AFT05MS006N part Drain current is varying.

[NarendraNasana]

Hi Team,

I used AFT05MS006N part for my design. i am operating at 118MHz with 12V VDD and 125mA of IDQ. Drain current is not constant it is varying with time hence my gain also varying. Which is not expected.

With 7.5V and 100mA IDQ also both Current and Gain varying.

Can you please suggest solution.

[LPP]

Sure it is a thermal issue. Quiescent current at fixed VGS voltage depends on the temperature.

To fix this problem a circuit should be tied to massive hit sink/heat spreader that will not allow local heating of the transistor. In addition to this, use thermal tracking gate bias circuit to keep optimum bias point in every temperature and bias voltage combination. Please refer to this application note on the topic:
https://www.nxp.com/docs/en/application-note/AN1643.pdf

Best Regards,
Pavel
NXP CAS