RF HIGH Power amplifier

[aks.rfms]

I TESTED THE RF POWER AMPLIFIER WHICH IS RATED OF 250 W CW.
the optimum power received with three stages is 53.2 dBm in simulation but while testing i got it only 51 dBm and after that it went to hard core saturation.
Devices used is free scales.
Kindly let me know how to enhance the gain and bring out the power level to 53 dBm and same time have to bring the chain from saturation.
Its 3 stage to achieve 250 W CW and at the freq of 1.35 GHz.

individual gain io the pri-amplifier is enough as below

first - 15 dBm
second- 17 dBm
third - 21.5 dBm

kindly help me to solve this.

 

[vfone]

First of all the gain should be rated in dB, and not dBm.
I don't know how did you count the gain of the stages, but normally the lowest gain should get from the highest power stage (output), and the highest gain from the input stage.
At this high power and relative high frequency there are many places where you can lose 2dBm of output power, like PCB layout, mismatch losses, impedance matching, etc.

 

[BigBoss]

As the final stage may enter into saturation, second or even the first stage can also enter into saturation.If you have already designed first and second stages carefully with a reasonable saturation margins, it should be checked the final stages' conditions ( load impedance, matching, supply, OP etc. ).
It's hard to say without more information..

 

[davenn]

and what is the device(s) ??

 

[Osman Ceylan]

Did you consider heating effect of the transistor at the simulation? It causes power and gain changes.

 

[aks.rfms]

no sir... how to considerd it in simulation?

 

[aks.rfms]

In the rf amplifier design....

I am using ADS 2012 and using device MW6S004NT..

In the DC analysis ..
How to select the bias point if i want the max PAE with optimum power gain.

 

[vfone]

To get maximum PAE is not enough to do a DC bias analysis. You have to do also a load-pull simulation.

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[aks.rfms]

sir , if in the data sheet its mention that device with

1-2000 MHz CW
4 watts.
28 Volts,

Where the max. rating is mention as -0.5 to +68 Volts
.
then whats is the meaning of 28 Volts ,whether its compulsory to make it operate on 28 VDC or we can vary it also?
on what factor it may depends and what may be the effect if we increase it and decrease it?

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To get maximum PAE is not enough to do a DC bias analysis. You have to do also a load-pull simulation.

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hello sir, if in the data sheet its mention that

1-2000 MHz
4 Watts
28 Volts
CW RF Power MOSFET

and maximum rating range is given as -0.5 to +68 VDC

Then what is the meaning of this 28 Volts.??

Whether its compulsory to bias the device with 28 Volts.or we can vary it also.???

on what factor it may depend.????

and if i increase it and decrease it that what may be the corresponding effects.??????

 

[Dan Mills]

Consider the drain voltage when the fet is turned off, with a correctly tuned load the drain voltage will swing up to roughly twice the DC value, so a 28V part will probably have the drain voltage peak at somewhere around 55V in saturation, and that assumes a correctly matched load.

In the event of load mismatch you can see way more voltage then that on the drain, and 68V is not that far away if you have a pathological load.

28V is the voltage at which they measured the device behaviour, all the parameters are voltage sensitive to a greater or lesser extent, move too far from this and you need a network analyser to find out what the device parameters are at your new voltage.

You can lower the supply voltage but check the graphs in the datasheet as the device may or may not be qualified at lower voltage, usually the complex part of S21 gets worse as the reverse transfer capacitance increases at lower drain voltage, and obviously the power available decreases for any given drain match, but it can buy you much better reliability into badly behaved loads.

Regards, Dan.