- After studying this section, you should be able to:
- Describe Different biasing modes used in optocouplers:
- • Saturation Mode.
- • Linear Mode.
- • Analogue Mode.
- List advantages & disadvantages of transistor vs. diode optocouplers:
Optocouplers or opto isolators are used for passing signals between two isolated circuits using different methods, depending mainly on the types of signals being linked. A computer system and its peripheral devices may need a digital signal, such as pulse width modulation signal driving a motor. In this case the optocoupler will be used in Saturation Mode.
A switched mode power supply may need a DC sample voltage of varying value to be fed back from the output to a voltage control system in the input circuit of the power supplywhilst maintaining complete electrical isolation between the input and output circuits. In this case Linear Mode will be used, as the control circuit will need to detect small changes in DC voltage.
To link circuits such as audio amplifiers where signal voltages are rapidly changing, but saturation and distortion need to be avoided, optocouplers can transfer signals using Analogue Mode so that audio can be safely transmitted, for example from an audio input device to a high powered amplifier.
Fig. 5.1.1 Saturation Mode
In saturation mode, the optocoupler output transistor is either turned fully 'on' (saturation conditions), or fully 'off' (non-conducting). Optocouplers working in saturation mode are widely used to protect the output pins of micro controllers for example, where they may be used to drive output devices such as motors that may need more current and/or higher voltages than can be supplied directly from the micro controller port.
The micro controller is then effectively only driving an infrared LED, either with signals such as pulse width modulation, stepper motor data or simple on and off signals. The isolation provided by the optocoupler means that the micro controller is also protected from any externally produced high voltages, such as the back emf that may be produced when switching off an inductive load such as a motor. Optocouplers also find uses in modems providing isolation between computers and the external phone lines.
Fig. 5.1.2 Linear Mode
Optocouplers can be used for voltage feedback in circuits such as switched mode power supplies, where the LED is illuminated by a sample of the output voltage so that any voltage variations cause a variation in the illumination of the optocoupler LED and therefore a variation in the conduction of the optocoupler's output transistor, that can be used to signify an error to the power supply control circuitry, allowing it to compensate for the output variation. A practical example of this feedback and the electrical isolation it provides by using an optocoupler in linear mode can be seen in our Power Supplies Module 3.4 where IC3 (a 4N25) provides a sample of the output voltage to be fed back to an error amplifier controlling the voltage regulator circuit within IC1, providing automatic voltage control, whilst giving complete electrical isolation between the 5V DC output circuit and the higher voltage input circuit.
Fig. 5.1.3 Audio Input in Analogue Mode
Like linear mode, the phototransistors used in analogue mode are not allowed to saturate, but a steady DC bias voltage of around half of the supply voltage is modulated by an audio, as shown in Fig. 5.1.3, or some other rapidly varying signal. This produces a varying current in the LED, which in turn produces a varying current in the output component of the optocoupler. This may be a phototransistor or very often a photodiode. The phototransistors used in optocouplers for audio purposes may also make use of a base connection available on some optocouplers to apply a suitable bias to the phototransistor to enable an undistorted audio signal output to be obtained. Specialised audio optocouplers duch as the IL300 shown in Fig. 5.1.4 may use one or more photodiodes in order to provide a more linear response than those using only phototransistors.
Fig. 5.1.4 The IL300 Audio
In addition to providing a more linear (less distortion) response the second diode is used to provide (isolated) feedback to the input circuit so that the IL300 can automatically compensate for variations in CTR due to changes in temperature and/or aging of the input LED.
Fig. 5.1.5 Audio Input in Analogue Mode
Phototransistor vs. Photodiode Optocouplers
Optocouplers using phototransistor outputs can pass analogue audio signals up to a frequency of a few tens of kHz. Varying the infra red light beam from the LED at these frequencies then causes the amount of current generated at the base of an output phototransistor to vary, with the transistor output following and amplifying the variations at the input.
However optocouplers using phototransistors do not have such as good a linear relationship between the changes in light input and output current as photodiode types, as illustrated in Fig. 5.1.5 therefore some distortion of the signal may occur. Photodiode output devices are preferred for use in most audio (and some digital) applications, even though their output signal amplitudes are much less than is possible with the amplification provided by a phototransistor; the reason for this is the phototransistor's distortion and poor performance at high frequencies.
This is due to the phototransistor having a much-enlarged base area, which whilst increasing the light sensitivity, also greatly increases the capacitance of the base/emitter junction. This increased capacitance is also made much worse because of the 'Miller Effect', which causes the base/emitter capacitance of a transistor to be multiplied by the current gain (hfe) of the transistor. Therefore higher frequencies are progressively reduced in amplitude, because the reactance of the base/emitter capacitance reduces as frequency increases much above the audio range.
Digital signals are also affected by this effect because the square waveforms of digital signals will contain many high frequency harmonics that contribute to the fast rise and fall times of the square wave, so that the rising edges of the signal become rounded and the switching time between 0 and 1 becomes longer.
High speed digital optocouplers, usable at frequencies in the hundreds of kHz and those used for audio operation usually use photodiodes as their sensing element because although some extra amplification must be provided, either externally or within the optocoupler chip itself, this is offset by having fast rise and fall times for digital operation, and a more linear response, producing less distortion when used with analogue audio.
The main function of an optocoupler, whatever type of signal is used, is to provide complete electrical isolation between the input and output circuits. An important advantage of optocouplers, compared with transformers, also often used for isolation purposes, is that optocouplers can be used with either AC or DC signals whereas transformers can only operate with AC.