- Joined Jan 17, 2007
https://en.wikipedia.org/wiki/Bipolar_junction_transistorTurn-on, turn-off, and storage delay
The Bipolar transistor exhibits a few delay characteristics when turning on and off. Most transistors, and especially power transistors, exhibit long base-storage times that limit maximum frequency of operation in switching applications. One method for reducing this storage time is by using a Baker clamp.
Thanks, interesting thing about the Baker Clamp...Hello,
From the wiki:
I do some real world measurements of this circuitStorage time (ts) is the time required for the BJT to come out of saturation. This is the time required for the VC to reach 10% of its high-state value (Vcc)
That arrangement has a name (which I can't remember offhand) - it works best of all if you use a Shottky-barrier diode.
Speedup capacitor.A baker clamp is the one using diodes, but what about the one using an RC at the transistor's gate? does it have a name too?
Thanks, Ian... I did explore the wikipedia/google ... but I'm not sure I understand it thoroughly...Exploring google/Wikipedia is left as an exercise for the student.
The 2369 is sometimes chosen for avalanche transistor ultra-short pulse generators.Speedup capacitor.
Note that some BJTs are designed to have a low saturation delay, expressly for their use as a switch. Specific impurities are diffused into the base region to increase the carrier recombination rate, thus reducing storage time.
The 2N2369, for example, has a saturation delay of only 15ns, a factor of ten or more improvement over most general purpose BJTs.
It's more than just the gate-source capacitance, it's the carriers generated in the base region that must be removed or recombined before the transistor turns off........................
I'm guessing that the transistor's gate behaves like a capacitor then (of course it does... the gate must be charged to a certain potential before it starts conducting... right?) but I'm still at a loss understanding why an RC arrangement at the gate (see bottom figure in post #4) speeds the transistor's turn-off time... is it because the capacitor is helping discharge the gate when the source signal is pulled down?
I think I get it now... kind of clicked when you mentioned it looks like a (very brief) short circuit...... looks like a short circuit during the switching edge for a higher base current. It works to improve both the on and off times for BJT's, FET's signal diodes, whatever.
When I serviced PC monitors, I often found the driver chip (UC3842 was the most common) drives the MOSFET gate via a forward diode, that allows the inclusion of an emitter follower to give more current drive while discharging the gate capacitance.I think I get it now... kind of clicked when you mentioned it looks like a (very brief) short circuit...
But what's that last comment about FETs? Is placing an RC arrangement at the gate advisable too? I thought that a FET's gate capacitance was one of those issues one had to deal with when driving it... so adding a cap at the gate makes it kind of confusing for me...
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by Luke James
by Luke James