How does an optocoupler amplifier circuit work?

Nowadays, all electronic products use optocouplers, so what is an optocoupler? An optocoupler is an electrical-to-optical-to-electrical conversion device that uses light as a medium to transmit electrical signals. It consists of two parts: a light source and a light receiver. The light source and the light receiver are assembled in the same sealed housing and separated from each other by transparent insulators. The pins of the light source are the input terminals, and the pins of the light receiver are the output terminals. Common light source sources are light-emitting diodes, and the light receivers are photodiodes, phototransistors, etc.

What is a coupling circuit?

In order to realize the transmission of energy and signals, the method of connecting various functional circuits is a coupling circuit. Generally, coupling circuits usually have one or more functions such as filtering, energy storage, isolation, and impedance transformation.

Several coupling circuits:

Optocoupler (optical coupler, English abbreviation OC) is also called photoelectric isolator, abbreviated as optocoupler. Optocouplers use light as a medium to transmit electrical signals. It has good isolation effect on input and output electrical signals, so it is widely used in various circuits. At present, it has become one of the most diverse and widely used optoelectronic devices. Optocouplers generally consist of three parts: light emission, light reception and signal amplification. The input electrical signal drives the light-emitting diode (LED) to emit light of a certain wavelength, which is received by the photodetector to generate a photocurrent, which is further amplified and output. It completes the conversion of electricity-optical-electricity, thereby playing the role of input, output, and isolation. Since the input and output of the optocoupler are isolated from each other and the electrical signal transmission is unidirectional, it has good electrical insulation and anti-interference capabilities.

An optocoupler is a semiconductor optoelectronic device that encapsulates a light-emitting device and a photosensitive device in the same shell, and transmits electrical signals through the conversion of electricity → light → electricity. Among them, the light-emitting devices are generally light-emitting diodes. There are many types of photosensitive devices. In addition to photodiodes, there are also phototransistors, photoresistors, photothyristors, etc. Optocouplers can be combined into many series of optocouplers from different types of light-emitting devices and photosensitive devices according to different requirements.

Comparator A1 compares the reference voltage of ZDl (node ​​A) with the output voltage through the voltage divider circuit R7 and R8, thereby controlling the conduction state of Q2, which can define the current of the light-emitting diode D1 and the photosensitive transistor Q1 through optical coupling. collector current. Q1 then defines the pulse width and output voltage, compensating for any tendency to change the output voltage. As the use time of the photoelectric coupler increases and the transmission ratio or gain decreases, in order to prevent control failure, it is necessary to provide sufficient driving current margin for Q2. There are many types of optocouplers, the most common ones are photodiode type, phototransistor type, photoresistor type, photocontrolled thyristor type, photoelectric Darlington type, integrated circuit type, etc. (The appearance includes metal round shell packaging, plastic packaging dual in-line, etc.).

How does it work?

Fundamental

A power signal is applied to the input end of the photocoupler to cause the light source to emit light. The intensity of the light depends on the size of the excitation current. After this light irradiates the packaged photoreceiver, a photocurrent is generated due to the photoelectric effect, which is output by the photoreceptor. The terminal is drawn out, thus realizing the conversion of electricity, light and electricity.

Optocoupler mainly consists of three parts: light emission, light reception and signal amplification. The light emission part is mainly composed of light-emitting devices. Light-emitting devices are generally light-emitting diodes. When a forward voltage is applied to a light-emitting diode, it can convert electrical energy into light energy and emit light. The light-emitting diode can be driven by DC, AC, pulse and other power supplies. However, the forward voltage must be applied to the light-emitting diode when used. The light receiving part is mainly composed of photosensitive devices. Photosensitive devices are generally photosensitive transistors. Photosensitive transistors work by using the principle that when a reverse voltage is applied to a PN junction, the reverse resistance changes from large to small under light irradiation.

The optical signal amplification part is mainly composed of electronic circuits. The pins of the light-emitting device are the input terminals, and the pins of the photosensitive devices are the output terminals. When working, an electrical signal is added to the input end to make the core of the light-emitting device emit light. The photosensitive device generates photocurrent after being exposed to light and is amplified by the electronic circuit and output, thereby realizing the conversion of electricity → light → electricity, thus realizing the input and output circuits. electrical isolation. Since the input and output circuits of the optocoupler are isolated from each other, and the electrical signal has the advantages of unidirectionality during transmission, the optocoupler has good anti-electromagnetic wave interference capabilities and electrical insulation capabilities.

Basic working characteristics

1. The common mode rejection ratio is very high

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

2. Output characteristics

The output characteristics of the optocoupler refers to the relationship between the bias voltage VCE applied to the photosensitive tube and the output current IC under a certain light-emitting current IF. When IF=0, the light-emitting diode does not emit light. At this time, the photosensitive transistor set The electrode output current is called dark current, which is generally very small. When IF>0, under the action of a certain IF, the corresponding IC basically has nothing to do with VCE. The change between IC and IF is linear. The output characteristics of the photocoupler measured with a semiconductor tube characteristic grapher are similar to the output characteristics of ordinary transistors. The test connection is as shown in Figure 2. In the figure, the three wires D, C, and E correspond to poles B, C, and E respectively, and are connected to the instrument socket.

3. Isolation characteristics

1.Isolation voltage Vio (Isolation Voltage)

The insulation withstand voltage value between the input and output ends of the optocoupler.

2. Isolation capacitor Cio (Isolation Capacitance):

Capacitance value between input and output of optocoupler device

3. Isolation resistance Rio: (Isolation Resistance)

The insulation resistance value between the input and output terminals of a semiconductor optocoupler.

4. Transmission characteristics:

1.Current Transfer Ratio CTR (Current Transfer Radio)

When the operating voltage of the output tube is a specified value, the ratio of the output current to the forward current of the light-emitting diode is the current transfer ratio CTR.

2. Rise time Tr (Rise Time) & fall time Tf (Fall Time)

Under the specified working conditions of the optocoupler, the light-emitting diode inputs the pulse wave of the specified current IFP, and the output tube outputs the corresponding pulse wave, from 10% to 90% of the amplitude of the output pulse front edge, and the required time is the pulse rise time tr. The time required from 90% to 10% of the output pulse trailing edge amplitude is the pulse fall time tf. Other parameters such as operating temperature, power dissipation, etc. will not be repeated one by one.

5. Optocoupler can be used as 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. In this way, the phototransistor receives an optical signal that increases or decreases in the bias current, and its output current will follow the input signal. The voltage changes linearly. Optocouplers can also work in a switching state and transmit pulse signals. When transmitting pulse signals, there is a certain delay time between the input signal and the output signal. The input and output delay times of photocouplers with different structures vary greatly. I believe that with the continuous development of science and technology, future optocouplers will become more and more efficient.