Coupling refers to the transfer of energy from one circuit to another circuit. Coupling is very common in analog circuits and digital circuits. Weak signals can be coupled to amplifier circuits for amplification, and amplified signals can also be output through coupling.

Coupling is a bridge connecting two functional circuits, which can realize the transmission of signals and energy. Common coupling circuits include direct coupling circuit, capacitive coupling circuit, photoelectric coupling circuit and transformer coupling circuit.

Here are some examples of how coupling works in circuits. coupling

1 Direct coupling

In direct coupling, two functional circuits are directly connected, and there is a direct influence between the two stages. In the amplifier circuit below, the input signal is directly coupled to T2 for two-stage amplification after being amplified by the triode T1, and then directly coupled to T3 for three-pole amplification.

Because all levels of the circuit directly connected, will affect each other, often used in low-frequency circuits and large-scale integrated digital circuits. Direct coupling

2 Resistance-capacitance coupling

In the resistance-capacitance coupling circuit, the two-stage circuit is isolated by the capacitor, and the signal is coupled to the next level through the capacitor. Because the capacitor has the function of separating the direct flow and AC, the Q point of the circuit at all levels will not affect each other, and can not amplify the slowly amplified signal, so it is often used for alternating high frequency signals. Resistance-capacitance coupling

3 Transformer coupling

In the transformer coupling circuit, the two stages are isolated by the transformer, the transformer needs to transmit signals through the way of electromagnetic induction, so it is not suitable for DC signals, generally used in AC signals.

It has poor low frequency characteristics and large volume.

But the two stages are independent of each other, static operating points will not affect each other, can achieve impedance transformation. Transformer coupling

4 Photoelectric coupling

Photocoupler is a device used for photocoupling, which is integrated with light-emitting diode and photosensitive triode. Of course, you can also build photocoupled circuits with individual light-emitting diodes and photosensitive triodes.

Light-emitting diode converts electricity into light, photosensitive transistor converts light back into electricity, photoelectric coupling is also isolated coupling mode, can well suppress interference signals