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## [AMPS] Re: Drive Level in amplifiers

 To: [AMPS] Re: Drive Level in amplifiers jono@webspun.com (Jon Ogden) Tue, 24 Mar 98 19:41:06 -0600
 ```Dick, You make a lot of sense here. You're right now that I think about it. The whole conjugate match idea is based on linear amplifier designs. And for a class AB or B amp, you do not have a linear situation. I have to admit I am beginning to be a bit embaressed in all this! In my college days, the onlt type of amplifiers that were ever covered in detail were class A (and I specialized in RF at U of I Urbana!). And in my design engineer days for Motorola, all I designed were Class A amplifiers. So my paradigm is Class A and that is all I think about. Now I have to sit and rethink things a bit. I am beginning to see why the input impedance of the tube does vary with drive. To quote Pooh Bear, "Think, think, think, think......." :-) 73, Jon KE9NA >[Richard W. Ehrhorn] > >Oh, boy! I've been biting my lip and staying out of the discussion of Zin >variation versus drive power - and I agree with George. > >BUT I can't resist commenting on the "conjugate match" issue, even though I >do know better. > >Unless the basic electrical engineering principles that I was taught in >EE101 (probably out of Terman or Ryder) have been repealed, Thevinen and >similar equivalent circuit principles on which the "conjugate matching" >idea is based are specifically defined as applying only to LINEAR devices >and circuits. > >In class AB, B, and C power amps such as discussed here THE TUBES (or >transistors) ARE NOT LINEAR DEVICES. Plate current flows over much less >than 360 degrees of the rf cycle and is OFF the rest. The tube is as much a >switch as a linear device. > >In fact, since the source R and load R are equal in a conjugate match, half >the total available power is dissipated in Rs and only half can be >delivered to RL: maximum theoretical efficiency = 50%. A tube running class >A draws relatively uniform Ip over the entire rf cycle; it IS a linear >device. If I remember right, maximum efficiency typically realized in a >class A amp is around 30%. > >IMHO the most fundamental way to define the function of the input matching >network of a class AB2 or B amp is that it (plus all the circuitry >intervening between it and the plate/collector/drain of the driver devices) >must transform the input impedance of the power tube(s) into whatever load >impedance those driver devices must see to deliver their desired output. >Alternately, since nearly all transceivers are designed to work best into a >nominal 50 ohm resistive load, the power amplifier input network should >present a 50 ohm resistive input. > >I know there are many fans of the conjugate matching idea out there, but no >one has explained credibly to me how a 50 ohm SOURCE resistance would allow >more than 50% of the total drive power to be delivered to the load. The >remarkable concept of a NON-DISSIPATIVE Rs has been offered, but it's sort >of beyond my comprehension. > >During preparation of the amps chapter of the 1995 ARRL Handbook I included >this argument, but the issue apparently had been so controversial that >League diplomats thought it best to forgo that discussion. Probably I'll do >the same hereafter! > >73 to all! Dick W0ID (ex-W4ETO) ------------------------------------- Jon Ogden KE9NA http://www.qsl.net/ke9na "A life lived in fear is a life half lived." -- FAQ on WWW: http://www.contesting.com/ampfaq.html Submissions: amps@contesting.com Administrative requests: amps-REQUEST@contesting.com Problems: owner-amps@contesting.com Search: http://www.contesting.com/km9p/search.htm ```
 Current Thread [AMPS] Re: Drive Level in amplifiers, Rich Measures [AMPS] Re: Drive Level in amplifiers, Jon Ogden [AMPS] Re: Drive Level in amplifiers, km1h @ juno.com [AMPS] Re: Drive Level in amplifiers, Phil Clements [AMPS] Re: Drive Level in amplifiers, Rich Measures [AMPS] Re: Drive Level in amplifiers, Richard W. Ehrhorn [AMPS] Re: Drive Level in amplifiers, Jon Ogden <= [AMPS] Re: Drive Level in amplifiers, Phil Clements