# How do transistors have three legs when there are two circuits going through?

I keep seeing a not-transistor outlined like this: NOT transistor http://www.waitingforfriday.com/images/thumb/4/44/Slide12.PNG/400px-Slide12.PNG

But every picture of an actual transistor looks like this: Transistor http://nefarius.at/wp-content/uploads/2012/04/P4140062.jpg

I can't help but notice four lines exiting the diagram, but only three legs exiting the physical transistor. The same seems to be the case for all the transistors I've looked at. What's the missing leg?

You mis-identified what a transistor is.

The transistor is one component in that circuit that you referenced. The symbol for an NPN transistor is:

Look closely and you will see one of these in there.

Each of the "squiggly" components is a resistor. You posted a circuit consisting of 2 resistors and 1 transistor.

• Ah, so what I physically see is only a small part of the diagram? I know that logic gates are available as IC chips, but I then assume that there is no other single, physical component version of the various logic gates? I.e. you build the whole diagram yourself, or you get an IC? – Henry Stone May 1 '16 at 15:49
• But the descriptions of the internal workings of a transistor that I find still say that there are two circuits involved: The base consists of one circuit, its current going in and then out of the transistor, and the signal is another circuit, going in and out. That's still two going in, two going out. To have only three legs on the transistor, some of those must be merged, mustn't they? – Henry Stone May 1 '16 at 15:53
• @HenryStone They are merged. In an NPN transistor, the emitter current is the sum of the base current and the collector current. Typically, the base current is a small fraction of the collector current, and so the emitter current is approximately the collector current. When the transistor is operating in the forward active regime, the ratio of $I_B$ and $I_C$ is $\beta$, the current gain of the transistor. – uint128_t May 1 '16 at 16:34
• @HenryStone - The "internal workings" you talk about are a model of what's going on, not a literal description. Do some searching on the physical construction of, for instance, an NPN transistor, and you'll find that it's just a chunk of (really, really pure) silicon or germainium, with some areas made slightly less pure. The way it works (carrier drift, recombination, etc) can be modelled as two circuits, a current sensor and a dependent current source, but that's not the physical reality. "All models are wrong, but some are useful." – WhatRoughBeast May 1 '16 at 17:30

There is no missing leg.

You are slightly confused because the schematic doesn't show the bounds of the transistor itself.

The connection "Out" in your diagram doesn't come out of the transistor. It is connected to the wire between the top resistor and the transistor.

Imagine the housing of the transistor around the schematic representation (like in the diagram below where the dotted line represents the housing.)

simulate this circuit – Schematic created using CircuitLab

The transistor has only three legs. The output comes from the junction between R1 and the transistor.

The "circuits" you have in mind are as follows:

1. Base of the transistor (from the left) to the emitter (bottom)
2. Collector of the transistor (top) to the emitter (bottom)

I've labeled the diagram and added arrows to show the two current flows.

When a voltage on the base causes current to flow through the base to the emitter (1.), the transistor allows current to flow from the collector to the emitter (2.).

The current flow through (1) can be very small. Transistors have what is called current gain, whereby a small current through (1) causes a larger current through (2.)

If no current flows through (1) then no current flows through (2.)

So, for the two "circuits" you are thinking of, both must use the emitter. This is called a common emitter circuit. The wikipedia link includes a lot of info on the common emitter circuit, as well as links to the common collector and common base circuits that can also be built with BJT transistors.

A transistor is actually a 3-terminal device. In the schematic you've shown, input is connected to the 'base' terminal, through a resistor. The terminal called 'emitter' is connected to the ground. The 'collector' terminal has 2 connections, the resistor and the output pin.