Optocoupler Operating Characteristics

1. High common mode rejection ratio

Inside the optocoupler, since the coupling capacitance between the light-emitting tube and the light receiver is very small (within 2pF), the common-mode input voltage has little effect on the output current through the inter-electrode coupling capacitance, so the common-mode rejection ratio is very high.

2. Output characteristics

The output characteristic of the photocoupler refers to the relationship between the bias voltage VCE applied to the phototransistor and the output current IC under a certain luminous current IF. When IF=0, the light-emitting diode does not emit light. The electrode output current is called dark current, which is generally very small. When IF>0, under a certain IF, the corresponding IC is basically irrelevant to VCE. The change between IC and IF has a linear relationship, and the output characteristics of the photocoupler measured by the semiconductor tube characteristic graph instrument are similar to the output characteristics of ordinary transistors.

3. The photocoupler can be used as a linear coupler

Provide a bias current on the light-emitting diode, and then couple the signal voltage to the light-emitting diode through a resistor, so that the phototransistor receives an optical signal that increases or decreases on the bias current, and its output current will follow the input signal. The voltage changes linearly. The photocoupler can also work in the switch state and transmit the pulse signal. When transmitting pulse signals, there is a certain delay time between the input signal and the output signal, and the input and output delay times of optocouplers with different structures vary greatly.