How to control the rf output of an amplifier

Your amplifier doesnt have an output enable you could use a rf switch.

http://www.minicircuits.com/products/Switches.shtml

Are you using an off the shelf amplifier or making one?

Am designing an amp model for myself.

Hi Rolly. There a number of ways to control power of an RF amplifier. I can and if you want, make up some circuits, but One favorite is to use a dual gate MOS FET before the amplifier and control the Gate 2 voltage. A good FET for this is a BF1100. This is an enhancement mode device and will have no output with G2 biased to zero volts. Another method is to use RF PIN diodes. These can operate at higher power than a dual gate MOS FET. Another you may like is to use either a single ended or push pull pair of higher power MOS FETS such as the PD55008. In this you will control the gate voltage to control the gain. You will need to be sure that you don't over drive the stage to get smooth gain control. Another good one is to use the good old Motorola MC1350P. These were obsolete for a while but other manufacturers have taken them (thank goodness). Cheers.

Hi Rodney,
Thanks for your suggestions, but the amp designed here is for 10 - 1000 MHz 100W CW now to achieve the desired variable power output from the amp what circuit/module do i have to add here.?

as said earlier my requirement here is a variable pot to select four different power levels i.e. 25W, 50W, 75W & 100W & this later will be filtered out for better harmonics at the output.

now how do i get this happen, would appreciate if you can help me on it.

Regards

OK firstly let me say that to get a single amplifier to work between 10 and 1000 MHz would be quite a feat indeed. My expertise finishes at about 50 MHz or so. To get your four power levels is not very hard and what you will need to add is a reflectometer which consists of a ferrite transformer and capacitors. Its been a while since I built one but you will see them in the ARRL Handbook and other places. The reflectometer bridge circuit is used in a feed back circuit (DC) and feeds back a DC value from the forward power output terminals which represents forward power. This is then (usually amplified and fed back to the gain control stage I mentioned last time.. Sounds complex and i have simplified things here as well. However the actual circuit is extremely simple. You should have no problems finding examples you can use. .

Ok this really sounds great.!! is that fine if you can you share this actual circuit or link that you have, just need to understand how do i integrate this in my project.

I will certainly do a search on it & thanks very much for your help.

Regards

I can send you some information as soon as I can. However, you need to know that the physical structure of the reflectometer will be completely different 1t 1000 MHz that it will between 10 and say 50 MHz. You will also need a second or even a third amplifier to cover the range. The rule of thumb is about a decade of frequency coverage per amplifier. Thus say the first would normally be 106 to 30 MHz The second from about 30 to 150 MHz (at a guess this would be OK) and a third from 300 to 1000 MHz. You may get away with two from say 10 to 100 MHz and from 100 to 1000 MHz, but you may be pushing the boundaries.I can help you with information to 30MHz no problems but beyond that, you will need oher assistance. Cheers for now.

Here is some information on reflectometers you may need and some advice on what you are about to do.

Use this area for your question body