Figure 4-13.-Variations of with for each configuration.

Vacuum-tube amplifiers are easier to service on

the bench.

IMPEDANCE MATCHING IN AMPLIFIERS

Amplifiers must sufficiently amplify an input signal

to the point where it may be applied to a power amplifier.

Due to their high input impedance, electron tubes cause

few impedance matching problems. While the input

impedance for transistors may be high in some

configurations, transistor circuits are nevertheless more

troublesome in this respect. A more detailed discussion

will now be given on how the effects of the input

impedance determine the choice of transistor

configurations.

The most desirable method of matching source

impedance to input impedance is by transformer

coupling; however, this is not always practical. When

Figure 4-13.-Variations of

with

for each configuration.

the preamplifier must be fed from a low-resistance

source (20 to 1,500 ohms), without the benefit of

transformer coupling, either the common base (CB) or

the common emitter (CE) configuration may be used.

The CB configuration has an input impedance that is

normally between 30 and 150 ohms; the CE

configuration has an input impedance that is normally

between 500 and 1,500 ohms.

If the signal source has a high internal impedance,

a high input impedance can then be obtained by using

one of the three following circuit arrangements.

The easiest configuration to use would be the

common collector (CC). The input resistance of the CC

configuration is high because of the large negative

voltage feedback in the base-emitter (BE) circuit. As the

input voltage rises, the opposing voltage developed

Figure 4-14.-Simplified schematic of a CE preamplifier with

series resister.

across the load resistance (fig. 4-12) substantially

reduces the net voltage across the BE junction. By this

action, the current drawn from the signal source remains

high impedance. If a load impedance of 500 ohms is

low. From Ohm's law, you should know that a low

used, the input resistance of a typical CC configuration

current drawn by a relatively high voltage represents a

will be over 30,000 ohms. The disadvantage of the CC

configuration, however, is that small variations in the

current drawn by the following stage cause large

changes in the input impedance value.

The variation of input impedance, as a function of

load impedance, for the CE, CB, and CC configurations

is shown in figure 4-13.

The CE configuration maybe used to match a high

source resistance by the addition of a series resistor in

the base lead. The BE junction resistance (represented

by in fig. 4-14) for a typical CE configuration is

Figure 4-12.-Simplified schematic of a CC preamplifier.

approximate y 1,000 ohms if a load resistance of 30,000

4-15