Figure 4-15.-Degenerated CE configuration.

Figure 4-15.-Degenerated CE configuration.

Figure 4-16.-Basic transistor output stages using two

transformers.

ohms is used. The input resistance

may be increased

by reducing the load resistance

For instance,

mismatch of the transistor and the speaker. The output

decreasing the load resistance to approximately 10,000

transformer in a basic audio output stage (fig. 4-16)

ohms will increase to approximately 1,500 ohms, as

seen in the curve of figure 4-13.

matches the low impedance of the speaker coil to the

high impedance of the output transistor. The primary of

Another method of increasing the input resistance

the transformer has more turns for high impedance; the

of a CE configuration is shown in figure 4-15. This

secondary has fewer turns to provide the low impedance

t y p e of circuit is the DEGENERATED CE

required by the speaker coil.

configuration. If an unbypassed resistor

is inserted

in the emitter lead, the signal voltage developed across

For a certain transistor, you can find the resistive

this resistor opposes the input signal voltage.

value (impedance) that provides maximum gain with

As in the case of the CC configuration, this

minimum distortion by consulting a transistor manual.

negative-feedback voltage or degenerative voltage

Typical values range from approximately 2,000 to 5,000

causes an increase in the input resistance. With a

ohms for one transistor, and they are doubled for

bypassed resistor in the emitter lead, the input resistance

push-pull outputs. Power to the secondary of a

of the CE configuration would be 2,000 ohms if a load

transformer is nearly equal to the power supplied by the

resistor of 500 ohms were used (fig. 4-13). With an

primary due to the near unity coupling of the iron core

of 500 ohms, the input

unbypassed resistor

in the transformer. The amount of power that an output

will appear as approximately 20,000

resistance

transformer can handle is determined by the current and

ohms. The input resistance may be made to appear as

voltage ratings of its windings and by allowable losses.

any desired value (within practical limits) by the proper

choice of

and

Like the CE circuit with the series

resistor, the total input resistance of the degenerated CE

IMPEDANCE RATIO

circuit will remain relatively constant with a varying

load. However, the advantage of the degenerated CE

Recall that the output voltage of a transformer varies

configuration is that the unbypassed resistor

also

directly with the turns ratio and that the output current

acts as an emitter stabilizer and aids in stabilizing the

varies inversely with the turns ratio. The proportions in

transistor bias.

equation form are

AUDIO OUTPUT STAGE

The voice coil of a dynamic speaker requires a

Multiply these proportions to get

low-impedance source, but the impedance of an output

transistor is high. If the speaker is connected directly to

the collector of the transistor, there is almost no audio

because the transistor has no gain, due to the loss in

4-16