Electronics notes/Transistors: Difference between revisions
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There are many ways to build an [[astable multivibrator]] (a flip-flop). | |||
Two transistors, plus some some capacitors to control speed, is one of them. | |||
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Latest revision as of 23:02, 21 April 2024
Component families
BJT family
Transistor behaviour (BJT)
Transistors have four distinct modes of operation:
Mode | NPN | PNP | Notes |
---|---|---|---|
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)) |
Active (forward active) |
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 |
Reverse (reverse active) |
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.
Gain
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.
https://learn.sparkfun.com/tutorials/transistors/operation-modes
unijunction transistors (UJT)
Identifying a bipolar transistor's legs
FET family
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
https://en.wikipedia.org/wiki/Insulated-gate_bipolar_transistor
Phototransistor / optocouple / opto-isolator
A phototransistor is a transistor with amount of light controlling the base - and exposed.
Uses:
- 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
...and more.
See also:
Behaviour and uses
BJT and FET similarities and differences
On conductance
BJTs and FETs as a switch
Simple logic
Transistor output stages
Basic amplifiers
On amplifier classes
Multiple transistor
Compound pairs
Darlington pair
Sziklai pair
Cascode
Current mirror
Long tailed pair
Semi-sorted
Oscillators
As a diode
Pre-biased transistors
Pre-biased transistors
- tend to have a resistor on the base
- A resistor on die is harder to do, so this has looser tolerance. ...but for digital switching, you won't care much.
- May have a second resistor, between base and emitter
Pre-biased transistors are mostly used to reduce component count.
The resistor values can be chosen to - effectively - limit the current through the component, which means they can protect a load, protect an input socket, or such, and the transistor may not even fry if you short them to ground without a load.