Introduction

So far in our discussion of amplifiers, a vital component in the design has been the coupling capacitor which transmits the a.c. signals but removes the steady d.c. voltage present at the input and output of each stage. This is necessary in order to avoid one stage upsetting the operation of adjacent ones.

A two-stage capacitor-coupled amplifier is shown in fig. 8.1 together with quiescent d.c. voltages. It is clear that C2 is isolating the collector of T1 (which needs to sit at 4.5 V for correct operation) from the base of T2, which is only 0.6 V above the grounded emitter, being a forward-biased junction. Making a direct connection between stages, omitting C2, would have the unfortunate result of clamping the collector of T1 only about 0.6 V above 0 V and passing a 2 mA base current into T2 through T1 collector load, permanently bottoming T2. The design would not be a success!

Coupling capacitors can, however, be eliminated by special d.c. amplifier design which is employed in virtually all present-day circuitry. There are two main reasons for this. The first, very practical, reason is that large capacitors cannot be fabricated on ICs, the maximum being a few tens of picofarads. The second reason is that the coupling capacitor inevitably leads to attenuation and phase shift at low frequencies: after all, there is no clear distinction between low-frequency a.c. and slowly changing d.c. and it is impossible to provide isolation from the latter without affecting the former.