ElectricalWorkbook

Figure 1: RC Differentiator Circuit.

A simple RC differentiator circuit consists of a series combination of a capacitor C and a resistor R (see Figure 1). The input signal is fed across the combination and output is obtained across the resistor. Values of C and R are so selected that at the normal operating signal frequencies, the reactance of the capacitor is very large as compared to the resistor. Thus the current flowing in the differentiator circuit is to a large extent decided by the capacitive reactance; the current will be leading the input voltage by 90° approximately. Due to this current, the voltage developed across the resistor also leads the input voltage by 90°.

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The transistor when used as a switch must, therefore, be able to operate in cutoff region (open switch) and saturation region (closed switch) only.

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Fig. 1. RC coupling.

RC Coupling is the simplest and cheapest type of coupling scheme as shown in Fig. 1 and consists of a series coupling capacitor between the amplifier output points and the load circuit. The load may be a simple resistor, a loudspeaker, an antenna in case of a transmitter or another amplifying stage but RC coupling is generally used when a second amplifying stage is used as a load. The series capacitor acts as a short circuit for signal frequencies thereby passing the entire signal to the output, but for DC the capacitor behaves as open circuit. Therefore DC is not passed to the output.

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Fig. 1. Transformer coupling.

A transformer may also be used for coupling the output of an amplifier to the load as shown in Fig. 1. The transformer ensures that the AC signal is transferred to the load but the DC cannot be passed on to the secondary. The circuit is costlier than RC coupling but has the advantage that the turn ratio of the transformer can be so adjusted as to provide impedance matching and hence maximum energy transfer to the load can be ensured.

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