- After studying this section, you should be able to:
- Describe the use of optocouplers in analogue mode:
- Recognise the advantages and disadvantages of different analogue optocouplers.
- • Photodiode devices.
- • Phototransistor devices.
- Describe the use of DC bias in optocoupler circuits.
- Understand the use of voltage amplifiers and buffers with optocouplers.
Using Audio Optocouplers
Fig. 5.3.1 IL300 Optocoupler
In audio systems isolation between inputs and higher voltage/current equipment is usually provided by audio transformers, however it is also possible to use specialised audio optocouplers such as the IL300, which uses an infra red LED to illuminate one photodiode as an output device and a second photodiode to provide feedback, ensuring improved linearity and wider frequency range than phototransistor or photoresistor alternatives. Using feedback from a second photodiode with characteristics closely matched to those of the output photodiode also overcomes a basic problem with optocouplers. Without some sort of feedback, variations in the current transfer ratio will affect the performance of the optocoupler. Variations can occur due to changes in ambient temperature and to ageing of the infrared LED. Optocouplers such as the IL300 from Vishay illustrated in Fig. 5.3.1 can therefore claim both better performance and stability over the lifetime of the circuit. Consequently these specialised devices are considerably more expensive than simple general-purpose optocouplers.
Fig. 5.3.2 4N25 Optocoupler
Using the 4N25 for Audio
However it is also possible to use some of the much cheaper, general purpose phototransistor optocouplers for audio applications, such as the popular 4N25 illustrated in Fig. 5.3.2 also from Vishay, which is normally used for low frequency digital signals (in saturation mode) or DC applications (in linear mode) for limited audio applications.
This is because the 4N25 has a connection for the phototransistor base available, as well as collector and emitter. This allows the three different methods of connection shown in Fig 5.3.3 where the phototransistor can be operated in common emitter mode (a), common collector (emitter follower) mode (b), or used as a photodiode (c).
Fig. 5.3.3 4N25 Connection Choices
When the phototransistor output is taken from the base connection (with the emitter left unconnected) as shown in Fig. 5.3.3(c) the base/collector junction is being used as a photodiode. As the phototransistor now does not function as an amplifier the 'Miller Effect' (where the value of the junction capacitance is multiplied by the current gain of the transistor) does not apply, so the effectively large capacitance of the base/collector junction is greatly reduced, allowing the optocoupler to work at higher speeds, which means in the case of audio applications, a wider bandwidth is available, although this mode of operation greatly reduces the output signal amplitude.
In any of the configurations shown in Fig 5.3.3, the choice of load resistor has a significant effect on the output signal; the higher the value of RL the greater the amplitude of the output signal but the narrower the bandwidth, so the value of RL chosen is a compromise that depends on the purpose of the circuit.
For the 4N25 to provide isolation for audio signals the input to the infrared LED must be appropriately biased with a DC voltage, so that when a modulating AC (audio) signal is applied, the current through the LED can be varied without the optocoupler output reaching either saturation or cut off. This is really an extension of the linear mode of operation, and can be applied using either one of two basic configurations, phototransistor or photodiode.
Fig. 5.3.4 Audio Isolator Using Phototransistor Mode
Method 1. Basic Audio Optocoupler.
The circuit shown in Fig. 5.3.4, adapted from a design in Newnes Electronic Circuits Pocket Book by Ray Marston (ISBN 10:1-4832-9192-8) uses the 4N25 connected as a phototransistor to pass audio signals whilst isolating the input and output circuits.
An LM324 op amp is used here to drive the LED input of the 4N25. R1 and R2 form a potential divider to set pin 3 (the non-inverting input of the op amp) to half of the supply voltage and the audio input signal is superimposed on this DC bias voltage via C1 to modulate the current through the LED.
Because the LM324 is connected in voltage follower mode with the LED forming part of the negative feedback loop, pin 1 exactly follows the voltage variations at pin 3, so varying the current though the LED. With no signal applied the standing current though the LED is about 0.5 to 1mA depending on the value of R3 and the supply voltage used for the input circuit.
Fig. 5.3.5 Audio Bandwidth Using Phototransistor Mode
With separate input and output supplies set at 12V and an AC input signal of 3Vpp, an output signal of around 6Vpp is obtained when RV1 is adjusted to set pin 4 of the 4N25 to about half supply (6V). Depending on supply voltages and the amplitude of signals used, fine adjustment of RV1 also provides a means of achieving minimum distortion.
However, because Fig. 5.3.4 uses a 4N25 in phototransistor mode, the useful bandwidth of the circuit is rather limited, as shown in Fig. 5.3.5 making it useful for speech, but lacking gain at frequencies above 8kHz, a typical effect due to the large base/emitter junction capacitance of the phototransistor.
Fig. 5.3.6 Audio Isolator Using a 4N25 in Photodiode Mode
Method 2 Wide Bandwidth Audio
Fig. 5.3.6 shows an improved circuit with a full audio bandwidth, achieved by using the 4N25 in photodiode mode where only the collector base junction is used, forming a photodiode with much less capacitance than with the full phototransistor. The downside of this method is that the 4N25 output is now reduced to milli-volts rather than volts. For this reason it is necessary to add a buffer amplifier (IC3a) having high input impedance, immediately after the optocoupler.
Fig. 5.3.7 Audio Bandwidth Using Photodiode Mode
No voltage amplification takes place in the buffer stage, but this is followed by a x100 voltage amplifier to produce an output of approximately 7.5Vpp from an original input of 1.5Vpp. The audio bandwidth of the circuit is now increased to cover a useful range from about 400Hz to 18kHz as shown in Fig. 5.3.7; still not hi-fi but quite useful as part of an audio amplifier for many isolation purposes.
In the experimental (breadboard) version of this circuit different supply voltages between +5V and +24V were used for both input and output sections of the circuit without any problems apart from the lower supply voltages requiring signals of reduced amplitude to avoid distortion.
Noise generated around the optocoupler on the breadboard could be a problem but may be reduced or eliminated by careful PCB design, and extra decoupling of the supply lines close to the ICs.