100W HF AMP - issues- suggestions please!

[spriya]

Hi, 

I’m currently testing a 100W HF amplifier board but am unable to achieve the expected 100W output power. Below are the details of the test setup and procedure.

Test Setup:

• As shown in the attached picture, a Vector Network Analyzer (VNA) is used as the RF source.
• A 5W driver amplifier provides the necessary input drive level to the 100W amplifier.
• The output power of the 100W amplifier is measured using a power meter.
• The attached pictures also show the heat sink and fan mounting setup.

VNA ( AS IP SOURCE) --> DRIVER AMP --> 100W HF AMP--> 100W POWER METER

Bias Procedure:

1. The input and output ports of the 100W amplifier board were terminated with 50 ohms.
2. The potentiometer was set fully counterclockwise, and then the 50V DC supply was powered on.
3. The potentiometer was slowly adjusted clockwise until a bias current of 100mA was achieved.
4. The input termination was then removed, and the signal source (driver amplifier) was connected to the input. The output termination was replaced with the 100W power meter.

Observations:

• During step 4, when the input source was connected, the current increased from 100mA to 200mA. However, when the input signal source was disconnected, the current dropped back to 100mA. The same behavior was observed when connecting the signal generator input directly without the driver amplifier.
• Despite this, we continued with the test, turning on the RF input signal and ensuring sufficient drive level (1-2W) at the input. We observed an output power reading of 50W, but after about 20 seconds, the output dropped to 0W. The current reading was around 3.5A at 50W output. Even when we increased the input power, the maximum output we could achieve was 50W.
• No significant fluctuations were observed in the DC supply voltage or current, and the board did not heat up excessively.

We’re having trouble identifying the cause of the issue. Any suggestions on how to improve our test setup or tips on debugging would be greatly appreciated!

Thanks

Regards

Sathya 

[jcve...@gmail.com]

Hi Sathya,

Thanks for sending the photos. I can see two things that might be an issue.

1. Mounting. The stand-offs are too long they should be 1/4" (6.4mm) so that the mounting hole in the MOSFET lines up with the hole in the PCB. There is a diode, D1, on the back side of the case that should be thermally coupled to the MRF101AN to adjust the bias current as the MOSFET heats up and in your setup the diode, D1, is not near the MOSFET. 

2. Frequency range. The VNA appears to be sweeping from 35 - 70 MHz. If you are actually operating the amp in this range, you should expect reduced power. For testing, it would be best if the signal source is set to a fixed frequency in the 10 -20 MHz range. The maximum frequency for this amplifier is 54 MHz and the output power is reduced above 30 MHz.

The power fading is due to something heating up. If you are running the amp at the indicated frequencies, it's probably C2. The PCB needs a good electrical grounding connection to the heatsink. I cannot tell from the photo if the stand-offs are a conducting material and the nuts appear to be nylon insert lock nuts. All of this should be replaced by aluminum or zinc plates steel hardware with 6.35mm stand-offs.

Regards,

Jim