# 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.

## BJTs and FETs

### Bipolar family

#### Transistor behaviour, circuit styles

 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 are linear in a region, but nonlinear overall.

They 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

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

These can be seen as transistors that are triggered via light (LED-and-phototransistor combination) instead of using conductors.

Often used for isolation of currents, to avoid communicating some of the voltage noise, or to have simple (one-way) interaction 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, and the transistor's collector and emitter.
• 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, but occasionally it's rather convenient to be able to trigger both ways (e.g. from the same and from the isolated circuit).

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.