Q. What are the advantages of an RC coupling over a direct coupling? Draw a basic Rc coupling network and explain.

Figure -RC-coupled transistor amplifier

The network of R1, R2, and C1 enclosed in the dashed lines of the figure is the coupling networkR1 acts as a load resistor for Q1 (the first stage) and develops the output signal of that stage The capacitor, C1, "blocks" the dc of Q1's collector, but "passes" the ac output signal. R2 develops this passed, or coupled, signal as the input signal to Q2 (the second stage). This arrangement allows the coupling of the signal while it isolates the biasing of each stage. This solves many of the problems associated with direct coupling. RC coupling does have a few disadvantages. The resistors use dc power and so the amplifier has low efficiency. The capacitor tends to limit the low-frequency response of the amplifier and the amplifying device itself limits the high-frequency response. For audio amplifiers this is usually not a problem; techniques for overcoming these frequency limitations will be covered later in this module. The direct-coupled amplifier is not very efficient and the losses increase as the number of stages increase. Because of the disadvantages, direct coupling is not used very often.

This explains why the low frequencies are limited by the capacitor. As frequency decreases, XC increases. This causes more of the signal to be "lost" in the capacitor. The formula for XC also shows that the value of capacitance (C) should be relatively high so that capacitive reactance (XC) can be kept as low as possible. So, when a capacitor is used as a coupling element, the capacitance should be relatively high so that it will couple the entire signal well and not reduce or distort the signal