____________________________________________________________________________ Amplifiers
IMPEDANCE CONSIDERATIONS FOR AMPLIFIERS
5-52. Remember that efficiency and impedance are important in amplifiers. have been
shown that any amplifier is a current-control device. Now there are two other principles
you need to know. The first principle is that there is no such thing as "something for
nothing" in electronics. That means that every time you do something to a signal it costs
something. It might mean a loss in fidelity to get high power. Some other compromise
might also be made when a circuit is designed. Regardless of the compromise, every stage
will require and use power. The second principle is "do things as efficiently as possible".
The improvement and design of electronic circuits is an attempt to do things as cheaply as
possible, in terms of power, when all the other requirements (fidelity, power output,
frequency range, and so forth) have been met.
5-53. The most efficient device is the one that does the job with the least loss of power.
One of the largest losses of power is caused by impedance differences between the output
of one circuit and the input of the next circuit. Perhaps the best way to think of an
impedance difference (mismatch) between circuits is to think of different-sized water
pipes. If you try to connect a one-inch water pipe to a two-inch water pipe without an
adapter you will lose water. An impedance-matching device is like that adapter. It allows
the connection of two devices with different impedances without the loss of power.
5-54. Figure 5-12 shows two circuits connected together. Circuit number 1 can be
considered as an AC source (ES) whose output impedance is represented by a resistor (R1).
It can be considered as an AC source because the output signal is an AC voltage and comes
from circuit number 1 through the output impedance. A resistor in series with the source
represents the input impedance of circuit number 2. The resistance is shown as variable to
show what will happen as the input impedance of circuit number 2 is changed. The chart
below the circuit shows the effect of a change in the input impedance of circuit number 2
(R2) on current (I), signal voltage developed at the input of circuit number 2 (ER2), the
power at the output of circuit number 1 (PR1), and the power at the input to circuit number
2 (PR2).
5-55. Two other important facts are shown in this chart. First, the power at the input to
circuit number 2 is greatest when the impedances are equal (matched). The power is also
equal at the output of circuit number 1 and the input of circuit number 2 when the
impedance is matched. Second, the largest voltage signal is developed at the input to circuit
number 2 when its input impedance is much larger than the output impedance of circuit
number 1. However, the power at the input of circuit number 2 is very low under these
conditions. So you must decide what conditions you want in coupling two circuits together
and select the components appropriately.
5-56.
Two important points to remember about impedance matching are as follows:
Maximum power transfer requires matched impedance.
To get maximum voltage at the input of a circuit requires an intentional
impedance mismatch with the circuit that is providing the input signal.
23 June 2005
TC 9-62
5-13
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