What are the common causes of gain drift over temperature in an RF amplifier?

Gain drift over temperature in an RF amplifier is caused by the temperature dependence of the active device's electrical characteristics and, to a lesser extent, the passive matching network components. The primary mechanisms are: transconductance change (the transistor's gain (gm) decreases with increasing temperature in most semiconductor technologies due to reduced carrier mobility, causing a gain decrease of approximately -0.01 to -0.03 dB/C for GaAs and GaN FETs, and -0.005 to -0.015 dB/C for SiGe HBTs), bias point shift (if the bias circuit does not compensate for threshold voltage changes with temperature, the operating point drifts, changing the gain; GaAs pHEMT threshold voltage shifts approximately -1 to -2 mV/C), matching network detuning (capacitor and inductor values change with temperature, shifting the impedance match and therefore the gain; ceramic capacitors can change 5-15% over a -40 to +85C range depending on dielectric type), and transmission line impedance change (substrate dielectric constant changes with temperature, altering transmission line impedance and electrical length). Over a military temperature range (-55 to +125C, a 180C span), total gain variation of 3-6 dB is common for an uncompensated amplifier. Temperature compensation techniques include: using a temperature-compensated bias circuit (mirrors or temperature-sensing networks that adjust the bias to maintain constant gm), adding a temperature-variable attenuator in the signal path, and using digital gain control with temperature sensor feedback.