[SOLVED] Help for pushpull amplifier

[fxhfzy]

I am designing a 200W,IMD3<-40dBc amp for 57-63MHz operation with freescale transistor MRF6VP2600.
I finished the circuit building, found a stable bias at Id=2.3A,Gain=24dB(f=60MHz) and measured the VSWR with network analyze(Pin=20dBm),vswr1=1.4,vswr2=7.2 @ f=60MHz.
Then I began to increase the input power(f=60MHz),I increased Pin 2dBm everytime.Gain remained 24dB until Pout=37dBm,it droped to 4dB and couldn't recover.
I have taken the transistor down,it seems to be secondary breakdown.Is there anyone could tell me why the transistor breakdown and what shall I do next?

The schematic circuit is as follows:

The amp picture is as follows:

I have upload the datasheet and ADS simulation files

 

[vfone]

Seems that you are using a different schematic than the one recommended by Freescale as a test circuit.
I don’t know how this will affect the output impedance at high output power, but I think is good to place at the output at least the LPF matching network recommended by Fresscale (Z19, C5, Z20).

The quality of the output capacitors is very important getting a rugged PA, and is good to use the high current/voltage ones recommended by Freescale.
I don’t think the small green trim cap that you use at the output can handle the high RF voltage and current.

Also check for low frequency oscillation at high output power and different load impedances, which can damage your transistors.

 

[BigBoss]

vswr2 is too much . ( 7.2 ) therefore , the amplifier can go into oscillation...
The oscillation will decrease power handling capability of the amplifier until the oscillation disappears..

 

[vfone]

According to Freescale this device should survive at VSWR = 10:1, @ 50 Vdc, 600 Watts Peak, in pulse mode.

I would use from the beginning the recommended test schematic by Freescale.

 

[ChaoFan]

what is the price of this transistor?

 

[fxhfzy]

This transistor can be requested as samples,so I don't know the price of the transistor.

 

[ChaoFan]

Thank you! However, they don't ship free samples to small company......

Added after 1 minutes:

In my opinion, you can try BLF574, I think it's linearity is better due to my experience.

 

[Boowei]

Hi fxhfzy
Which company do your 25 ohm coax(TL2,TL3) buy from? what is the type?
I want to design a pushpull amplifier(30-512MHz,P-1cw=120W).

 

[Chinaski]

Hi Boowei
In Europe and for small quantities I use Barend Hendriksen site (Holland)

**broken link removed**

Good luck