Gate Voltage Sequencing

Hi Ed,

Yes, it is safe to do what you are suggesting. To be honest, the steps described in the datasheet are more focused on how you would (carefully) turn on one of these devices in the lab. Taking the gate voltage directly to its final value is safe as long as it is done before VDD turns on. 

Have you considered using the HMC980 Active Bias Controller as an alternative?

Eamon

Hi Ed, one of the downsides of applying a fixed gate voltage to a device like ADPA7008 is that the resulting quiescent current will vary from part to part. If you look at the datasheet for ADPA7008, you will notice that performance can vary a lot vs quiescent supply current (e.g. OP1dB and OIP3, gain to a lesser extent). If you use an active bias controller such as HMC980, it will servo the gate voltage and force the current on every device to a setpoint (the setpoint current is set by a resistor on one of the HMC980 pins). So you end up with better device-to-device repeatability. 

One benefit I forgot to mention was that HMC980 will take care of safely sequencing the VDD and VGG voltages? Regarding downsides, there is one thing that you need to be aware of. When you wrap the HMC980 around the ADPA7008 and set the drain current (by attaching a resistor to the ISENSE pin (pin 20)), the current you set will always flow through the RF amplifier, regardless of whether RF power is present or not. Also, you need to set the current based on the current you expect to need at the highest output power level you expect to operate at.  Take a look at Figure 34 in the ADPA7008 datasheet. Notice that if you want to operate at saturation (32 dBm), the current increases to almost 2.4 A. So if you are going to set the drain current of the ADPA7008 using HMC980 and you want to support output powers up to 32 dBm, the setpoint current has to be set to 2.4A. Now, if during operation, you decide for some reason to back off the output power to, say, 25 dBm, you reduce the input power to around +8dBm. However, 2.4 Amps of drain current will still be flowing. So you could argue that this is a bit wasteful. If you were doing the same thing with the gate voltage fixed, the drain current would scale up and down as needed. But you are getting better device to device repeatability and built-in safe turn on/off. So it's a tradeoff. 

I'd like to try pasting in an image.

HMC980 is available in die form. HMC920 is only available in a package. - Eamon