Using AD7293 Power Amplifier Current Controller to Control the Drain Current of a GaN PA Operating in Pulsed Mode

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Question: 
 Can I use the AD7293 to control the ADPA1122 RF GaN PA (or other similar RF GaN Power Amplifiers) in closed loop mode, that is, where the AD7293 adjusts the gate voltage to regulate the drain current to setpoint value? How quickly can I turn on and off the PA when operating in closed loop mode? Does AD293 have an open loop mode where I can keep the gate voltage fixed? 
 Answer: 
 When operating in closed loop mode, AD7293 adjusts the gate voltage to regulate the drain current. So current is sensed by the shunt resistor and diff amp. It is compared to the setpoint from the DAC and the internal integrator voltage increases or decreases the gate voltage until the setpoint current is reached. It does not know or care that it is controlling an RF amplifier or that much of that dc current is being converted into RF energy. 
 
 Let's say we want to operate ADPA1113 at 46 dBm at 2 GHz. In order for Pout to reach 46 dBm, the Idd current must be 3.3A. (see Figure 30 in the ADPA1113 datasheet). So the setpoint current from AD7293 must be 3.3A. So if you are operating steady-state (i.e. loop is settled, VDD=28V, Pin=20dBm, Pout= 46 dBm, the gate voltage is around -1.5 V and the current into the VDD pin is 3.3A), it will work ok. However, you have to be super careful. 
 
 PAE, under these conditions is 43%. That means that 92.4 Watts is going into the device (28Vx3.3A) and 39.8Watts (46 dBm) is leaving the part as RF energy. So 52.6 Watts have to dissipate in the device. All good. However, if you remove the RF input power while the control loop is still active, you now have 92.4 Watts of power dissipating in the device since no dc power is being converted to RF energy. This will cause the die temperature to shoot up and potentially get into dangerous territory. 
 
 So, if you are turning the device on and off by adjusting the setpoint voltage in the DAC in AD7293, I think that it's important that the RF input power to the PA is present before you turn on the DAC and after you turn it off. 
 
 This brings us to the next challenge which is settling time. This closed loop system is going to have an associated settling time. Depending on the loop dynamics, that settling time will probably be in the hundreds of microseconds to milliseconds range (see http://www.analog.com/an-136 1). That may be ok for steady-state applications where the PA is intended to be on all of the time. But it's not fast enough for for radar applications where settling times of 1us or less are typically required. 
 
 It is possible to operate the AD7293 is an open loop mode where the Integrator is bypassed and the bipolar DAC connects directly to Bi-Vout pin. In this mode, you would set the gate voltage to give you a quiescent current of 200 mA (with no RF present). When RF is applied, the gate voltage does not change but the drain current will increase as needed. You can enable and disable the PA by switching the DAC voltage from is nominal level that give an Idq of 200 mA (-2.65) to pinch-off (around -3.1V) In this mode, the DAC voltage will have a settling time of around 1us. So that may be fast enough for some pulsed applications. 
 
 You could still monitor the drain current using the external shunt resistor to tweak the gate voltage from the DAC. But this would be done in software and would require register reads and writes.

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AD7293: FAQ