HMC7357 pHEMT gate control with filtered PWM not working as expected

Hello all,



I am working with a pHEMT amplifier, HMC7357. To control the amp I need to supply gate voltages of -1.4 and -0.8V. Ultimately I want the gate to be driven by a microcontroller's PWM output. To do so, I am experimenting with the multiple feedback amp (MFA) pictured above. The load resistor, R5, represents the gate resistance given in the amp's datasheet. To test the functionality of the MFA I first measured the drain current (Id) through the amplifier using a bench top power supply to power the gates. I then compared those results to the drain current, Id, using the filtered PWM from above.

Vgate (V) Drain Current w/ Benchtop Power supply on gate (mA) Drain current with filtered PWM on gate (mA)
-1.4 2 2
-1.11 269 330
-0.82 849 1103

Despite measuring the same gate voltage (using both an oscilloscope and DC voltmeter) the same FET, under the same drain bias (6V), drew different amounts of current depends on whether the gate was driven by the benchtop power supply or my filtered pwm output.
I checked the noise remaining on the filtered PWM output, there is still ~10mVp-p on the oscilloscope. However, a 10mV offset to the benchtop power supply isn't sufficient to make the current equal.

Initial Vgate (V) Adjusted Benchtop supply Vgate to match Id Id (mA)
-1.11 -1.06 330
-0.82 -0.72 1040

The chart above shows how much I had to adjust the bench top supply's Vgate in order to make drain currents match the PWM case. To me, its very confusing i can measure the same DC voltage across the amp (using a benchtop supply vs filtered pwm output), but measure very different drain currents. Anyone have any thoughts as to why this might be happening?

added notes: PWM is 450Hz, 5V peak, 20mA max supply. All measurements done with NO RF input.

Happy to provide more context,
Thanks in advance!



Added part number to title
[edited by: aandrews at 5:14 PM (GMT -4) on 27 Oct 2020]