What is the function of Coupling Capacitors?

A coupling capacitor is used to connect AC input of one stage to successive stage, while DC voltage is not allowed to pass (blocked). Stages of two circuits may have different biasing voltages and may be operating at different DC levels. By definition, coupling capacitor is used to transmit AC signal from one node to another. Voltage, current, resistance values within each stage are not affected. Whenever a capacitor is connected in series with a load, it is called a coupling capacitor. A coupling capacitor is defined in broader way as a capacitor that is used to connect AC signal of one circuit to another circuit.

DC electronic circuits and amplifiers produce AC signals, which are passed on in successive stages for amplification and processing. There are number of stages (circuit segments) performing different operations on Ac signals. These stages work under different conditions and different DC biases. Coupling capacitors are provided in series with output of a stage and input of next stage to block effect of DC voltages to be passed on.

A capacitor has high impedance to low frequencies and blocks them, and allows high frequencies to pass to next stage. Value of coupling capacitor depends on the frequencies to be passed on. A very low value of capacitance is needed for higher frequencies, and values in pF range are common. Low frequencies need high value of capacitor in microfarad range.

The coupling capacitor has its own impedance, self-inductance and transient behaviour, which must not affect circuit performance significantly. Transient changes in DC at input end can be passed on to output side, and should be minimized.  Capacitor value and its impedance frequency behaviour decides the choice of an optimum capacitor. Its design value depends on the lowest frequency to be passed through.

Coupling capacitor decreases low frequency gain of system being coupled. The capacitor, along with input impedance of next stage, forms a high pass filter. A sequence of such filters results in cumulative filter with a cutoff frequency, which could be higher than individual filters. Coupling capacitor can also introduce non-linear distortion at low frequencies. This is not an issue at high frequencies since voltage across capacitor is almost zero. If the signal frequency passing through is lower than RC cutoff frequency of capacitor, a voltage can develop across its terminals. This may affect the capacitance itself in some types, and capacitor should be selected keeping this in view.

Reactive nature of capacitor means its response to different frequencies is different. For selection of capacitor for coupling / DC blocking, one has to consider its impedance, equivalent series resistance, and series resonant frequency.

AC coupling is used in modern digital circuits is used to transmit circuits transmit AC signals with zero DC components, known as DC balanced signals. For this reason, these circuits produce DC balanced waveforms.

Following is rough guideline for capacitors to be used for different frequencies:

Signal Frequency	Coupling capacitor
100 Hz	10 μF
1000 Hz	1 μF
10 KHz	100 nF
100 KHz	10 nF
1 MHz	1 nF
10 MHz	100 pF
100 MHz	10 pF

Gimmick Loop

This is made by twisting two wires running close-by, and increases with length of wire. The capacitance may be varied by tightening or loosening the wires. Gimmick capacitance may also be reduced by cutting or shortening the wire. This capacitance may be less than 0.5 pf / cm of wire length.

Parasitic Capacitive Coupling

This is unintended coupling between two conducting paths. For instance, two wires running close-by, or two conducting tracks on PCB will have capacitance between them. This could create noise problem due to interaction between sections of circuits. Spacing the conductors apart will reduce this coupling. Ground lines may be run between these conductors, so that any capacitive coupling is with ground, and not between these parts. If a high-gain amplifier output couples capacitively to its input, it may become an oscillator.

Often, double sided printed circuit boards have parallel tracks running on their two sides. These can create problems in smooth working of circuit, and measures have to be taken to avoid them or account for them in design.

Measurements of small impedances can be seriously affected because of capacitance between lead wires running from measuring instrument to the device / component under test. Guards are used on these wires to minimize the effect of this unwanted stray capacitance.

Applications of Coupling capacitors

1. Audio circuits
2. Where AC signal is the desired output from an electronic circuit.
3. Substations in power line communication.
4. PLCC equipment to connect carrier equipment and transmission line.

Types of capacitors used for coupling

1. Aluminium Electrolytic
2. Tantalum
3. Ceramic
4. Polypropylene
5. Polyester

Coupling capacitor in Power Line Carrier Communication (PLCC)

PLCC is used in modern electrical power systems for telemetry and tele-control.  Telemetry refers to measurements from remote locations. Transmission lines used to carry power are also used for communication between substations and substations and control centers. The communication is done via high frequency circuits. Signals generated at substations are fed to transmission lines and carried over long distances. However, they are separated from power frequency at nodes so that they do not interfere with normal supply and effective communication is maintained.

Line traps and coupling capacitors are used at desired points on transmission line. Line trap is an inductor, which forms a tank circuit with capacitor. High carrier frequencies (5 KHz to 500 KHZ) are shorted by capacitor and directed to control centers for communication purposes. Low power frequencies of 50 / 60 Hz are blocked by capacitor and passed through line trap along the transmission line and load. Tele-monitoring and teleoperation is thus achieved. Carrier communication is used to protect lines and system against faults, as also effective management of grid.