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

186 Chapter Six
Figure 6-8b
An example of a
simple “series-pass”
voltage regulator.
conditions. Figure 6-8b illustrates a simple method of maintaining a
constant voltage across a load. A raw DC power source is applied to
the collector of Q1, with a reference voltage source of 5.7 volts applied
to the base. Allowing for the typical 0.7-volt drop across Q1’s baseemitter
junction, about 5 volts should be dropped across the emitter
load (think of the emitter load as being an emitter resistor, such as RE
in Fig. 6-8a). For illustration purposes, a potentiometer is shown as a
variable load in Fig. 6-8b. Note that the setting of the potentiometer
does not control the voltage drop across the variable load. Rather, it is
maintained at about 5 volts as a function of the constant 5.7 volts
applied to the base of Q1 and the effect of Q1’s beta, which tries to
keep the emitter voltage equal to the base voltage (minus the 0.7-volt
base-emitter drop). By adjusting the potentiometer, both the emitter
current and the collector current will change radically in proportion
to differences in the load resistance, but the voltage across the variable
load will remain relatively constant. Because Q1 is connected in series
between the raw DC power supply and the load, it is often referred to
as a series-pass transistor.
At this point, you may be wondering why it is advantageous to incorporate
Q1 in the first place. Why not simply provide operational power
to the variable load directly from the stable reference voltage source The
answer to this very reasonable question is the simple fact that almost all
high-quality sources of reference voltages have very limited output current
capabilities. In other words, as soon as you begin to draw higher
operational currents from the reference voltage supply, the voltage output
will drop and it will cease to be a “reference voltage.” Therefore, you
need to make the reference voltage into a “controlling factor,” while
drawing the actual operational “power” from a different source. The regulator
circuit of Fig. 6-8b utilizes the current amplification factor (beta)
to control the “large” emitter-collector current flow with only a small