Transistor behaviour (BJT)
|This article/section is a stub — probably a pile of half-sorted notes and is probably a first version, is not well-checked, so may have incorrect bits. (Feel free to ignore, or tell me)|
Transistors have four distinct modes of operation:
|Cutoff||VB < VE and VB < VC||VB > VE and VB > VC||C-E current is 0 because it's an open circuit.|
Maximum VCE (VCE = VCC, the least flow through the collector (verify))
|VE < VB < VC||VE > VB > VC||C-E currrent is IB times hFE, i.e. input current times the gain|
meaning it's a current amplifier, also usable for amplifying signals
|Saturation||VB > VE and VB > VC||VB < VE and VB < VC||The C-E current is maximum, often close enough to short circuit|
(so) at this point it acts like a switch
also in that C-E current is already maximum, and higher base current has no further effect
|VE > VB > VC||VE < VB < VC||Current flows from E to C|
Current will be IB times the reverse gain
reverse gain is much smaller
(...and not often specced, because this is rarely used intentionally)
Note that between NPN and PNP, the conditions are just flipped.
A BJT's (forward) current gain, hFE, is ΔIC/ΔIB, a dimensionless value.
- If the input and output impedance is equal (it often is), this can be simplified to Iout/Iin, and can be given in dB
There is also a reverse gain, which will be much smaller, and is rarely characterized because it's not typically used.
When amplifying signals, note that transistors have a linear region, but are not linear from rail to rail.
In particular near 0V it does weird things, which conceptually is because the first ~0.6V are eated up by the diode-like nature of transistors. This is why you often see biasing (adding a little voltage) to put the signal you want in the linear region above that.
unijunction transistors (UJT)
Identifying a bipolar transistor's legs
Transistor behaviour (FET)
Voltage controlled resistor
On power loss
So why do we use both BJTs and FETs?
Insulated-gate bipolar transistors (IGBT)
A hybrid of the above, basically the high-current ruggedness of a Bipolar with the sensitivity of a FET
Phototransistor / optocouple / opto-isolator
A phototransistor is a transistor with amount of light controlling the base - and exposed.
- switching things on at night.
- galvanically isolated switching
- galvanically isolated communication - then often IR (and often modulated, to avoid environment light being confusing)
Optocouples are essentially a LED plus phototransistor isolated in an IC. These are often used for their galvanic isolation, e.g. avoiding ground loops, and are also useful when you want simple (one-way) interactions between circuits at different voltages.
Often appear as 4-pin or 6-pin ICs.
- The 4-pin variants give you the LED (cathode and anode) on one side, and the transistor's (collector and emitter) on the other.
- The 6-pin variant use 5 pins; it adds the transistor's base, which just works as an extra (non-isolated) trigger. In practice it may often be left unconnected
ICs with multiple optocouplers also exist.
Specs vary in details such as:
- current use
- output voltage
- how much voltage difference can be isolated
- rise/fall time (order of microseconds, so order of 1MHz speeds, slower than plain transistors)
- added components -- may e.g. be a darlington setup