Identifying transistor pinouts

[foozertone]

I know I can look online and find the pinouts for various transistors but my question is how do you identify which holes on the board are for the emmiter, collector... I've only seen a couple of layouts that identify which are which. Is there a way to tell from the schematic? (I'm sure there is) Please hip me.

Thanks

Jeff

[PerroGrande]

Unlabeled board + schematic:  Look on the schematic for the connections to each of the three pins of the tranny.  Match the connections on the PCB to discover which hole is for which pin on the board.   For example:

You have a grounded emitter on the schematic, the base biased with a voltage divider consisting of a 100K and 62K resistor, and a 10K resistor connecting the collector to the positive supply.  On the PCB, look for which of the three holes has a trace that when followed leads directly to ground.  This is the emitter.  Find the hole that has a trace that connects to a 10K resistor the other end of which can be traced to the positive supply.  This is the collector... etc. etc.

Another tip (although NOT guaranteed).   Many times on a PCB, the holes for a transistor will be in the shape of a triangle. MOST (not all) boards orient the transistor so the FLAT side of the tranny (assuming a TO-92 style plastic case) matches the two in-line holes.  The middle lead is the one that is "back".  However, this is NOT guaranteed to be the case (and obviously useless if all three holes are in a straight line), so check the connections against the schematic.

[foozertone]

sorry to be so ignorant but how does that relate to FET. Which is which? 

Thanks Perro, you cleared me right up.

Thanks

JEff

[PerroGrande]

Hi Jeff,

Generally speaking, when someone refers to a "Transistor" they are referring to a Bipolar Junction Transistor (a BJT).  This is the "regular" transistor -- a three terminal device consisting of a Base, Emitter, and Collector.  They can be made of two different types of semiconductor materials (Germanium or Silicon).  They are available in two different polarities (NPN or PNP).

When one refers to an FET, they are typically referring to a Junction Field-Effect Transistor (a JFET).  A JFET is also a three terminal device, but is different than a BJT in many ways.  The FET consists of a Gate, Source, and Drain and comes in two different polarities (N-Channel and P-Channel).  The JFET operates differently than a BJT, and the two are not interchangeable.

Another variety of FET is the MOSFET (Metal Oxide Semiconductor Field-Effect Transistor).  They are close cousins to the FET (in fact, sometimes they are called Insulated Gate Field Effect Transistors).  MOSFETs are also three-terminal devices consisting of a Gate, Source,and Drain.  (There *are* some FETs with a 4th connection for the body). However, they are not directly interchangeable with FETs in most cases.  They come in two fundamental varieties:  Depletion-Mode and Enhancement Mode, although the vast majority of MOSFETs are Enhancement Mode devices (JFETs are depletion mode).  Enhancement Mode FETs are available in either p-channel or n-channel polarity.

All three devices (BJT, FET, MOSFET) have different schematic symbols.  Their respective schematic symbols tell us which polarity device is required for a given circuit.

So -- in summary, all three devices are similar -- similar enough to all have the term "transistor" in the name, but their behavior within a circuit is notably different.

[foozertone]

okay so how do  I tell with a JFET(or other FET) which space in the board is Gate< drain etc,,,
I'm trying

Thanks

Jeff

[PerroGrande]

Jeff,

If the board isn't labeled, use the same technique -- match the traces with lines on the schematic.

"text-mode N-channel JFet schematic symbol"


       |_____D
G     |
---->|
       |_____S
       |

[ambulancevoice]

remember, the power supply always goes to the collector/drain usually with a bias resistor
the emitter/source is always connected to ground (sometimes with bias resistor, or a gain pot of some sort)
and the base/gate is always where the guitar signal go in