Hello,
I am trying to create a steady DC output to drive some pHEMTS, HMC7357. To control the gates I need to produce -1.4, and -0.8V and supply about 0.5-1 mA. To do so, I was planning on using a filtered PWM output from a microcontroller, followed by a -1 inverting op-amp circuit. I was playing around in LT-spice and i got some results I didn't expect. Perhaps someone can help me understand what is happening and why.

Above is a screen capture of my schematic and simple transient analysis. I am using a op-amp model from the TI website for the LM358. The input is a 500 Hz 50% duty cycle square wave with a magnitude of 5V. I am not plotting the input on the graph because the X axis time scale is long. The circuit acts as one expects, the PWM signal is filtered at each stage, V1,V2 and the output, Vout, is a steady -2.5V.

However, when i change the value of the feedback resistors, now 1K instead of 1 Meg, the value of the output changes drastically. From what I understand about IDEAL op-amps it shouldn't really matter what value of feedback resistors I use? clearly I am missing something fundamental to op-amps.
BONUS question: I tested the first circuit, with 1Meg feedback, on a breadboard. Without the pHEMT connected, I got the -1.4 and -0.8V as simulated and expected. Connecting the circuit to the pHEMT's gate:
With no driving power to the pHEMT, meaning virtually no current DC to the gate, the outputs work as expected.
When I apply a RF power, ~20 dBm causing at most 1mA gate current, the PWM output from the AVG drops, subsequently dropping both of my voltages, expected -0.8V to -0.7V and -1.4 to -1.3! None of my circuit elements should be drawing much, if ANY current, does anyone have any clues?
Happy to provide and more context,
Thanks for any and all help,
Sami
I am trying to create a steady DC output to drive some pHEMTS, HMC7357. To control the gates I need to produce -1.4, and -0.8V and supply about 0.5-1 mA. To do so, I was planning on using a filtered PWM output from a microcontroller, followed by a -1 inverting op-amp circuit. I was playing around in LT-spice and i got some results I didn't expect. Perhaps someone can help me understand what is happening and why.

Above is a screen capture of my schematic and simple transient analysis. I am using a op-amp model from the TI website for the LM358. The input is a 500 Hz 50% duty cycle square wave with a magnitude of 5V. I am not plotting the input on the graph because the X axis time scale is long. The circuit acts as one expects, the PWM signal is filtered at each stage, V1,V2 and the output, Vout, is a steady -2.5V.

However, when i change the value of the feedback resistors, now 1K instead of 1 Meg, the value of the output changes drastically. From what I understand about IDEAL op-amps it shouldn't really matter what value of feedback resistors I use? clearly I am missing something fundamental to op-amps.
BONUS question: I tested the first circuit, with 1Meg feedback, on a breadboard. Without the pHEMT connected, I got the -1.4 and -0.8V as simulated and expected. Connecting the circuit to the pHEMT's gate:
With no driving power to the pHEMT, meaning virtually no current DC to the gate, the outputs work as expected.
When I apply a RF power, ~20 dBm causing at most 1mA gate current, the PWM output from the AVG drops, subsequently dropping both of my voltages, expected -0.8V to -0.7V and -1.4 to -1.3! None of my circuit elements should be drawing much, if ANY current, does anyone have any clues?
Happy to provide and more context,
Thanks for any and all help,
Sami