Electronics notes/Transistors

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Component families

• 

  PNP BJT (bipolar junction transistor)

• 

  NPN BJT (bipolar junction transistor)

• 

  P-channel FET (field-effect transistor)

• 

  N-channel FET (field-effect transistor)

BJT family

Transistor behaviour (BJT)

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

• 

  PNP transistor

• 

  NPN transistor

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 (close to it). 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 - it's a current amplifier.
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)

Notes:

• between NPN and PNP, the conditions are just flipped.

• In precision circuits, there are a lot more footnotes to all of that.

• A BJT's gain (forward current gain), h_FE, is ΔI_C/ΔI_B, a dimensionless value.

If the input and output impedance is equal (it often is), this can be simplified to I_out/I_in, 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.

While current amplifiers, transistors are easily used as voltage signal amplifiers as well.

It then matters more that they 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).

You ten often want to use a linear-enough region. Which often starts above said diode-ish voltage, which is why you often see biasing (=adding a little voltage) to put the signal you want in that linear region.

https://learn.sparkfun.com/tutorials/transistors/operation-modes

FET family

Transistor behaviour (FET)

• 

  P-channel FET

• 

  N-channel 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

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:

• http://en.wikipedia.org/wiki/Optocouple

Behaviour and uses

Transistor output stages

See Transistor output stages

Semi-sorted

Pre-biased transistors

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

One symbol for a pre-biased transistor

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 (but don't count on it).