About the RF on/off time of a power amplifier!!

[waterytime]

power amp time constant issue



I have finished design a power amplifier with a LDMOS device. But the remaining problem is that the RF on/off time of the pA is required not long as 3us (by controlling the gate bias)!! This work is not what I am good at.Can anyone tell me how to realize that?

 

[vfone]

**broken link removed**

 

[waterytime]

Thank you. I’v viewed the datasheet FN8204, it is just a bias controller.

 

[biff44]

Well, a microwave fet will certainly turn on and off VERY quickly by applying the right gate control bias voltagel! There are really only 2 problems:
1) pinching off the gate will shut down most of the drain current, but not all of it. If you ae battery operated, the leakage current in a rf power fet, when pinched off, will probably still be too high
2) The switching time problem is mostly due to the gate bias line capacitors used. If you have a 1 uF capacitor to ground in the bias network, you will have to switch the voltage on that capacitor in less than 1 uS. That will take a lot of power supply current!

So the tradeoff is that you want to make any shunt bias network capacitors on the gate to be as small as possible WITHOUT letting the amp start to oscillate. The design is typically a resistor network set up between ground and the gate, and the gate to -5 volts, with resistor value ratio set up to give the deisred normal gate voltage. Then you have a transistor between the gate and the -5 V supply to pull it down quickly to -5 volts. Then, when you turn off that transistor, the resistor from the gate to ground works to pull up the gate voltage back to the steady state value. It is simply an RC time constant issue at that point.

So, you can turn OFF the rf fet quickly. But to turn it on quickly, you have to minimize the capacitance to ground, and make the resistors in the bias tree be small enough in absolute value to satisfy the RC time constant in 1 us. You might find that the -5 volt supply is supplying a lot of current when you are done.