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]
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  • +1
    •  Analog Employees 
    on Nov 19, 2020 1:21 PM 4 months ago

    Hi Sami H,

                        I think there is some confusion about the gate resistance. The gate resistance for VGG1 & VGG2 is ~ 6 meg ohms. The 1.5 k ohm resistor is an ESD resistor on the RF input. The resistor is isolated from the gate by the dc blocking capacitor.

    I am not familiar with a PWM type signal. I suspect the PWM voltage is slightly higher. Can you measure the PWM voltage using an oscilloscope  and compare it to the DMM?

    I suspect the average voltage reading of the DMM is slightly lower than the actual voltage present at the pin.

    Regards,

    Jim B

Reply
  • +1
    •  Analog Employees 
    on Nov 19, 2020 1:21 PM 4 months ago

    Hi Sami H,

                        I think there is some confusion about the gate resistance. The gate resistance for VGG1 & VGG2 is ~ 6 meg ohms. The 1.5 k ohm resistor is an ESD resistor on the RF input. The resistor is isolated from the gate by the dc blocking capacitor.

    I am not familiar with a PWM type signal. I suspect the PWM voltage is slightly higher. Can you measure the PWM voltage using an oscilloscope  and compare it to the DMM?

    I suspect the average voltage reading of the DMM is slightly lower than the actual voltage present at the pin.

    Regards,

    Jim B

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