Help me design a Power Amplifier Class C at 60MHz

[galilu]

mrf171 amplifier

Hi

I need to design PA Class C @ 60MHz, with output power 10Watt.
using GMSK signal (linearity not issue), I failed to get stable with high efficiency in same time , my procedure:
1)- input matching network.
2)- RF transistor (MRF171)
3)- output matching network
4)- LPF

Please I need help

 

[flatulent]

Re: Power Amplifier ...help

Stability is another criteria to design for. It does not come automatically with matching for the desired operating characteristics. It is caused by a combination of internal feedback (S12) of the transistor and external feedback from the board layout.

There are many ways to make the amplifier stable. They involve having the source and load impedance out of band be what is needed. Another method is to put a RLC feedback network from base to collector or gate to drain.

 

[galilu]

Re: Power Amplifier ...help

hi flatulent

I'm really not consider feedback/forward technique, since it complicated and need alot of optimization as I think. My aim is to build PA work properly with no damage :wink: , as transistor blowup be easily these day and take the experience of RF design.

If you can please give me rough analysis/method to get stable PA, it will be nice from.

Thanks in advance...

 

[flatulent]

Re: Power Amplifier ...help

This RLC network is standard practice on many amplifiers and is not a linearizing method. It shapes the gain out of band. There is no simple solution. Power transistors have data sheets with recommended circuits. Start there.

Attached is a screen dump of the power transistor links I have.

 

[goodboy_pl]

Re: Power Amplifier ...help

mosfets are high gain so try to decrease input resistance by means a parallel resistor at gate!
in this way your PA will be stable.

BEST!

 

[galilu]

Re: Power Amplifier ...help

hi

I'm using MRF171A, problem appear in matching circuit...when I get tuned operation around the operating frequency gain increase significant and I have stability problem. in Cripps book, he say that adding small Resistance will give unconditional stable operation :roll: ...Ok, after add this part I have the surprise, gain fall and some time I get dissipated PA.
my head burnout, as well as my PC...

My request to add some tweaking thing to get stability and max. gain (It's recommended for me to avoid feed back)

Thanks in advance...

 

[BigBoss]

Re: Power Amplifier ...help

hi

I'm using MRF171A, problem appear in matching circuit...when I get tuned operation around the operating frequency gain increase significant and I have stability problem. in Cripps book, he say that adding small Resistance will give unconditional stable operation :roll: ...Ok, after add this part I have the surprise, gain fall and some time I get dissipated PA.
my head burnout, as well as my PC...

My request to add some tweaking thing to get stability and max. gain (It's recommended for me to avoid feed back)

Thanks in advance...

Don't use conjugate matching circuit at the output. This will give you the maximum gain,but NOT the power. Unstability may come from....
Instead of that , you have to use "Power Matching at that level and frequency"

Unstability might have a reason that appears from either input/output matching.

Last point....before implement that amplifier, simulate with a certain simulator and check the either input and output stability cicles.

 

[goodboy_pl]

Re: Power Amplifier ...help

as i mentioned, one way to decrease oscillation is to decrease the power gain by reducing input parallel resistance if it does not solve the problem, check:

1. is yor matching network is low Q?
if not there is spurious oscillation danger.

2.are u using inductors or beads for supply rf de-coupling(supply filter)?
if yes perhaps it is the source of oscillation, u can solve this problem by paralling a resistor with the chock to decrease the Q factor the inductor and reduce oscillation possiblity.

for more practical infos refer to:
http://www.polyfet.com/rftopics.pdf

BEST!

 

[galilu]

Re: Power Amplifier ...help

Thanks all for advice...

Dear BigBoss,
I'm still in my first step in RF design, last information I have about matching are matching!!! Power matching as I get from search considered for Tube PA. Please correct my information’s.
About the simulator, I'm already make use of the wonderfully, most easy simulator, ADS. It’s perfect and save alot of time in measurement setup as well as the tuning property.

Dear goodboy_pl,

This advice will be considered, and really appreciated , in implementation. Honestly, I didn’t reach this level in ADS to simulate beads de-coupling, but I will try to learn it soon.

 

[lincoln123]

Re: Power Amplifier ...help

POwer amp needs a feedback network to be stable