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Old 1st April 2004, 11:47 PM   #1
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Default Diodes in Grid-Cathode Circuit

Just a quick question.

In DC coupled designs where a diode is used to prevent the grid from going too positive with respect to cathode, are there any diodes that are particularly good for this purpose? Or is any diode that meets voltage/current requirements as good as any other?

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Old 1st April 2004, 11:59 PM   #2
EC8010 is offline EC8010  United Kingdom
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In normal operation the diode will be reverse biased (switched off) and the depletion layer between the P and N type semiconductors forms the dielectric of a capacitor (just like the varactor diodes used for tuning your television). Varactor diodes work by changing the thickness of the depletion layer in inverse proportion to the diode voltage, so their capacitance is inversely proportional to voltage. Power diodes need bigger bits of silicon so their depletion layer has a larger area, increasing capacitance. You don't really want to add shunt capacitance that changes significantly with applied voltage, so a small-signal diode such as 1N4148 seems the best bet.
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Old 2nd April 2004, 09:00 AM   #3
Jax is offline Jax  Sweden
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On the other hand, such diodes are mostly seen between grid and cathode on cathode followers where the cathode is unbypassed. The voltage variation over the diode is small here.

1N4148 will do fine so I concur.
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Old 2nd April 2004, 09:41 AM   #4
dhaen is offline dhaen  Europe
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I agree that shunt capacitance is a bad thing, but probably the ratio of change of capacitance with voltage is worse. This makes me cautious about advocating the smallest signal types, whose ratio I tested some years ago and found a big enough change to make them useful as cheap varactors.

If you must use a clamp, how about trying one of the many small signal thermionic diodes Eg EB91 (2.5pF at any voltage)?

Of course all the arguments hinge on the source impedance of the driver stage, which in a good design should squash a few pf's here and there.
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Old 2nd April 2004, 12:27 PM   #5
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Quote:
Originally posted by dhaen
Of course all the arguments hinge on the source impedance of the driver stage, which in a good design should squash a few pf's here and there.
So I wondered what people did if they couldn't squash a few pF; Tektronix used small neons.
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Old 2nd April 2004, 12:42 PM   #6
dhaen is offline dhaen  Europe
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Quote:
Tektronix used small neons.
Was that for normal excursions or EHT flashover?
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Old 2nd April 2004, 01:02 PM   #7
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Thanks for the replies.

The preceding stages can handle a few pfs, but there is going to be a 4-5Vp GK voltage swing across one set of diodes.

dhaen, how much capacitance change were you seeing?

neons are the perfect solution. are there any in current production that would be suitable for this?
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Old 2nd April 2004, 01:30 PM   #8
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Quote:
Originally posted by dhaen
Was that for normal excursions or EHT flashover?
It looks as though it was for both. The 555 oscilloscope had neons between grid and cathode of the cathode followers driving the horizontal deflection plates, and also between the deflection plates as deflection indicators.

Runeight: I can't imagine that modern CRTs are immune to flashover, so protection must still be necessary. However, very few oscilloscopes now use CRTs, so those particular neons might not be in production. Telephones use neons etc for transient protection, and they will definitely still be in production.
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Old 2nd April 2004, 07:00 PM   #9
dhaen is offline dhaen  Europe
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Quote:
Originally posted by runeight
Thanks for the replies.

The preceding stages can handle a few pfs, but there is going to be a 4-5Vp GK voltage swing across one set of diodes.

dhaen, how much capacitance change were you seeing?

neons are the perfect solution. are there any in current production that would be suitable for this?
Don't panic! Now I've looked back, we're talking really low pf's here. Something in the range of:
IN4148: 5p to 1p for 10v change or so,
1n4001: 25p to 10p for 10v change or so.
You can see the 1n4148 is the best varactor, but probably the best clamp too, as the capacitance is so small.
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Old 2nd April 2004, 11:41 PM   #10
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1N4148's are 'ok', no doubt, but neons approach the theoretical ideal capacitance wise, plus they give visual feedback. There's companies that still make them - Newark Electronics stocks them - I buy the type with the lead wires.

You can get small neons rated for under a milliamp to at least several milliamps continuous, but they'll clamp significantly higher currents for shorter periods of time - it's a matter of heat dissipation with them mostly.
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