Electronics > Beginners
Low voltage drop transistor (or other device)
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Zero999:

--- Quote from: permal on December 02, 2018, 09:04:27 pm ---
--- Quote from: IanB on December 02, 2018, 08:48:06 pm ---
--- Quote from: permal on December 02, 2018, 08:42:10 pm ---Wait, what? Why greater voltage drop?
--- End quote ---

Because you have to put the transistor on the correct side of the load to obtain good switching properties.

For example, an NPN BJT has to go on the low side of the load in order to be a good switch. If you put it on the high side it won't work efficiently.

Similarly, a PNP BJT should go on the high side of the load.

By similar logic, the same is true of MOSFETs. According to the type of FET you need to put it on the correct side of the load. If you put it on the wrong side it won't switch efficiently.

--- End quote ---
Ok. So, the rule of thumb is that Ibe/Igd should never have to pass the load to reach ground for the transistor/mosfet to be an effective switch. Is that correctly understood?

--- End quote ---

Sorry, I missed the schematic, when I posted previously.

The drain.collector current in a MOSFET/BJT is dependant on the potential difference between the source/emitter and gate/base. An NPN/N-channel device starts to turn on then this potential difference is positive and a PNP/P-channel device needs it to be negative to turn on. With the load connected between the source/emitter and 0V and the gatebase tied to +V, the voltage at the source/emitter will rise, thus reducing the potential difference between the source/emitter and gate/base.

In short, when the load is connected between the emitter/source and 0V, the transistor will never fully turn on, unless the base/gate voltage rises above the positive supply. Look up emitter follower.

If the emitter or source is connected to 0V, with the load between the collector or drain and +V, the fully supply voltage can be applied between the gate and source or base and emitter via a suitable resistor and the transistor can turn fully on.

Because a P-channel/PNP device conducts with the gate or base potential difference is negative, it can be used to switch the positive line, but it will reverse the logic.

I'm tired now, so ,my wording is probably poor. No doubt someone else can do a better job.
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