Electronics notes/Transistors

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This is for beginners and very much by a beginner / hobbyist.

It's intended to get an intuitive overview for hobbyist needs. It may get you started, but to be able to do anything remotely clever, follow a proper course or read a good book.

Some basics and reference: Volts, amps, energy, power · batteries · resistors · transistors · fuses · diodes · capacitors · inductors and transformers · ground

Slightly less basic: amplifier notes · varistors · changing voltage · baluns · frequency generation · Transmission lines · skin effect

And some more applied stuff:

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Audio notes: See avnotes

Platform specific

Arduino and AVR notes · (Ethernet)
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Less sorted: Ground · device voltage and impedance (+ audio-specific) · electricity and humans · power supply considerations · Common terms, useful basics, soldering · landline phones · pulse modulation · signal reflection · Project boxes · resource metering · SDR · PLL · vacuum tubes · Multimeter notes Unsorted stuff

Some stuff I've messed with: Avrusb500v2 · GPS · Hilo GPRS · JY-MCU · DMX · Thermal printer ·

See also Category:Electronics.

Component families

BJT family

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:

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))
(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 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.


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

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


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

...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

See Transistor output stages

Basic amplifiers

On amplifier classes

Multiple transistor

Compound pairs

Darlington pair

Sziklai pair


Current mirror

Long tailed pair



As a diode

Pre-biased transistors