Electronics notes/Transistors

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This is for beginners and very much by a beginner. It's meant to try to cover hobbyist needs, and as a starting point to find out which may be the relevant details for you, not for definitive information.

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See also Category:Electronics.

BJT family

Transistor behaviour (BJT)

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

Transistors have four distinct modes of operation:

  • Cutoff
NPN: Vb < Ve and Vb < Vc
PNP: Vb > Ve and Vb > Vc
C-E is an open circuit, nothing happens.
Maximum VCE (VCE = VCC, the least flow through the collector (verify))
  • Active (a.k.a. Forward active)
NPN: Vc > Vb and Vb > Ve
PNP: Vc < Vb and Vb < Ve
The C-E currrent is hFE*IB i.e. proportional to the current into the base, amplifying current
  • Saturation
NPN: Vb > Ve and Vb > Vc
PNP: Vb < Ve and Vb < Vc
The C-E connection is essentially a short circuit already. The transistor acts like a switch
Saturation also describes the fact that increasing the current on the base no longer has an effect on the C-E current (because it's already maximum).
  • Reverse (a.k.a. Reverse Active)
NPN: Vb > Vc and Ve > Vb
PNP: Vb < Vc and Ve < Vb
the gain in this direction will be much smaller
this is rarely used intentionally

When amplifying signals, note that transistors are linear in a region, but nonlinear overall, and you often want to avoid 0V with a little biasing.



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.

As a switch

High side versus low side switching/driving
This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)


As a diode

Identifying a bipolar transistor's legs

FET family

Voltage controlled resistor

Transistor behaviour (FET)

Insulated-gate bipolar transistors (IGBT)

A hybrid of the above, basically the high-current ruggedness of a Bipolar with the sensitivity of a FET


Transistor behaviour, circuit styles (shared, differences)

Compound pairs

Simple logic

Phototransistor / optocouple / opto-isolator

A phototransistor is a transistor with amount of light being the base.


  • 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 typically 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
  • added components -- may e.g. be a darlington setup

...and more.

See also: