Re: LF: RE: Class E etc

[M0FMT <m0fmt@yahoo.co.uk>]

Hi Ken

 

I believe you can operate some thermionic devices in higher classes than C but they generally lack the frequency response of a modern FET. I won't go into figures but just think about it. The high efficiency output stage has to operate like a switch it is not actually an amplifier. The faster it changes state from fully on with ultra low resistance to fully off (which means a device that will operate well into the VHF) the higher the efficiency. If you have a device that is barely rated to say a couple of megs its not going to be much good (in terms of what we are talking about) at 500kc/s. Remember you are trying to get the magnetic field in the inductor to rise and collapse at a very fast rate in-order to generate a high electromagnetic field at the drive frequency. We are not talking sine waves here as a scope on the drain will show. Although a scope on the output side of the drain inductor will show quite a clean looking sine wave. Try it.

 

That chart at the end of that PDF file is very clever and is the guide I use to bring a PA into optimum performance.

 

What I was trying to say is that the math is all very well but if you are trying to make a practical and efficient CW TX for LF then there is enough empirical data from following the guide lines lines shown on the sites I have posted that will get you a good TX. If you want a doctorate in theoretical physics to bamboozle you colleagues then that will detract from the time it takes to build a very interesting high power, efficient TX design well within anybodies capabilities. The GW3UEP design being IMHO a very good (Scalable) example with no RadCom style unobtainium in the components listing.

 

73 es GL Pete M0FMT IO91UX

From: Ken <ken.h.wright@btinternet.com>
To: rsgb_lf_group@blacksheep.org
Sent: Monday, 11 June 2012, 22:09
Subject: LF: RE: Class E etc

Hi Pete.

About 40 years ago my integration was quite good, liked doing 2^nd order differential equations, partial integration and integration by parts, the one thing I had big problems with was 3 dimensional integration of a point in free space .   Looking at some of the sites my Class C MOSFET PA is probably more like Class E by default. The reason I like valves is because they are very forgiving devices unlike semiconductors, also the keep the shack and the cat warm.

 

73s

 

Ken

 

M0KHW

 

From: owner- rsgb_lf_group@blacksheep.org [mailto: owner- rsgb_lf_group@blacksheep.org ] On Behalf Of M0FMT
Sent: 11 June 2012 19:16
To: rsgb_lf_group@blacksheep.org
Subject: LF: Class E etc

 

 

Hi Ken

 

Ha ha Stefan, how good is your Integration Ken?

 

Here are a couple of sites that should answer your questions. You need a good dummy load and a good oscilloscope. Setting the conditions for the amplifier is to do with  A/ the correct driver preferably using a FET drive chip (like TC4422 non and TC4421 inverting) that will turn the FET fully on and fully off on each cycle. Very fast rise time with accurate on off timing (mark / space). Ensuring the max permitted gate voltage for the FET is not exceeded otherwise they pop, look out for transients, a diode clamp helps.  B/ Setting the shunt capacitance and inductance in the drain circuit can be done empirically See article below. Then follow it with a low pass filter, although the output wave will be surprisingly clean. Calculating the values is difficult as you don't necessarily know all the starting parameters and in any case the result is going to wide of the mark. The site below will gives some starting values. Best to use 'Scope measurements to set the output conditions. You are looking for fast, very fast On / Off rise times. Gate drive and understanding the drain circuit is the key.

These amplifiers are capable of incredibly good efficiency well above a dodgy class C amp. This means the heat dissipation from the FET is very low at high power giving cool heat-sinks!

 

The GW3UEP (do a web search) single ended design is capable good efficiency and high power depending upon the FET used and Drain supply voltage. I have modified one to run reliably at 140watts and is capable of almost twice that into a dummy load with a heat sink that is fairly cool. Adjusting L and C empirically using the 'scope to get the correct drain wave shape for max efficiency is the way to go. The article below shows you what you are looking for. I set my TX up into a 50 Ohm dummy load adjusting L and C to get the correct drain wave shape on the 'scope then measuring the peak voltage across the Dummy load. I then transfer my connector from the Dummy load to the input of the tuned transformer in the antenna up-lead. I adjust the coupling link to give the same peak voltage across the tuner link turns. You are not looking for maximum peak just the sames as across the dummy load indicating a 50ohm match. This is done by either adding or reducing the number of turns (about two turns) along with bringing the the transformer to resonance by measuring the peak up-lead current, here you are looking for the maximum peak.  

 

Oh yes and WIMA high voltage high value capacitors from Maplin work well.

 

Read the articles and search info on class E amplifiers. 

 

 

For some bed time reading but ignor the math look at the Tuning Procedure and the pictures at the end which tell it all really..... http://www.cs.berkeley.edu/~culler/AIIT/papers/radio/Sokal%20AACD5-poweramps.pdf

 

For design ideas

http://www.classeradio.com/

 

For a good practical circuit for a single ended amp that  is a scalable surefire design, no need to reinvent the wheel.

http://www.gw3uep.ukfsn.org/

 

For antennas and antenna feed methods a "look no where else site"

http://www.strobbe.eu/on7yd/136ant/

 

73 es GL Pete M0FMT IO91UX