RF choke, why not turned-off MOS?
very large resistance and small parasitics for rf application. But we rarely see
this, why?
Gee, that is a great idea. Next time you need an RF choke that has extremely high dc resistance, feel free to use it!
But how about the settling time constant, while you power down or power up
your circuit.
RF Chokes are meant to have Very High RF impedance and
no DC impedance. They also need to be able to handle any
where from 1mA to 10 Amps of current.
An Inductor it the perfect candidate.
If you connect a rf choke to a gate of mos, it needn't to handle the dc current.
If there is no inversion layer in the channel, the DC bias of the substrate is equal to GND, so it is impossible to conduct the VB to RF_IN.
The Vb still can conduc to RF_in because of the leakage current. It's not really infinite
ohms when a transistor turned of. It may be around Gohms.
The voltage conducted to RF_in is divided by two huge resistors, one is the
turned- off channel, and the other one is reversed p-n juction. So, we need
to make sure the resistance of channel is smaller enough than junction
resistance.
What is the advantage of an turned off MOS over a RF Choke? Can you get stand alone MOS devices to put in a power supply line? How much does it cost? How about static zaps?
There are hundreds of ways to do an arbitrary design task. If you are proposing a new way to do something, there should be advantages that you can enumerate. Just because you can do something doesn't mean that you should.
Dave