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| Transistor NOT gate slow output rise time |
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| ogden:
--- Quote from: Ian.M on February 10, 2019, 09:21:44 pm ---LTspice sim of the various BJT inverter ideas presented in this topic for comparison. The 100pF speedup capacitor seems to be the winner, but it does require a low impedance drive signal and has a nasty negative going glitch on the output due to the input falling edge. --- End quote --- Try combining capacitor and diode augmentations. To limit glitch - add input current-limiting 100 Ohm resistor. |
| Zero999:
--- Quote from: T3sl4co1l on February 10, 2019, 08:04:01 pm ---BTW, 2N7000 is a pretty big MOSFET, rated 1A (more than a 2N4401) and the gate is about 200pF equivalent. (Again, the capacitance is nonlinear, but in this case it's a simpler matter: the capacitance depends on Vds, and Miller effect must be included. The total is documented on the datasheet's gate charge graph.) --- End quote --- Try putting 1A through a 2N7000 and see how long it lasts for and a gate charge of 1nC is tiny, compared to larger MOSFETs. Even with a fairly decent gate resistance, it should switch fairly quickly. http://www.mouser.com/ds/2/149/2N7002-8405.pdf |
| ogden:
--- Quote from: Zero999 on February 10, 2019, 10:11:00 pm --- --- Quote from: T3sl4co1l on February 10, 2019, 08:04:01 pm ---BTW, 2N7000 is a pretty big MOSFET, rated 1A (more than a 2N4401) and the gate is about 200pF equivalent. (Again, the capacitance is nonlinear, but in this case it's a simpler matter: the capacitance depends on Vds, and Miller effect must be included. The total is documented on the datasheet's gate charge graph.) --- End quote --- Try putting 1A through a 2N7000 and see how long it lasts --- End quote --- Right. It's DRSon = 5Ohm @ 0.2A. For 1A it means that poor TO-92 shall dissipate (more than) 5W ;) BTW specification say DC 0.2A, pulsed 0.5A. Nowhere near 1A. |
| David Hess:
If you are driving the transistor base with a CMOS output which goes to ground, then the base-emitter shunt resistor is not as effective because the series resistor can already remove charge from the junction when the CMOS output is zero and the small capacitor in parallel with the series resistor makes this even more effective. But the baker clamp with a schottky diode from the collector to base to prevent saturation and associated storage time still helps for faster turn off. |
| Zero999:
--- Quote from: T3sl4co1l on February 10, 2019, 08:04:01 pm ---the gate is about 200pF equivalent. (Again, the capacitance is nonlinear, but in this case it's a simpler matter: the capacitance depends on Vds, and Miller effect must be included. The total is documented on the datasheet's gate charge graph.) Tim --- End quote --- I think the drain-source capacitance is more of a problem than the gate, especially with a 10k load. When the MOSFET turns off, the drain-source capacitance is going to take awhile to charge via 10k. |
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