Part 4: Transistor Switching
and other digital devices voltages are used to represent binary
values: a high voltage represents the number 1, a low voltage represents
the number 0. Consequently digital transistor circuits are designed
to act as switches which are either on or off.
When the base
voltage (VBB)is less than 0.6 volts, the transistor
is off and no current flows through the collector. Because there
is no voltage drop across the collector resistor, the collector
voltage (VCC) is equal to the supply voltage.
When the transistor is not conducting, it is said to be cut off.
circuits are designed so that the base voltage is always below cut-off,
or high enough to turn the transistor fully on. When the base voltage
is about 0.85 volts, sufficient base current flows to turn the transistor
fully on. The collector voltage drops to approximately half a volt
because of the voltage drop across the collector resistor. A transistor
which is conducting the maximum current is said to be in saturation.