turkey3_scratch :
I found this on Wikipedia:
I am trying to learn about transistors and CPU architecture so I have a few questions about this image.
1) what does the squiggle represent?
2) What is a base, collector, and emittor?
3) Why does it cut to the left and cut back near the base?
4) How is this transistor useful to arithmetic and logical units for calculating data?
Ah, you've discovered TTL logic. TTL is an old and depreciated form of constructing integrated circuits. Almost all modern digital electronics are fabricated using a variation of CMOS logic.
1. The "squiggle" is the electrical circuit symbol for a resistor. A resistor impedes the flow of current and dissipates energy as heat.
2. Base, Collector, and Emitter are three of the four terminals of a Bipolar Junction Transistor. BJT transistors are nominally current amplifiers.
The symbol that you see there, with the arrow pointing from the base towards the emitter, is an NPN transistor. In an NPN transistor a current across the base-emitter junction is used to control the current across the collector-emitter junction. If the base-emitter junction current is positive and high, then the collector-emitter junction will be really high. The current flowing out of the emitter terminal is the sum of the base-emitter current and the collector-emitter current.
A PNP transistor (identified by the arrow pointing from the emitter to the base) works the other way around. The current flowing into the emitter is the sum of the current flowing out of the base and out of the collector.
3. That's just the symbol. It is often enclosed within a circle to avoid confusion with normal wires.
4. Great question!
If my answer to #1 and #2 made any sense to you, you may have guessed that the transistor lets the designer control how much current flows from the node Vout to the reference point (ground). If Vin is at a suitably high potential, then current can flow [almost] freely across the transistor. If Vin is at a suitably low potential, then current can barely flow across the transistor at all.
Understanding what this allows the designer to do requires knowledge of Kirchoff's circuit laws, as well as Ohm's law. However, I will simplify this a little bit. When Vin is suitably high, an ideal BJT transistor appears to be a short circuit, it may as well not even exist. When Vin is suitably low, an ideal BJT appears to be an open circuit.
In the former case, all of the energy carried by the electrons would be dropped across the resistor and Vout would be equipotential to reference, or nominally 0 volts. In the latter case, the electrons would not be able to flow at all so no energy would be lost and Vout would be equipotential to supply, which may be 5v.
So, to describe the logical operation of that circuit,
If Vin is the equivalent of a logic high, Vout is the equivalent of a logic low.
If Vin is the equivalent of a logic low, Vout is the equivalent of a logic high.
The circuit above is an inverter, or a NOT gate.
The complexity of the logic can be increased by adding more transistors.
If two transistors are placed in series, both must be supplied with a logic high for current to flow, this forms a NAND gate.
If two transistors are placed in parallel, supplying either one with a logic high is sufficient to drag Vout to a logic low. This forms a NOR gate.
The arrangement of transistors that is responsible for connecting the output to the reference (ground) is called the pull-down network. In the example above, if nothing is pulling the logic down, it will be pulled up by the resistor.
If the diagram is switched around with a resistor on the bottom and a PNP transistor on the top it becomes a pull-up network with a pull-down resistor.
However, resistors are not flexible devices. They impede the flow of current so using them to pull logic up or down is very slow as they do not readily allow charge the flow through to the output; they also dissipate a lot of heat. This is why most modern circuits are constructed using both pull-up and pull-down networks. The networks are constructed in a complementary fashion so that at any point in time either the pull-up network is driving the output, or the pull-down network is driving the output. These kinds of circuits are called complementary logic, and when constructed with MOSFETs (a different type of transistor that is more suited for digital applications) the design is called CMOS or Complementary Metal Oxide Semiconductor. All modern microprocessors are designed primarily using CMOS logic.