## Thursday, February 3, 2011

### Lesson 27: General Class Exam Course G7B

Lesson 27 covers amplifiers, oscillators and digital circuits.  This lesson is a little deep in that it deals with some concepts that the average bear is not extremely familiar with.

Digital circuits process information using binary signals.  This means information is sent to the circuit through a series of ones and zeros.  This is relatively simple for circuits to process because the ones and zeros easily represent and "on" and "off" state, electronically.

Here are the digital circuits you need to know.

Flip-flop.  A flip-flop is a digital circuit with two stable states.  A flip-flop has two inputs: a "set" and a "reset".  They also have two outputs "Q" and "NOT Q".  The "set" and "reset" will each receive an input of either 1 or 0.  Depending on the combination of ones and zeros to the "set" and "reset", the "Q" will either output a 1 or a 0.  The "NOT Q" will produce the opposite of the one or zero the "Q" produces.  The fact that one of the two outputs is a 1 (on state) and the other is a 0 (off state) gives the flip-flop two stable states.

The other circuits require an understanding of the "NOT," "AND," and "OR" logic functions.

The "NOT" function is simply a function which produces the opposite of the input.  So if a 0 goes in, a 1 comes out.

The "AND" function takes two separate inputs and produces a single output.  For the "AND" function, if either one or both of the inputs is 0, then the output will be 0.  If both of the inputs are 1, then the output will be 1.

The "OR" function is similar to the "AND" function except the preference is to 1.  If either or both of the inputs are 1, then the output will be a 1.  If both inputs are 0, then the output will be 0.

These functions can be combined.  There is a question which asks about a NAND gate and one that asks about the output of a NOR gate.  The NAND gate is a digital logic integrated circuit which combines the "AND" and "NOT" function.  The NOR gate simply combines the "OR" and "NOT" functions.  For instance, in a NAND gate, if the two inputs receive a 0 input signal, then the output will be a 1.  The NAND gate takes the input signals and runs them through the "AND" function.  The product of the "AND" function with the two 0 inputs produces a 0 output.  Now add the "NOT" function to the "AND" function's output and the the output becomes the opposite, a 1.  The NOR gate is a combination of the "OR" function and the "NOT" function.  The "NOT" function simply outputs the opposite of the product of the "OR" function.

You will also need to be basically familiar with Class A and Class C amplifiers.  Specifically, a Class A amplifier has low distortion.  A Class C amplifier is very efficient and amplifies CW signals very well.

Good luck.  As always, please leave any suggestions, comments, or questions you may have in the comments box.  Thanks!

73,
Andy
KE4GKP