Page 164 - Complete Wireless Design

Amplifier Design

Amplifier Design 163

R and the emitter resistor’s bypass capacitor C and their small—but
E E
unavoidable—values of stray inductance, the base-biased emitter feedback cir-
cuit is not normally employed in microwave amplifiers; gain reduction and
possibly instability problems are caused by these reactances.
One of the more common of the low-cost bias schemes, with a higher tem-
perature stability than the above method, is the voltage divider emitter feed-
back biasing circuit of Fig. 3.63. This circuit is temperature stable because the
current through the voltage divider of R and R is significantly higher than
1 2
the base current, and any rise in the device’s temperature, which will increase
the base current, will not substantially vary the voltage across R , which is
2
equal to the voltage at the base in respect to ground; thus maintaining a con-
stant voltage from base to ground. In addition, just as in the base-biased emit-
ter feedback discussed above, when the emitter current rises with an increase
in the transistor’s junction temperature, the top of the emitter resistor will
turn more positive. But as the base is always around 0.7 V more positive than
the emitter itself, the base-emitter junction will now have an actual decrease
in the voltage dropped across it when referenced to the common emitter lead,
thus reducing I to its desired amplitude.
C
For sensitive applications, we can go even further to increase temperature
stabilization. The most common method is diode temperature compensation,
shown in Fig. 3.64. Two diodes, D and D , which are attached to the tran-
1 2
sistor’s heat sink or to the device itself, will carefully track the transistor’s

Figure 3.63 A voltage divider emitter feedback–biased
C-E amplifier with current flow.

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