Setting the Bias/Drain Current Level of HMC980

Question How do I determine the correct drain/bias current setting when using HMC980? Answer The setpoint drain current is set by the Rsense resistor which is connected between the ISENSE pin (pin 20) and ground. The equation that relates the drain current setting to resistance is Rsense = 150/Idrain (Rsense is in ohms and Idrain is in Amps). It's critical to understand that when the active bias controller is operating, the setpoint drain current always flows and is fixed regardless of whether the RF signal is absent or present or large or small it is. If you are controlling an amp that has a quiescent current of 300 mA (no rf signal present) that increases to 1 A when the amp reaches saturation (e.g. Pout = 30 dBm), you must set the drain current to 1 A if you plan on operating at an output power of 30 dBm. So the approach to determining the setpoint drain current should be. 1. Determine the maximum RF output power you will need from the amplifier. 2. Use the power sweep plots in the data sheet to determine the required current (Idd) to support this power level. Note that this current level can change with frequency. So you need to review all of the power sweep plots within your frequency range and pick the highest current that you see. 3. Plug this current into the (Rsense = 150/Idrain) equation to determine the value of Rsense Because the setpoint current always flows, active bias control can be seen as less efficient in a system where the RF output power varies over a large range. An alternative approach would be to not use and active bias controller and fix the gate voltage. In this configuration, the drain current will scale up and down as needed with the output power. However because the relationship between gate voltage and drain current varies with temperature and from device to device, this approach will result in more performance variance (primarily OIPdB and OIP3 which are affected by the drain current level) from part to part and over temperature.

Accuracy of HMC980 Bias Current Setting using Rsese

Question The HMC980 datasheet provides an equation for setting the bias current. (Rsense = 150/Idrain). If I use a high precision resistor, say, 0.1%, what will be accuracy of the resulting bias/drain current from device to device? Answer There is no a min/max specification on the accuracy of the HMC980's bias/drain current. However there is a plot in application note AN-1363 that provides some guidance on what part-to-part variation to expect. Figure 11 shows the bias current that results when six HMC980 devices were set to a drain/bias current of 100 mA. If we just focus on operation at 25 degC, we see that the current varies by device from 98.5 mA to 102 mA. This indicates a variance of approximately +/-2% range. Just to be clear, you should not take this a a min/max spec. But it does give you a sense for what level of accuracy to expect.