John Larkin a écrit :
Hey, I just used one (OK a 3906) and a mosfet wouldn't have worked. Maybe the 6SN7 would have worked, because I needed that grid current too at positive bias...
-- Thanks, Fred.
In addition to the other suggestions (lowering Rcollector and reducing base drive), you might consider adding an emitter resistor, 0.05xRcollector, in parallel with a capacitor, so the input can pull charge out of the base. It's less expensive than a Baker clamp.
Yeah, I'll have to order some, I've only got power mosfet's on hand. But honestly, at the moment the npn with diode is just fine.
George H.
OK, That's interesting. How does that help get the the charge out of the base-collector junction? (From the limited reading I was doing over the weekend that seems to be the cause of the slow time response due to saturation.) But I'll give it a try...
George H.
If the stage driving this transistor is low impedance in the low level, it's another path to sweep charge out of the base. You don't want the charge swept out the emitter because that requires Beta(+1)*charge. However, this won't often be as good as not allowing the saturation charge to build up in the base (Schottky clamp) in the first place.
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OK the emitter resistor just keeps everything a bit above ground, and that adds a bit of 'push' when the base is grounded? The Schottky does this too.. well at least keeps the collector further off ground.. (yeah, you know all this.) I found a bag of 2N7000's in a drawer today. That and a 1k drain resistor worked great. Should I put a bit of R in gate lead?
George H.
That's the way I see it. It increases the output impedance (and low level) of the driver. It's not something I've ever done.
ground..
Its purpose is to keep the base-collector junction from forward biasing. Yes, it also adds some to Vce(sat).
I don't see how it will help, in this case. There isn't the charge that can be swept out. I suppose it helps some to take Vgs negative.
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