Iq vs VGS for AFM907N 760-870 MHZ

[khalilchiha]

Hello,

We are trying to make a design based on AFM907N for the frequency band of 760MHz-870MHz.         So we made a PCB based on the evalkit AFM907N 760-870 MHZ REFERENCE CIRCUIT. We placed the same elements at the same positions with the same values (capacitors, inductors,..) and we made the same PCB routing. We fixed the the VDS at 7.5V and we start to increase the VGS in order to fix the quiescent current at 100mA (with no RF input). However when increasing the VGS slowly, we start to get Iq=10mA at 2.44V; Iq=20mA at 2.49V; Iq=40mA at 2.55V; Iq=48mA at 2.6V then when trying to increase slowly the VGS the Iq current jump to 700mA then 1A at VGS=2.65V. after that the AFM907N is damaged. We did the test on 3 components and we get the same behavior.              With the 4th component we tried to keep the Iq at 50mA and apply an RF_in. as a result we get attenuation instead of amplification. So we tried to increase again the VGS and the Iq jump to 1A and the component was blown.

Can you help us to understand what’s happen to the RF components ?

Do you have data on Iq vs VGS ?

How we have to proceed to succefully test the component ?

[laeliaralph]

You're experiencing issues with the AFM907N design, such as a sudden jump in quiescent current and component damage when increasing VGS slowly. Attenuation instead of amplification occurs when applying RF_in. For successful testing, check design stability, use stability enhancement networks, ensure accurate PCB layout, and consult an RF design expert if needed. Let me know if you need assistance with iq test free.

[LPP]

AFM907 is a LDMOS device. The drain current dependency on the gate-to-source voltage is quadratic. Parameters of the curve can be found from datasheet Table 5 "On characteristics". I found dependency as: Id=8*(V-Vgsth)^2 [A].

Sure, the device fails because your design is unstable. As the quiescent current rises, the gain increases and at certain moment the amplifier begins oscillation at arbitrary frequency. AFM907 is conditionally stable. It can oscillate at certain source and load impedances. A design must provide some stability enhancement networks to avoid problems. You must analyze your design and modify it to improve the stability.

To keep your device during initial board bring up, some precautions are required. Perform control of output or drain voltage using spectrum analyzer or a scope, attach nominal load to the output, set power supply current protection at reasonable level (about twice of the target quiescent current), begin testing at low drain voltage (6V), increase bias voltage to set target drain current, apply input RF signal, check small-signal gain. Once you make sure there are no problems, you can proceed to next steps.

I recommend you to read "Precautions for RF Power Transistors" in the RFTOPICS from Polyfet:

www.polyfet.com/pptadb.htm 

Best Regards,

Pavel

NXP CAS