Series RC Circuits

Series RC circuits are widely used for coupling signals from one stage to another. If the time constant of the coupling circuit is comparatively long, the shape of the output waveform will be almost identical to that of the input. However, the output DC reference level may be different from that of the input. The figure below, view A, shows a typical RC coupling circuit in which the output reference level has been changed to 0 volts. In this circuit, the values of R and C are chosen so that the capacitor will charge (during t₀ to t₁) to 20 percent of the applied voltage, as shown in view B. With this in mind, let's consider the operation of the circuit.

RC coupling.

At t₀ the input voltage is -50 volts and the capacitor begins charging. At the first instant the voltage across C is 0 and the voltage across R is -50 volts. As C charges, its voltage increases. The voltage across R, which is the output voltage, begins to drop as the voltage across C increases. At t₁ the capacitor has charged to 20 percent of the -50 volts input, or -10 volts. Because the input voltage is now 0 volts, the capacitor must discharge. It discharges through the low impedance of the signal source and through R, developing +10 volts across R at the first instant. C discharges 20 percent of the original 10-volt charge from t₁ to t₂. Thus, C discharges to +8 volts and the output voltage also drops to 8 volts.

At t₂ the input signal becomes -50 volts again. This -50 volts is in series opposition to the 8-volt charge on the capacitor. Thus, the voltage across R totals -42 volts (-50 plus +8 volts). Notice that this value of voltage (-42 volts) is smaller in amplitude than the amplitude of the output voltage which occurred at t₀ (-50 volts). Capacitor C now charges from +8 to +16 volts. If we were to continue to follow the operation of the circuit, we would find that the output wave shape would become exactly distributed around the 0-volt reference point. At that time the circuit operation would have reached a stable operating point. Note that the output wave shape has the same amplitude and approximately the same shape as the input wave shape, but now "rides" equally above and below 0 volts. Clampers use this RC time so that the input and output waveforms will be almost identical, as shown from t₁₁ to t₁₂.