HMC980 bias controller used with HMC608

Question

I am using the HMC980 bias controller with the HMC608 amplifier. 
 The input power to the amplifier is 3 dBm. 
 The bias conditions are: Rset=510 ohm (Idrain=300mA) delivering to the amplifier VDrain=4.8V and VG= -3.2V. 
 We are using the amplifier in an FMCW application. The output of the amplifier appears to oscillate at particular frequencies over the sweep (9.5 - 9.7 GHz). When we lower the drain current (Rset=580ohm), the oscillation goes away. I don't know if the issue is related to the biasing IC or the amplifier. 
 At the moment I am suspecting that the amplifier is dissipating too much power and that causes some nonlinear behaviour (refer to power dissipation figure on page 3 of datasheet). 
 Any advice regarding this problem will be appreciated.

ADIApproved · Accepted Answer

Hi, 
 I think Idrain is set too low at this input drive since power amplifier's current tends to expand with higher output power. Since HMC980 is biasing the amplifier at a constant current, driving it at Pin = 3 dBm will likely saturate the amplifier causing nonlinearities. I recommend bias it at Idrain = 600 - 650mA since it is the current drawn under constant voltage biasing. 
 Regards, 
 David