 How to Bias Optocoupler: The Complete Course

How to bias optocoupler is even simpler than bipolar junction transistors. However such device is not well discussed in the universities making people believe that it is a difficult device to deal with. Here I will reveal the methods and techniques on how to bias optocoupler. Before jumping to the main topic on how to bias optocoupler, let me touch some basics on optocoupler.

Optocoupler is sometimes called an opto isolator or photocoupler. It can provide circuit isolation and prevent noise to enter from the other circuit to another circuit. The input side of an optocoupler is a light source which commonly a light emitting diode. The output circuit is a photo transistor. Below figure is a schematic symbol of an optocoupler with input and output is identified. For more information on how optocoupler works, read this.

Because the input circuit is coupled to the output by means of light, there is no physical or electrical connection between input and output. This makes optocoupler a number one choice for isolation.

If a BJT has its current gain beta (β), an optocoupler has its current transfer ratio (CTR). CTR is defined as the ratio of the collector to the forward current expressed in percent. The collector current is the current in the photo transistor while forward current is the current in the diode. Mathematically, CTR Current transfer ratio is the one linking the input and the output circuits. CTR is prided in the device datasheet and the same with BJT; it is a value that varies with few factors. It is advisable to select an optocoupler with a tight CTR range so that the output will not swing much. Read this to know the factors affecting CTR.

Learning from Example on How to Bias Optocoupler

Variation 1: Output is Taken from the Collector with Collector Resistor and No Emitter Resistor

Input Circuit Analysis

The first example circuit we gonna use to discuss thoroughly how to bias optocoupler is shown below. By doing KVL from Vdd down to ground in the circuit below we can get the forward current equation.  Using the given values, the forward current is then (Note: Optocoupler forward voltage is ranging from 0.7V to 1.4V. In this example I am using 1V forward voltage.)

Output Circuit Analysis

Perform KVL on the output side  Expressing Vout  In this circuit configuration actually Vout is equal to VCE.

To solve Ic, we will use the CTR equation.  Then By using the actual values of the circuit, the collector current and Vout are  (I used CTR of 95% because based on the datasheet of PC817A at 1.33mA the CTR is around 95%)

Verifying through Circuit Simulation

Below are the simulation results using actual components. The level of Vout is around 3.74V as well which coincides with the computation result.

Analysis at Saturation

The same with a bipolar junction transistor, the photo transistor will also saturate. When this happens, the collector current will no longer be dependent to the forward current and the CTR of the device. In saturation, the collector current is fixed as limited by the collector resistor and cannot anymore increase despite the increase of the forward current. For the above circuit, at saturation the collector current is The value of VCEsat is very low and can be ignored. Then the collector current is can be written as Variation 2: Output is taken at the Collector with Collector Resistor and Emitter Resistor

Input Circuit Analysis

The analysis on the input side is the same with the previous configuration. Actually all the following circuits have the same analysis with this and the first one. This i why I said earlier that how to bias optocopler is simpler than BJT.  Using the given values, the forward current is then (Optocoupler forward voltage is ranging from 0.7V to 1.4V. In this example I am using 1V forward voltage.)

Output Circuit Analysis

Unlike BJTs, the collector and the emitter current of an optocoupler is the same.                It is because there is no electrical connection between the input and the output making the transistor an open base. Expressing Ic Expressing VCE  Formulating the equation of Vout  Or it can be expressed as Using the given in Figure 5 the value of Ic is The value of Vout is Solving for VCE Using the other equation for Vout, Verifying through Circuit Simulation

Below is the level of the output voltage Vout when simulating the circuit using the given values.

Analysis at Saturation

At saturation, the level of VCE is approaching zero volts. Then the expression of Ic is then  Ic during this time is no longer dependent to the device CTR and the value of If. Increasing the level of the forward current will not corresponds any increase to the level of the collector current.

Variation 3: Output is taken at the Emitter without Collector Resistor and has Emitter Resistor

Figure 8 below is another circuit variation for optocoupler. In this circuit there is no collector resistor and the output is taken from the emitter. The analysis is still very easy because of the fact that the base of the transistor is open making the collector and the emitter current equal.

Input Circuit Analysis

The analysis on the input side is the same with the previous configuration.  Using the given values, the forward current is then (Optocoupler forward voltage is ranging from 0.7V to 1.4V. In this example I am using 1V forward voltage.)

Output Circuit Analysis Expressing VCE Expressing Vout Solving for Ic Using Vcc=5V, VCE is then Vout is then Verifying through Circuit Simulation

Analysis at Saturation

At saturation, the level of VCE is approaching zero volts. Then the expression of Ic is then  Ic during this time is no longer dependent to the device CTR and the value of If. Increasing the level of the forward current will not corresponds any increase to the level of the collector current.

Variation 4: Output Is taken from the Emitter and there are Re and Rc

Input Circuit Analysis

The analysis on the input side is the same with the previous configuration.  Using the given values, the forward current is then Optocoupler forward voltage is ranging from 0.7V to 1.4V. In this example I am using 1V forward voltage.

Output Circuit Analysis Expressing VCE But Ie is equal to Ic, then Expressing Vout  Solving for Ic using CTR equation Using the values in the schematic, VCE is then Solving Vout Verifying through Circuit Simulation

Analysis at Saturation

At saturation, the level of VCE is approaching zero volts. Then the expression of Ic is then  Ic during this time is no longer dependent to the device CTR and the value of If. Increasing the level of the forward current will not corresponds any increase to the level of the collector current.

After learning the four circuit variations above I am sure that you can already design and analyze your own optocoupler circuit. The above analysis proved that how to bias optocoupler is simpler that BJT because there is no base current. The key on the analysis is the CTR which is linking the input and output circuitry for linear operation. For operation as a switch such that the optocoupler will saturate, the equations are also given above which are very straight forward. How to bias optocoupler is that easy!

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