Tab Electronics Guide to Understanding Electricity ... - Sciences Club

192 Chapter Six
Figure 6-8e
Several examples of
Darlington (i.e. betaenhanced)
transistor
pairs. The circuit on
the left is the more
standard configuration,
while the circuit
on the right provides
improved DC and
temperature stability
characteristics.
However, by adding the temperature-stabilizing resistor, the base impedance
of the Darlington pair is lowered considerably. Since the base input
of a Darlington pair literally consists of “two” base-emitter junctions, the
required forward threshold voltage is doubled (i.e., the base lead would
have to be biased with about 1.4 volts for base-emitter current flow,
instead of the normal 0.7-volt minimum for a single transistor). Depending
on the type of circuit in which a Darlington pair is being used, there
can be other problems as well. In high-frequency applications, the speed
limitations of one transistor tends to add to the speed limitations of the
second, making Darlington pairs relatively slow in comparison to single
transistors. Also, with some critical circuitry, the apparent difference in
the internal capacitances can adversely affect operation.
Final Comments Even though the uses of transistors are quite varied,
the basic fundamentals of transistor operation are quite simple. Within
every circuit you have examined in this section, the actual transistor
operation consisted of the following basics:
1. When the transistor was forward-biased, there was a 0.7-volt drop
across the emitter-base junction.
2. The base current was multiplied by the beta (H fE
) parameter
promoting a collector current flow equal to the product of
the two.
3. The emitter voltage was the same as the base voltage, minus the
0.7-volt base-emitter voltage.
4. The “total” current flow through the transistor originated in the
emitter, with a minor portion flowing through the base circuit,
while the vast majority flowed through the collector circuit.