![]() ![]() The only time you really have it used is in power devices, where the base is reverse biased during turn off to turn off the device faster, as then the charge in the base region is actively removed.Turn on voltage might be a peak of 2V at 3A base current, and turn off is a peak reverse voltage of around the same magnitude, provided by a driver transformer. I used a high gain transistor (probably BC 817-40 with a \\beta\ of 400) and a base current drive of about ten times the collector current - i.e. Reverse biased operation is rarely specified, as most of the time transistors are not operated in there, it is more a stand off voltage set that it will survive. The saturation voltage of the transistor affected the output voltage of the divider. If you get it hot enough ( over 150C) then the leakage is going to be enough to turn it on. It is very easy to have leakage current charge the very high impedance mosfet up to conduction ( or to keep it there if it is on and there is no discharge path) while the BJT needs either enough collector voltage so that leakage turns it on or really high gain ( darlington without integral discharge resistors) so that even low leakage is enough to do the same. actually currently i put 5V dc-dc converter there driving some other load.Ĭurrent operated BJT and voltage operated MOSFET. please assume its just a normal load like resistor etc. ![]() ![]() i quick drew it in tina spice and i dont see any proper symbol to use. ps: circuit and pnp datasheet are provided below for reference. its out of my expectation, although the pnp switch works, but i certainly missing something and i cannot guarantee that the PNP is in saturation mode, and i cannot guarantee it will dissipate less heat (efficient) for higher load. i've changed R1 R2 to different value closed to the calc value but the measured Vbe is still the same, Vbe measured to be around -0.3V only. This is what I know about NPN BJTs (Bipolar Junction Transistors): The Base-Emitter current is amplified HFE times at Collector-Emitter, so that Ice Ibe HFE Vbe is the voltage between Base-Emitter, and, like any diode, is usually around 0,65V. thinking there will be much current going from 15V to GND, i upped the value to R1 = 1K, R2 = 3K. my heart content says i want Vbe to be around -5V max, Ib max is 10mA, so i made some unknown calculation resulting R1 = 500ohm, R2 = 1.5Kohm approximately. R1, R2 also will make a voltage divider to avoid Vbe greater than max rating ie around -5V is the idea correct? so i made a circuit using 2SB1443 PNP. I want to make a high pnp switch to a load 0.5-2A, but to ensure proper saturation operation to avoid heat waste (as in linear mode operation), i need to set R1, R2 value properly. ![]()
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