X-GM-THRID: 1232221858640183843 X-Gmail-Labels: rsgb lf Delivered-To: daveyxm@gmail.com Received: by 10.115.73.6 with SMTP id a6cs321885wal; Thu, 29 Mar 2007 11:31:13 -0700 (PDT) Received: by 10.67.98.9 with SMTP id a9mr1610141ugm.1175193072419; Thu, 29 Mar 2007 11:31:12 -0700 (PDT) Return-Path: Received: from post.thorcom.com (post.thorcom.com [193.82.116.20]) by mx.google.com with ESMTP id e34si2343507ugd.2007.03.29.11.31.09; Thu, 29 Mar 2007 11:31:12 -0700 (PDT) Received-SPF: neutral (google.com: 193.82.116.20 is neither permitted nor denied by best guess record for domain of owner-rsgb_lf_group@blacksheep.org) DomainKey-Status: good (test mode) Received: from majordom by post.thorcom.com with local (Exim 4.14) id 1HWzJa-0003Qh-Se for rs_out_1@blacksheep.org; Thu, 29 Mar 2007 19:25:38 +0100 Received: from [193.82.116.32] (helo=relay1.thorcom.net) by post.thorcom.com with esmtp (Exim 4.14) id 1HWzJa-0003QY-Ch for rsgb_lf_group@blacksheep.org; Thu, 29 Mar 2007 19:25:38 +0100 Received: from web25412.mail.ukl.yahoo.com ([217.146.176.230]) by relay1.thorcom.net with smtp (Exim 4.63) (envelope-from ) id 1HWzJa-0006mb-1t for rsgb_lf_group@blacksheep.org; Thu, 29 Mar 2007 19:25:38 +0100 Received: (qmail 75152 invoked by uid 60001); 29 Mar 2007 18:25:32 -0000 DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws; s=s1024; d=yahoo.co.uk; h=X-YMail-OSG:Received:Date:From:Subject:To:In-Reply-To:MIME-Version:Content-Type:Content-Transfer-Encoding:Message-ID; b=S2fa2qtcZuzyukaelSR8+JvZjy8GKN6AXtgtCZJCJm7tdWLCM3RBAKDuJDV1CuqjinTp1hprd6Q3Wky7v2cooojlTyOKdRBmcMOyN71Mdzp4Y9P2QishFXK+DJ/d9Y0A8cyoEH4l+F+8s6VpzuRquMpMvuw3u9VjPppnKXu1mgY=; X-YMail-OSG: Q.5dWlwVM1lncw4dUPTROOD37BMRO_Pbbga050rr7kVwXcA2dcfDhMGEgplN4yFrYg-- Received: from [213.78.182.231] by web25412.mail.ukl.yahoo.com via HTTP; Thu, 29 Mar 2007 19:25:31 BST Date: Thu, 29 Mar 2007 19:25:31 +0100 (BST) From: brian hodgson To: rsgb_lf_group@blacksheep.org In-Reply-To: <460B4585.7070504@telus.net> MIME-Version: 1.0 Message-ID: <400989.73836.qm@web25412.mail.ukl.yahoo.com> DomainKey-Status: good (testing) Subject: Re: LF: Why 34nb20 blocking cap? Content-Type: multipart/alternative; boundary="0-1609227511-1175192731=:73836" Content-Transfer-Encoding: 8bit X-Spam-Checker-Version: SpamAssassin 2.63 (2004-01-11) on post.thorcom.com X-Spam-Level: * X-Spam-Status: No, hits=1.1 required=5.0 tests=HTML_MESSAGE, MAILTO_TO_SPAM_ADDR autolearn=no version=2.63 X-SA-Exim-Scanned: Yes Sender: owner-rsgb_lf_group@blacksheep.org Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group X-SA-Exim-Rcpt-To: rs_out_1@blacksheep.org X-SA-Exim-Scanned: No; SAEximRunCond expanded to false Status: O X-Status: X-Keywords: X-UID: 3992 --0-1609227511-1175192731=:73836 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8bit Hi Guys - I also like to use the blocking capacitor circuit. I've had a few FETs go gate-drain short (probably from drain voltage spikes) - without the capacitor the driver chip gets the full HT voltage. - Brian (G3YKB) Scott Tilley wrote: J. This is to ensure DC doesn't drive the FETs and blow them up in the event that for some reason the exciter stops and the driver provides a constant DC signal to the FETs gate. This clamping type arrangement is to ensure only + going pulses drive the FETS and this is a critical FET safety feature. I have seen designs without it but I'm sure the users must have a box of spare FETs handy and a warm soldering iron... Jump into your way back machine and do a circuit analyst of it and you'll see what I mean. The cap AC couples the drive, the diode ensures that it is clamped as a + DC pulse train... Therefore, a constant DC bias on the gate can't happen... G0MRF was the first I noticed using this technique in amateur design of 2200m class D amps. 73 Scott PS - check your diode polarity!!! J. Allen wrote: > Hello Tech types. > > While I am working on the amp, I noticed that some of the FET drive > circuits are fed directly from a driver device (G3YXM circuit p66 LF > Today) and some use a blocking capacitor followed by a diode (G0MRF > circuit, p72). > > Why are the capacitors and diodes required in one FET circuit and not > in the other? > > Thanks, > > J. > > --------------------------------- All New Yahoo! Mail � Tired of unwanted email come-ons? Let our SpamGuard protect you. --0-1609227511-1175192731=:73836 Content-Type: text/html; charset=iso-8859-1 Content-Transfer-Encoding: 8bit
Hi Guys - I also like to use the blocking capacitor circuit. I've had a few FETs go gate-drain short (probably from drain voltage spikes) - without the capacitor the driver chip gets the full HT voltage.  - Brian (G3YKB)

Scott Tilley <sthed475@telus.net> wrote:
J.

This is to ensure DC doesn't drive the FETs and blow them up in the
event that for some reason the exciter stops and the driver provides a
constant DC signal to the FETs gate. This clamping type arrangement is
to ensure only + going pulses drive the FETS and this is a critical FET
safety feature. I have seen designs without it but I'm sure the users
must have a box of spare FETs handy and a warm soldering iron...

Jump into your way back machine and do a circuit analyst of it and
you'll see what I mean. The cap AC couples the drive, the diode ensures
that it is clamped as a + DC pulse train... Therefore, a constant DC
bias on the gate can't happen...

G0MRF was the first I noticed using this technique in amateur design of
2200m class D amps.

73 Scott

PS - check your diode polarity!!!


J. Allen wrote:
> Hello Tech types.
>
> While I am working on the amp, I noticed that some of the FET drive
> circuits are fed directly from a driver device (G3YXM circuit p66 LF
> Today) and some use a blocking capacitor followed by a diode (G0MRF
> circuit, p72).
>
> Why are the capacitors and diodes required in one FET circuit and not
> in the other?
>
> Thanks,
>
> J.
>
>


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