Help required for GSM Amplifier

adnan

Hi,

I have designed a GSM amplifier using MRFE6S9125N.The band width is 915 to 960 MHz . The out put power is 40 watts. I followed matching networks given in the datasheet on FR4. This amplifier is working well without any problem. It ' gain was set  to 16  or17 db by using  VNA. This amplifier is working well for months.

After this i tried to design the same amplifier for 880 to 915 MHz band with some changing in matching network capacitors value and  location. I tested its gain on VNA and it was 18-19db (input power 0dbm from VAN ) at 24~28 volt dc and 600mA idq. I tested it at 40 to 65 watts power several times and the duration of each test  was from  1 to 2 hour with antenna and with dummy load.

Today when i turned it  on it showed no power on power meter .After debugging  i found that gate was showing short to ground.

I want to ask few questions


1) After running it model in ADS LSSP with 30 dbm input power and with 50 ohm terminations it shows that the amplifier is not unconditionally stable at 10 MHz , and also from 280 to 450 MHz. However it is stable for the gsm band

Do i need to stabilize it at for the whole band?

2) What is the reason of amplifier damage? Some times only gate is damaged and some times both drain and gate are shorted to source

3) The input power is 30dbm. Can any power level above this  damage amplifier?

4)Can i design amplifier at low gains say 15db by changing input and output matching networks?

xlking

about the damage problem, check your circuit and make sure the transient voltage at Gate/Drain is below 12/66 V, sometimes the voltage will be sudden high when you shut down the power due to the high current .
1. I think its better to make the amplifier stable in the whole band.
3. the input power of 30dbm is ok, but be careful above this level and transient voltage , notice the gate voltage should be less than 12V.
4. you should design the gain around 20dB to get high output power, which can also relieve your input power to make low input voltage.

adnan

Thanks for reply.

I want to ask another question , MRF6v2150N is a vhf amplifier with 25 db gain at 220 MHz and 23 db at 450MHz according to datasheet.
Here is a comparison of input and output capacitance of both amplifiers MRFE6S9125N and MRF6v2150n.

MRF6v2150n MRFE6S9125N

input capacitance ------------------- 163pf 350Pf
output capacitance -------------- 93pf 63pf

MRF6v2150n simulation provide gain more than 15 db at 925MHz.

can i use it for 880 to 915 MHz with reduced gain?

adnan

"about the damage problem, check your circuit and make sure the transient voltage at Gate/Drain is below 12/66 V, sometimes the voltage will be sudden high when you shut down the power due to the high current . "

I have a VARISTOR with break down voltage of 5.5v  at the gate side and a 36 volt TVS at the drain.  in case of high voltage spikes they should work?

xlking

I don't use MRF6v2150 before, but I think it is ok for 900MHz with reduced gain.

Varistor has lower speed than TVS, its response time is ns, so it can't work well under short time spikes when the frequency is 900MHz.

adnan

Thanks for reply.

Will update after solving issues.