zenering a jfet gate
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From: LM on 7 May 2010 14:05 On 7 touko, 20:28, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Fri, 7 May 2010 09:52:31 -0700 (PDT), cassiope > > > > <f...(a)u.washington.edu> wrote: > >On May 5, 9:35 am, John Larkin > ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >> Any comments on the behavior/effects of zenering the gate of a jfet? > > >> John > > >In the old Tek vertical amplifier inputs, Tek used a protection diode > >to prevent zenering for > >negative input overloads. > > >I've never checked to see how/whether the device conducts or not in > >this mode. > > >Is this necessary? I'm sure most of us just copied that part, > >assuming that the gates > >couldn't take as much current as could be delivered by the input > >current limiting resistor. > >With today's presumably smaller structures, it would probably be worse > >unless the > >zener voltages were proportionately reduced. > > I have a design where it's barely possible that under some transient > or startup condition I could zener a BF862 gate by a couple of hundred > uA for a short period. It's a super-sensitive node so I'd prefer not > to hang unnecessary diodes on it, or add the Johnson noise of a > resistor. The BF862 is rated for -20 and seems to zener about -24. > There's probably other knobs I can turn to keep the negative gate > voltage from getting over 20, but I was curious if anyone had any > experience, or if there was anything comparable to bipolar damage from > zenering the base. > > John Isn't there some low-capacitance over-voltage zeners/transzorps somewhere? ST?
From: Fred Bartoli on 7 May 2010 19:14
LM a �crit : > On 7 touko, 20:28, John Larkin > <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Fri, 7 May 2010 09:52:31 -0700 (PDT), cassiope >> >> >> >> <f...(a)u.washington.edu> wrote: >>> On May 5, 9:35 am, John Larkin >>> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >>>> Any comments on the behavior/effects of zenering the gate of a jfet? >>>> John >>> In the old Tek vertical amplifier inputs, Tek used a protection diode >>> to prevent zenering for >>> negative input overloads. >>> I've never checked to see how/whether the device conducts or not in >>> this mode. >>> Is this necessary? I'm sure most of us just copied that part, >>> assuming that the gates >>> couldn't take as much current as could be delivered by the input >>> current limiting resistor. >>> With today's presumably smaller structures, it would probably be worse >>> unless the >>> zener voltages were proportionately reduced. >> I have a design where it's barely possible that under some transient >> or startup condition I could zener a BF862 gate by a couple of hundred >> uA for a short period. It's a super-sensitive node so I'd prefer not >> to hang unnecessary diodes on it, or add the Johnson noise of a >> resistor. The BF862 is rated for -20 and seems to zener about -24. >> There's probably other knobs I can turn to keep the negative gate >> voltage from getting over 20, but I was curious if anyone had any >> experience, or if there was anything comparable to bipolar damage from >> zenering the base. >> >> John > > Isn't there some low-capacitance over-voltage zeners/transzorps > somewhere? ST? Yup. I've found some real nice ones. John, if you're interested email me. (can't go public on this) -- Thanks, Fred. |