Transistor Families

Some very very basic info regarding transistors.
(I'm writing this for a friend who is just beginning electronics so skip through if needed).

You will find "trannies" everywhere when it comes to synthesizers.
They are a fundamental building block of electronics & are great for making switches and for amplification.

There are two main types:
1) BJT's or Bipolar Junction Transistors (Current controlled)
2) FETs or  Field Effect Transistors. (Voltage controlled)

1)The BJT's are the most common.
    They are made by joining pieces of P-type to N-type material.
     (P-type & N-type materials are found in diodes).
    You will probably recognise BJT's as either NPN or PNP.

The BJTs have 3 leads : the base, the collector, and the emitter.
 These are are current-controlled devices (not voltage). This means that a small current entering in the base region of the transistor causes a much larger current flow from the emitter to the collector region.
You can see why these are so useful as switches and as amplifiers.

NPN = The majority current carrier are electrons.

(Here, the base is made of P-type material)

Probably the most common general purpose NPN transistor I have in my store is the 2N3904.
Its great for driving LEDs, making noise & current sources, and for amplification

PNP = the majority current carrier are holes.
(Here, the base is made of N-type material)

 Probably the most common general purpose PNP transistor I have in my store is the 2N3906

In both these BJT's current flows from one terminal to another (under the influence of current at a third terminal).
A small current at the third terminal controls a much larger current.at the other 2 terminals.
The "Base" is the controlling terminal.
The arrow indicates which direction current is flowing through the transistor.


------------------------------------------------------------------------------------

2) FETs can be divided into two further groups:
     a) JFETs - Junction FETS.
     b) MOSFETs --- Metal Oxide Semiconductor FETS

The main thing to remember is that FETS are voltage controlled. (unlike BJT;s which are current controlled).
FETS are really good in switch applications, as amplifiers and as voltage controlled resistors
They have 3 regions: a gate, a source, and a drain.
There is a channel or path between the source and the drain terminals.
A voltage placed at the gate controls current flow from the source to the drain of the transistor.

FETs can have an n-type or p-type channel.

Field Effect transistors have very high input impedance..... meaning the FET's gate input draws very little current from a circuit's power source.

The disadvantage of FETs over BJTs is that they won't provide the same amplification that could be gotten from bipolar transistors. Also, their very high input impedance (making them very sensitive to input voltage signals) means that they can be easily damaged by static electricity.

JFETs

Out of these two subgroups the most common one I come across in the synth world is probably the JFET.
The MOSFET however has an even higher input impedance than the JFET.... thus even less loading in a circuit.
The problem with JFETs is that it is much harder to make consistent JFETs than to make consistent bipolar devices. Therefore matching them is important.
 Here is a good link all about matching JFETs
http://djjondent.blogspot.com/2016/10/all-about-jfets-matching-for.html


A JFET is usually ON when there is no voltage between its gate and source terminals.
A voltage at the gate increases the channel resistance and reduces the drain-draw source current.

MOSFETs

Metal-oxide semiconductor FETs are really important in computers.
Most memory ICs are arrays of thousands of MOSFETS.
They are cheap, really easy to make, consume negligible power and can be made extremely small.
They are either N or P type.
Unlike JFETs, the gate of a MOSFET has no electrical contact with the source and drain.
(there is a layer of silicon-dioxide separating the gate from the rest of the trannie)
-------------------------------------------------------------------
Transistor Package  types
Transistors come in many shapes and sizes.
TO = Transistor Outline
Sometimes the component you need can be purchased in more than 1 package type and the leads may have different positions and orientations.

Probably the two most common through hole transistor packages you will come across will be the TO-92, the TO-18 and the TO-220.

Terminal identification [BJT]; Pin 1 Emitter, Pin 2 Base, Pin 3 Collector.
It's got a flat face & 3 leads.Note that not all transistors use this ID.
Because of its small size, its not great for dissipating heat and therefore not recommended for devices with high power consumption.

Below is an example of a TO-18 package

The above example is an Z414 IC, but lots of transistors also use this package.
It's made of a metal case (or can).
The tab is located 45° from pin 1, which is typically the emitter.... in the case of a BJT
If this is a JFET, then pin 1 will be the source.

When looking at stuff on data sheets, always be aware of the perspective. If you are looking from the top through the package, the pins may be shown in reverse. Just remember the metal tab marks pin 1.
Next closest to pin 1, is pin 2. 
 -----------------------------------

 Next is the TO-220 package

This package is used in high power applications. TO = Transistor outline but you will find this used in lots of other types of components such as voltage regulators and integrated circuits
The metal tab at the top is a heat sink. This metal tab is usually electrically connected to the device's internal circuit. Therefore you must place some form of insulator between the pad and the rest of your circuit if they contact one another. 
..............................................................................................

This page will be constantly added to over time and hopefully grow with lots of useful synth info
....... ... I'll place a link to it in the DIY page.

Links
+ Transistor Matching
+ JFETS - matching for synthesizers