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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 24th Jul 2020, 9:16 pm   #1
buggies
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Default JFET sanity check please...

The small circuit extract below shows a 2N5245 FET with no resistor on the gate. The FET is used as an RF buffer and amplitude modulator.
I can't think of any good reason - thinking 100k to 1M?
There is no "R" number missing from the sequence of components.
The circuit comes from the radiomuseum and I have been unable to locate another on the internet.
Although this refers to the circuit of a Nombrex 41 signal generator, I have picked this forum department as I don't actually have that piece of gear - just rehashing an old biscuit-tin genny.
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Old 24th Jul 2020, 10:32 pm   #2
Herald1360
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Default Re: JFET sanity check please...

As a JFET rather than IGFET they may just "get away with it". I'm intrigued as to why the gate capacitor should need to be 62.2pF. Wouldn't 68pF have done just as well?

It's not even an E192 value!
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Old 25th Jul 2020, 2:47 am   #3
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Default Re: JFET sanity check please...

Try reading it as C6, 2.2pf and the value seems less silly.

As for any DC bias path to the gate, I think it's being left to bias itself with self-rectification acting against stray leakage..... Yuk!

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Old 25th Jul 2020, 5:36 pm   #4
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Default Re: JFET sanity check please...

Thanks for confirming my suspicions - taking an alternative route now based on circuit of Trio-Kenwood SG-402 which looks more solid.
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Old 25th Jul 2020, 7:09 pm   #5
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Default Re: JFET sanity check please...

Yes, stray leakage for me, too.

Rather flakey, as leakage is indeterminate! But with a J-FET rather than a MOSFET, there's a chance that they could get away with it.
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