SLIGHTLY EDITED NOTE FROM IAN WHITE (G3SEK) ========================================== Date: Mon, 11 Jun 2001 21:59:09 +0100 To: "Richard C. Bandla" From: Ian White Subject: Re: Transistor PA Bias Circuits Hello Rick Sorry for the delay in replying. >Greetings from Ottawa, Canada. Zack Lau pointed me to your web page because >of a question that I asked him. I really enjoyed your web info on biasing >Transistor PA's. > >I am retired from telco and with the thousands of hours of spare time >(joke) I volunteered to kit up 10 of the 222 MHz xvrtr by Zack published in >July'93 QEX magazine - that is nearly finished, i'm just waiting >impatiently for the PC boards to clear customs. > >I was thinking that with all the junk commercial vhf equipment around, that >it should be fairly easy to come up with a real cheap PA for 222 MHz rather >than dish out the cash for those pricey brick PA's. The pricey brick PAs are becoming pricier still - Mitsubishi doubled their prices last November. They say they are going to replace all the narrowband bipolar modules with a much smaller range of broadband FET modules (and one can have some sympathy with that) but the prices of the FET modules will probably be higher still. All very bad news! >The Motorola Maxor80 is >plentiful around here and I think it has a nice little 25w FM PA, with pin >diode TR switching, etc. It has both the driver and the final mounted on a >hefty heat sink. So using your article, I thought that I might try to >re-bias that PA to work as a 222 MHz amp. Of course some of the coils will >need shortening but i thought that it should be worthwhile. > >Does the driver to the PA normally need any rework in terms of biasing, etc? > Yes - every stage in the amplifier has to be biased for linear operation. >I have included a link to my web page with the schematic of the Maxor80. >http://www.qsl.net/ve3cvg/222xvrtr >If you have time (i know, i know) I'd really appreciate your comments on >the modification. > The Motorola range of mobile radios is virtually unknown in the UK, so I'm working on pure theory here... For the driver, it looks like you need to lift the cold end of the base choke L202, bypass it to ground and insert the bias circuit there. For the PA, you need to add a base choke, to make it very much like the driver circuit. The only difference is that all the current levels in the PA transistor and bias circuit will be higher than in the corresponding parts of the driver stage. To design the bias circuits using the information on the web page, you'll need to know the collector and base currents in Class C. (The base current can be measured with a low-resistance meter between the bottom of the base choke and ground.) The Class AB currents may be slightly different, but that's a good place to start. Likewise, a good starting value for the no-signal collector current would be 10-15% of the max-signal value. I hope these suggestions will be helpful. 73 from Ian G3SEK Editor, 'The VHF/UHF DX Book' 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek