+ permal
Here is a mini teach-in covering Bipolar Junction Transistors (BJTs) (normal transistors) and Metal Oxide Silicon Field Effect Transistors (MOSFETS). To keep it simple, this teach-in only covers the two transistor types in their linear mode (amplifiers) and omits the switching mode. But have no fear, once you have got the linear mode, the switching mode is pretty straight forward.
BJTs are available in complimentary types, NPN and PNP. Likewise MOSFETs are available in complimentary types, N type and P type. The only difference between the two types is that all voltages and currents are the opposite polarity. To simplify matters only the N versions of BJTs and MOSFETs will be covered further. So there will be NBJTs and NMOSFETs.
In most fields of engineering you can form simple models of components that describe their fundamental operation, and it is very important not to let any of the more esoteric aspects cloud your view.
NBJTsAn NBJT has three terminals, emitter, base, collector.
In a linear circuit the collector is always more positive than the emitter, 5V to 50V say.
The base is like a diode connected to the emitter.
To make a diode conduct current you put 600mV across it, with the base being the more positive, If you put less than 600mV across the base emitter no current will flow, but if you put more than 600mV across the base emitter a large current will flow.
The current (Ib) flowing from the base to the emitter will be amplified by the NBJT and will cause a higher current to flow from the collector to the emitter.
The ratio of the collector current(Ic) and the base current (Ib) is the current gain of the NBJT, called HFE in data sheets.
And that completes the basic model for an NBJT.
But there is one very important characteristic to remember about NBJTs: the collector current will always be HFE * Ib, regardless of what the collector/emitter voltage is. This characteristic has some very important consequences, and is fundamental to many linear NBJT circuits.
NMOSFETsAn NMOSFET has three terminals, source, gate, drain. These are analogous to the emitter, base, collector of an NBJT
In a linear circuit the drain is always kept more positive than the source, say 5V to 50V.
The gate is completely insulated from the rest of the NMOSFET and cannot pass any current. This is radically different to the BJT base.
If the gate voltage is the same as the drain voltage no current will flow from the drain to the source.
As you make the gate more positive than the source a voltage will be reached when a small current starts to flow from the drain to the source, say 100uA. This gate/source voltage is the threshold voltage (Vth) of the NMOSFET.
As you increase the gate/source voltage so the drain/source current will increase, exponentially.
And that is it for NMOSFTs
But there is one very important characteristic to remember about NMOSfets: provided that VGS is kept constant the drain current will always remain constant regardless of what the collector voltage is. This characteristic has some very important consequences and is fundamental to many NMOSFET linear circuits.
Here endeth the first leson.