With simple gate and combinational logic
circuits, there is a definite output state for any given input state. Take
the truth table of an OR gate, for instance:
For each of the four possible combinations of input states (0-0, 0-1,
1-0, and 1-1), there is one, definite, unambiguous output state. Whether
we're dealing with a multitude of cascaded gates or a single gate, that
output state is determined by the truth table(s) for the gate(s) in the
circuit, and nothing else.
However, if we alter this gate circuit so as to give signal feedback from
the output to one of the inputs, strange things begin to happen:
We know that if A is 1, the output must be 1, as well. Such is the
nature of an OR gate: any "high" (1) input forces the output "high" (1). If
A is "low" (0), however, we cannot guarantee the logic level or state of the
output in our truth table. Since the output feeds back to one of the OR
gate's inputs, and we know that any 1 input to an OR gates makes the output
1, this circuit will "latch" in the 1 output state after any time that A is
1. When A is 0, the output could be either 0 or 1, depending on the
circuit's prior state! The proper way to complete the above truth table
would be to insert the word latch in place of the question mark,
showing that the output maintains its last state when A is 0.
Any digital circuit employing feedback is called a multivibrator.
The example we just explored with the OR gate was a very simple example of
what is called a bistable multivibrator. It is called "bistable"
because it can hold stable in one of two possible output states,
either 0 or 1. There are also monostable multivibrators, which have
only one stable output state (that other state being momentary),
which we'll explore later; and astable multivibrators, which have no
stable state (oscillating back and forth between an output of 0 and 1).
A very simple astable multivibrator is an inverter with the output fed
directly back to the input:
When the input is 0, the output switches to 1. That 1 output gets fed
back to the input as a 1. When the input is 1, the output switches to 0.
That 0 output gets fed back to the input as a 0, and the cycle repeats
itself. The result is a high frequency (several megahertz) oscillator, if
implemented with a solid-state (semiconductor) inverter gate:
If implemented with relay logic, the resulting oscillator will be
considerably slower, cycling at a frequency well within the audio range. The
buzzer or vibrator circuit thus formed was used extensively in
early radio circuitry, as a way to convert steady, low-voltage DC power into
pulsating DC power which could then be stepped up in voltage through a
transformer to produce the high voltage necessary for operating the vacuum
tube amplifiers. Henry Ford's engineers also employed the buzzer/transformer
circuit to create continuous high voltage for operating the spark plugs on
Model T automobile engines:
Borrowing terminology from the old mechanical buzzer (vibrator) circuits,
solid-state circuit engineers referred to any circuit with two or more
vibrators linked together as a multivibrator. The astable
multivibrator mentioned previously, with only one "vibrator," is more
commonly implemented with multiple gates, as we'll see later.
The most interesting and widely used multivibrators are of the bistable
variety, so we'll explore them in detail now. |
