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

Transistors
179
will be very stable as illustrated, because the voltage gain associated with
the DC quiescent operational levels is held at 10, but it will not be very
repeatable.
Repeatability applies to the capability of being able to reproduce the
identical set of operating characteristics within multiple identical circuits.
For example, suppose you constructed the circuit of Fig. 6-7f, compared
the input signal to the output signal, and discovered the actual AC
voltage gain to be 180. If you then removed Q1 from the circuit and
replaced it with another identical 2N3904 transistor, would the AC voltage
gain still be 180 In all probability, it would not. Transistors are not
manufactured with exact beta values; rather, they are specified as meeting
a required range or minimum beta value. Two identical transistors
with the same part number could vary by more than 100% in their actual
beta values. Since the AC voltage gain of Fig. 6-7f is largely dependent on
the beta value of Q1, you could construct a dozen identical circuit copies
of Fig. 6-7f and achieve variable AC voltage gains ranging from 120 to 250.
Therefore, the repeatability of such a circuit is said to be poor.
In its simplest form, you can think of any single-stage transistor
amplifier circuit as a “block” with an input and an output. Like all electronic
circuits having inputs or outputs, there will always be some finite
input impedance and output impedance. So far in this section, you have
examined the internal functions of transistor amplifiers, but it is also
important to understand the effect such amplifiers have on external
devices or circuits.
Referring back to Fig. 6-7e, the AC signal source in this illustration
could represent a wide variety of devices. It could be a lab signal generator,
a previous transistor amplifier stage, a signal output from a radio,
the electrical “pickup” from an electric guitar, and the list goes on and
on. It represents any conceivable AC voltage that you want to amplify
for any conceivable reason. Regardless of what this AC signal source represents,
it will have an output impedance. In order for transistor amplifiers
to function well in a practical manner, the output impedance of
the intended signal source must be compatible with the input impedance
of the transistor amplifier. Otherwise, your intended amplifier may
turn out to be an attenuator (i.e., a signal reducer).
Before going into a detailed description of input and output impedances,
it is important to understand that all power supplies look like a
low impedance path to circuit common (or ground potential) to AC signals.
In other words, the internal impedance of all high-quality power
supplies must be very low. As a means of understanding this principle,
you can try a little experiment with a common 9-volt transistor battery