BJT stands for bipolar junction transistor.

BJT transistors are used primarily in amplifying or switching applications (e.g. power supplies.)

FET stands for the field-effect transistor.

FET transistors rely on an electric field to control conductivity of a channel.

JFET stands for the junction gate field-effect transistor (or JUGFET), and is the simplest type of the field effect transistor.

See also: wikipedia articles about transistors and JFET transistors.

Common transistor circuit symbols:

	PNP		P-channel
	NPN		N-channel
BJT		JFET

Most CPUs are made of MOS transistors.

MOS stands for is metal-oxide semiconductor.

MOSFET is most commonly used type of FET, the field-effect transistor. The MOSFET has the advantage over JFET because of MOSFET's extremely low gate current (measured in picoamperes.)

MOSFET transistors operate logically, just like wall switches.

Transistor has three terminals:

• Gate terminal is controlling the opening and closing of a physical gate.

• Source

• Drain

Electrons flow from the source terminal towards the drain terminal.

See also: wikipedia article about field effect transistor

Common MOSFET circuit symbols:

When source and drain are connected like piece of wire we have a closed circuit between the source and the drain.

• This happens when the gate is supplied 2.9 volts.

When connection between source and the drain is broken, we say it is an open circuit.

• This happens when the gate is supplied with 0 volt

Common N-type transistor circuit symbols:

Works in exactly the opposite fashion from the N-type transistor.

When gate is supplied with 0 volts, we have closed circuit like a wire between the source and the drain.

When the gate is supplied 2.9 volts, P-type transistor acts like an open circuit with broken connection.

Common P-type transistor circuit symbols:

Includes two MOS transistors.

The circle in the NOT-gate symbol on circuit drawings is called an inversion bubble.

A bubble on the input or output wire of another circuit symbol indicates an inversion of that signal:

• 0 becomes 1, and vice versa.

Schematic circuit based on 4 transistors.

See also: basic gates

OR gate is constructed by adding an inverter to the NOR gate.

See also: OR gate

See also: AND gate

Instead of drawing individual transistors, we will draw abstract symbols representing various types of gates as shown:

Basic logic gates:

     _______
     _     _
     A AND B   =  A OR B

In plain English this is

• "The outcome is true when
  it is not the case that both A and B are false."

The law is equivalent to

• "The outcome is true when
  at least one of A and B is true."

The relationship is also called De Morgan duality:

    not (P and Q) = (not P) or (not Q)
    not (P or Q) = (not P) and (not Q)

See also: Wikipedia article about DeMorgan Laws and XOR function

Multiple inputs can be combined into a single unit of AND, OR, NAND, and NOR gates.

The truth table can be constructed without a problem as well:

Mux is a two-level (two-input) multiplexer.

It's function is to select one of the inputs and connect it to the output.

Input A is the addressing input, specifying which of the two data inputs, X0 or X1, will be transmitted to the output X.

See also: two input multiplexer

S is a selector signal.

If S = 0 then the output is identical to input A.

If S = 1 then the output is identical to input B.

Multiplexers are not limited to two data inputs.

If two addressing inputs used, we can multiplex up to four data signals.

With three addressing inputs, we can multiplex eight signals, and so on.

Four-input multiplexer is a digital circuit that combines four input signals, one of which is selected by two separate address inputs to be sent to the single output.

The number of signal inputs is equal to 2^k, where k is the number of addressing inputs.

As the number of inputs increases, so is the number of selectors that are required for the mux to operate.

The opposite of the multiplexer circuit, logically enough, is the demultiplexer.

The one-input demultiplexer circuit takes

• one single data input IN, and

• one (or more) address selector inputs A.

The demultiplexer circuit can be transformed into a decoder.

The decoder determines type of the input bit pattern, therefore it is a circuit useful to interpret bit patterns.

Address inputs A and B specify a binary number.

The output matches the specified binary address input as follows:

• only one single output is set to 1, while the rest of the outputs are 0s.

An example of a bit pattern would be an OPCODE that specifies the type of the CPU instruction.

If the number of addressing inputs (A, B, ...) is equal to k, then the number of output lines becomes 2^k.