A mosfet output stage has an input impedance that at DC to low frequencies is in the range of maybe 10,000 Megohm plus. Of course as you start to drive it with higher frequencies and higher slew rates, a bit of effort is required to charge and discharge the internal mosfet capacitances.
I have been messing around with a paraphase driver / phase splitter that simply uses a 470 ohm above and below a smallish mosfet, the whole thing running off a 100v rail with 25v dropped across each resistor so there is about 50mA flowing. As the fet is turning on the drive current is only really limited by the fet itself, but as it is turning off the drive current depends on the 470 ohm resistors. The required output is +/ - 20v peak per each output.
The output fets, (STY34NB50) http://us.st.com/stonline/books/pdf/docs/6005.pdf running in **source follower** mode have a Ciss = 5900 pF, Coss = 880 pF, Crss = 80 pF. Are the 470 ohm resistors low enough or is there anything much to be gained by going lower?
GP.
I have been messing around with a paraphase driver / phase splitter that simply uses a 470 ohm above and below a smallish mosfet, the whole thing running off a 100v rail with 25v dropped across each resistor so there is about 50mA flowing. As the fet is turning on the drive current is only really limited by the fet itself, but as it is turning off the drive current depends on the 470 ohm resistors. The required output is +/ - 20v peak per each output.
The output fets, (STY34NB50) http://us.st.com/stonline/books/pdf/docs/6005.pdf running in **source follower** mode have a Ciss = 5900 pF, Coss = 880 pF, Crss = 80 pF. Are the 470 ohm resistors low enough or is there anything much to be gained by going lower?
GP.