Question regarding FET with low Drain to Source capacitance - FET_termination.jpg (0/1)

Fred,

Thank you for your response. In my setup, I use a video generator that is capable sweeping from 1 MHz to 30MHz with an amplitude of

100IRE or 714mV. I view the sweep on a oscilloscpe and observe the amplitude attenuation. In the case of this MOSFET design, the attenuation of the signal goes down -3dB(Approx. 0.5v from 714mV) at 15.4MHz. Since the signal is so small across FET's drain to source ends, it is possible that the capacitance can be high per the data sheet. In fact where I am at on the drain to source, Coss is 75pF, Ciss is 50pF and Crss is 30pF. The capacitance across G-D, G-S and D-S are:

Cgd = Crss = 50pF Cgs = Ciss -Crss = 20pF Cds = Coss - Crss = 45pF

Miller Cmout= Cgd(Av+1)/Av = 60pF with Av=1

Miller Cmin = 0 and Cgs = 0 since the source is gnd as the gate is also ground.

Therefore,

Capacitance across the circuit where the FET's Drain to Source is connected to:

Total Capacitance = Cmout+Cds +Cgs = 60pF+45pF+30pF = 135pF

Pretty close to what I measured, 137PF. Honestly, I was surprise with this large number. From all the feedback I got from everyone and my measurements, it seems that the Bipolar approach may be a better choice. Also, from one of the feedback, John, I am also concerned with the DC drift on the video signal that I also observed on the video analyzer, VM700.

If you disagree with these findings, please send me your feedback since it helps me to understandand find a proper solution to this design.

Thanks Caesar

Thanks Win for you reply,

I will look into your suggession regarding using a MOSFET with CMOS and " bipolar photoMOS optocoupler relay switches".

Thanks Caesar

LRCR wrote: > Fred, >

That Aromat AQY221 in WH's post looks like just the thing- and fully characterized to 10MHz- looks to have slow rolloff beyond that.

Thanks Fred. I will check on this part.

Caesar