The r-c combinations formed the network capacitors

360° 270°					f
Figure 11.7
11.5		LOW-FREQUENCY ANALYSIS—
In the low-frequency region of the single-stage BJT or FET amplifier, it is the R-C combinations formed by the network capacitors CC, CE, and Cs and the network re-sistive parameters that determine the cutoff frequencies. In fact, an R-C network sim-ilar to Fig. 11.8 can be established for each capacitive element and the frequency at which the output voltage drops to 0.707 of its maximum value determined. Once the cutoff frequencies due to each capacitor are determined, they can be compared to es-tablish which will determine the low-cutoff frequency for the system. Our analysis, therefore, will begin with the series R-C combination of Fig. 11.8 and the development of a procedure that will result in a plot of the frequency response with a minimum of time and effort. At very high frequencies,
				Figure 11.8

				+	+
				Vi	R
XC � �2�1� � 0 �fC			–		–
				Figure 11.9	R-C circuit of Fig-
				ure 11.8 at very high frequencies.
XC � �2� 1� � �fC 2�( 1� � � �0)C voltage appears across the output terminals.


0	Aυ Vo Vi
Figure 11.11
11.5					503

f

with the magnitude of Vo determined by

Vo ��R�2��RVi

The frequency at which this occurs is determined from

Gv � 20 log10 Av � 20 log10		� �3 dB

while at Av � Vo/Vi � 1 or Vo � Vi (the maximum value),

i R � R� � �jXC 1 � j( 1� ��XC/R) 1 � j(1 1� ��/�CR) 1 � j(1 1

/2�fCR)�



