Return-Path: Received: (qmail 17506 invoked from network); 7 Mar 2003 17:17:26 -0000 Content-Transfer-Encoding: 8bit Received: from netmail01.services.quay.plus.net (212.159.14.219) by mailstore with SMTP; 7 Mar 2003 17:17:26 -0000 Received: (qmail 16895 invoked by uid 10001); 7 Mar 2003 17:17:25 -0000 X-Priority: 3 X-MSMail-Priority: Normal Received: from post.thorcom.com (193.82.116.70) by netmail01.services.quay.plus.net with SMTP; 7 Mar 2003 17:17:25 -0000 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 X-SQ: A Received: from majordom by post.thorcom.com with local (Exim 4.12) id 18rLS7-0003lX-00 for rsgb_lf_group-outgoing@blacksheep.org; Fri, 07 Mar 2003 17:16:11 +0000 Received: from [147.197.200.9] (helo=hestia.herts.ac.uk) by post.thorcom.com with esmtp (Exim 4.12) id 18rLRy-0003lO-00 for rsgb_lf_group@blacksheep.org; Fri, 07 Mar 2003 17:16:02 +0000 Received: from gemini ([147.197.200.44] helo=gemini.herts.ac.uk) by hestia.herts.ac.uk with esmtp (Exim 3.22 #1) id 18rKQO-0002iV-00 for rsgb_lf_group@blacksheep.org; Fri, 07 Mar 2003 16:10:20 +0000 Received: from [147.197.232.252] (helo=rsch-15.herts.ac.uk) by gemini.herts.ac.uk with esmtp (Exim 3.33 #1) id 18rKQM-0005Jh-00 for rsgb_lf_group@blacksheep.org; Fri, 07 Mar 2003 16:10:18 +0000 Message-ID: <5.1.0.14.0.20030307152021.00a5d568@gemini.herts.ac.uk> X-Sender: mj9ar@gemini.herts.ac.uk X-Mailer: QUALCOMM Windows Eudora Version 5.1 Date: Fri, 07 Mar 2003 16:10:13 +0000 To: rsgb_lf_group@blacksheep.org From: "James Moritz" In-reply-to: <29BDD4F529FCD311B631009027357C4E07F4ED5E@btss103a.swh.sk> MIME-Version: 1.0 X-MailScanner: No Virus detected Subject: Re: LF: Preamplifier...G3LNP loop Content-Type: text/plain; charset=us-ascii; format=flowed X-Spam-Status: No, hits=-9.8 required=5.0tests=EMAIL_ATTRIBUTION,IN_REP_TOversion=2.50 X-Spam-Checker-Version: SpamAssassin 2.50 (1.173-2003-02-20-exp) X-SA-Exim-Scanned: Yes Sender: Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group X-SA-Exim-Rcpt-To: rsgb_lf_group-outgoing@blacksheep.org X-SA-Exim-Scanned: No; SAEximRunCond expanded to false At 13:42 07/03/2003 +0100, you wrote: >Hi LF-ers... > >Just a question for any user of the G3LNP loop (Radcom 99). I replaced >2N3819 (not available in Slovakia) fets with BF245. The current of the >complete preamplifier is only 10mA with 6V on drains of fets and 13V on NPN >TR3/TR4. I'm not sure if it's enough. > >73 de Rich OM2TW The FET bias sounds OK, but there is a flaw in the design of the G3LNP preamp - with the biasing scheme used, only one of the output NPN transistors will be biased "on", depending on which FET has the higher drain voltage. This will lead to increased distortion. The solution is to: -Remove the 1kohm/1uF between the output transformer centre tap and ground (C4 and R5 in the LF experimenters handbook), and ground the centre tap directly. - Add 2kohm resistors in parallel with 0.1uF capacitors in series with the emitters of each output transistor and the transformer terminals. The 2 output transistors are now independently biased, so small differences in FET characteristics will only slightly unbalance the output stage. The output stage current will be quite low, about 3mA in each transistor, which will lead to distortion products if large signals are present at the output; reducing the 2kohm resistors will increase the bias current. 470ohms will give about 12mA per device, which would increase the available output roughly 12dB before distortion occurs. The drain voltage of the FETs will depend on the individual FET Idss, and the FETS should be selected for drain voltage of about half the supply voltage (Idss ~ 3mA), so 6V sounds about right. The collector voltage on the NPN output devices will be equal to the supply voltage. The emitter voltages should be about 0.6 - 0.7V less than the drain voltages. Hope this makes sense... Cheers, Jim Moritz 73 de M0BMU