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

Transistors
may recall from Chapter 4, a silicon forward-biased diode will produce a
reasonably constant voltage drop of about 0.7 volt, regardless of how
much forward current it is passing. Since diodes D1 and D2 are held in a
constant forward bias from the V cc
power supply (through R1), the base
voltage of Q1 will be the sum of the two 0.7-volt drops, or approximately
1.4 volts DC. The base-emitter junction of Q1 will drop about 0.7 volts of
this 1.4 volt base bias, leaving approximately 0.7 volts across RE. Therefore,
using Ohm’s law, the emitter current of Q1 is
E 0.7 volt
I
0.001 amp or 1 milliamp
700 ohms
R
185
Of course, about 1 200
th of this 1-milliamp emitter current will flow
through the base (assuming that Q1 has a beta of 200), but if you consider
this small base current to be negligible (which is appropriate in many
design situations), you can say that about 1 milliamp of current must
also be flowing through the collector. Note that the variable controlling
the collector current flow is the constant voltage dropped across RE,
which is held constant by the stable voltage drops across the two diodes.
In other words, the collector resistance has nothing to do with controlling
the collector current. You should be able to adjust the 10-Kohm potentiometer
(i.e., the collector resistance) from one extreme to the other with
almost no change in the approximate 1-milliamp collector current flow.
You may want to construct the circuit of Fig. 6-8a for an educational
experiment. If so, construct RE a 620-ohm resistor (700 ohms is not a
standard resistor value—I chose this value in the illustration for easy
calculation). If you don’t have the 1N4148 diodes, almost any generalpurpose
diodes will function well. When I constructed this circuit, my
actual collector current flow came out to 0.9983 milliamps at the minimum
setting of the potentiometer (I was lucky—actual results seldom
turn out that close on the first try). By adjusting the potentiometer to its
maximum resistance value, the collector current decreased to 0.9934 milliamps.
This comes out to a regulation factor of 99.5% (i.e., the regulated
current varied by only 0.5% from a condition of minimum load to maximum
load), which is considered very good.
Many types of electronic circuits require a very accurate and steady
amplitude level of DC operational voltage. The problem with a simple
“raw” (i.e., unregulated) power supply, such as the types discussed in
Chapter 5, is that the output voltage(s) will vary by about 10−30% as
load demands change. Consequently, voltage regulator circuits are needed
to hold voltage levels constant regardless of changes in the loading