# Difference between revisions of "Electronics notes/Transistors"

 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. Some basics and reference: Volts, amps, energy, power · Ground · batteries · resistors · changing voltage · transistors · fuses · diodes · varistors · capacitors · inductors · transformers · baluns · amplifier notes · frequency generation · skin effect And some more applied stuff: IO: IO and wired communication · localish communication · wireless (ISM RF, GSM, RFID, more) · 802.11 (WiFi) · 802.15 (including zigbee) Some stuff I've messed with: Avrusb500v2 · GPS · Hilo GPRS · Bluetooth serial · JY-MCU · DMX · ESC/POS notes 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)
When
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
When
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)
When
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.

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.

#### 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)

## 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 being the base.

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 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.