I would say, Tektronix is among few American companies that had excellent designers. I can name 2 more: Collins Radio and Altec Lansing.
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Old 18th August 2009, 03:33 AM   #9
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Hi Anatoliy,
Quote:
I would say, Tektronix is among few American companies that had excellent designers. I can name 2 more: Collins Radio and Altec Lansing.
You can't leave out Agilent/HP from your list! I dare say that stuff is every bit as good as your other mentions. Tektronix - absolutely!

I dare say there are many others out there, too many to list. I'm talking about design excellence here. Hughes Aerospace and Itty-Bitty (IBM) are also examples of fine design work left out of the list.

Hi Steve,
Quote:
I was told "It won't work."
Did you get any reasons, or just a flat statement? That circuit is indeed a classic and does work, even as drawn without the finer points of "just-in-case" protection. Of course, bleeding edge audio work can not include anything that may affect the sound at all.

-Chris
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Old 18th August 2009, 03:51 AM   #10
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Quote:
Originally posted by Lumba Ogir
Hi,
what are the JFETs supposed to be proactively protected against by low capacitance diodes, trimpots, capacitors and reverse biasing? Swine flu? They have an outstanding ability to protect themselves when just being biased suitably and cleanly.
Voltages that might cause the gate to channel oxide barrier to fail, and ESD discharges. The citation was for test equipment inputs.

The pot is used to set the operating current, and by inference probably to set the q point of the output to 0V. And yes it works just fine, and as SY indicated a resistor from wiper to negative supply will prevent fet frying mischief if the wiper goes bad.

Incidentally the wide variation in transconductance from fet to fet makes this circuit a reasonable idea if fets cannot be selected.
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