zenering a jfet gate
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From: John Larkin on 5 May 2010 12:35 Any comments on the behavior/effects of zenering the gate of a jfet? John
From: Jim Thompson on 5 May 2010 12:45 On Wed, 05 May 2010 09:35:47 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > > >Any comments on the behavior/effects of zenering the gate of a jfet? > >John Depends on the current and repetitiveness. Ultimately causes electromigration. I don't know what degrades with a JFET (maybe the gate gets leaky), but a similarly-structured BJT loses beta. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | The only thing bipartisan in this country is hypocrisy
From: whit3rd on 6 May 2010 15:00 On May 5, 9:45 am, Jim Thompson <To-Email-Use-The-Envelope-I...(a)On-My- Web-Site.com> wrote: > >Any comments on the behavior/effects of zenering the gate of a jfet? > Depends on the current and repetitiveness. Ultimately causes > electromigration. I don't know what degrades with a JFET (maybe the > gate gets leaky), but a similarly-structured BJT loses beta. Shouldn't it be a lot less damaging than to a BJT? There's no surface-of-the-silicon high field involved in the JFET, and the field direction is the same as normal bias (so none of the precautions against surface contamination are voided). The special-purpose two-terminal JFETs used for current regulators are completely specified for normal operation in Zener (actually, avalanche) breakdown. As long as you don't overheat them, they just go back and forth across the knee indefinitely. The JFETs in an IC amplifier, of course, might have other issues.
From: cassiope on 7 May 2010 12:52 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. HTH...
From: John Larkin on 7 May 2010 13:28
On Fri, 7 May 2010 09:52:31 -0700 (PDT), cassiope <fpm(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 |