4.11. INPUT AND OUTPUT COUPLING249The ﬁnal circuit diagram is shown in the “Practical Analog Circuits” chapter, “Class Acascode ampliﬁer . . . ” (page 442,433).• REVIEW:• See Figure 259,4.94.• Select bias circuit conﬁguration• Select RC and IE for the intended application. The values for RC and IE should normallyset collector voltage VC to 1/2 of VCC.• Calculate base resistor RB to achieve desired emitter current.• Recalculate emitter current IE for standard value resistors if necessary.• For voltage divider bias, perform emitter-bias calculations ﬁrst, then determine R1 andR2.• For AC ampliﬁers, a bypass capacitor in parallel with RE improves AC gain. Set XC≤0.10REfor lowest frequency.4.11Input and output couplingTo overcome the challenge of creating necessary DC bias voltage for an ampliﬁer’s input signalwithout resorting to the insertion of a battery in series with the AC signal source, we used avoltage divider connected across the DC power source. To make this work in conjunction withan AC input signal, we “coupled” the signal source to the divider through a capacitor, whichacted as a high-pass ﬁlter. With that ﬁltering in place, the low impedance of the AC signalsource couldn’t “short out” the DC voltage dropped across the bottom resistor of the voltagedivider. A simple solution, but not without any disadvantages.Most obvious is the fact that using a high-pass ﬁlter capacitor to couple the signal sourceto the ampliﬁer means that the ampliﬁer can only amplify AC signals. A steady, DC voltageapplied to the input would be blocked by the coupling capacitor just as much as the voltagedivider bias voltage is blocked from the input source. Furthermore, since capacitive reactanceis frequency-dependent, lower-frequency AC signals will not be ampliﬁed as much as higher-frequency signals. Non-sinusoidal signals will tend to be distorted, as the capacitor respondsdifferently to each of the signal’s constituent harmonics. An extreme example of this would bea low-frequency square-wave signal in Figure 260,4.95.Incidentally, this same problem occurs when oscilloscope inputs are set to the “AC cou-pling” mode as in Figure 261,4.97. In this mode, a coupling capacitor is inserted in series with themeasured voltage signal to eliminate any vertical offset of the displayed waveform due to DCvoltage combined with the signal. This works ﬁne when the AC component of the measuredsignal is of a fairly high frequency, and the capacitor offers little impedance to the signal.However, if the signal is of a low frequency, or contains considerable levels of harmonics overa wide frequency range, the oscilloscope’s display of the waveform will not be accurate. (Fig-ure 261,4.97) Low frequency signals may be viewed by setting the oscilloscope to “DC coupling” inFigure 260,4.96.