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Old 29th March 2007, 09:35 PM   #1
CBS240 is offline CBS240  United States
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Default Which circiut is better?

Hi

Both fet and bipolar have their pro's and con's. I'm trying to determine which one is better suited for a cascode VAS output, BJT or mosfet. In circuit A, the PNP's in question are monolithic duel array. In circuit B, the P-channel mosfet's are also a monolithic duel array so one can assume the transistors are equal in character. I guess it is really a comparison between the common base and the common gate for this application. It may come down to which is better at high frequencies considering dynamic capacitances (related to Vce or Vds) and compensation. Maybe just some trial and error. These 2 components would be a drop in replacement if a PCB is made. Any thoughts on this subject from you experts out there?


Thanks


EDIT: These are the components in question, bias is 3mA per leg, +/-V is 25V.
http://www.fairchildsemi.com/ds/ND%2FNDC7003P.pdf

http://www.fairchildsemi.com/ds/FM%2FFMBSA56.pdf
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Old 30th March 2007, 02:58 AM   #2
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Interesting idea,BJT and MOSFET in cascode rather than JFET.
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Old 30th March 2007, 04:22 PM   #3
CBS240 is offline CBS240  United States
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Hi

Yes well, j-fets are harder to match or find in monolithic pairs so as to have equal characteristics. J-fets generally don't like lots of volts either. The concern was the effect on the diff transistors driving the source-gate capacitance, assuming the gate voltage is fixed. Although the Cin value appears to be only 66pf in the data sheet. Somewhere I remember seeing an amp circuit that used mosfet as VAS, but I can't remember where it is.
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