Part II Choosing and Using Photo Sensors Selection of the right photo sensor is the first step towards designing an optimal sensor-based system. The second step, and indeed a very important one, is the design of the front-end electronics to perform opto-electronic conversion. That is what this concluding part of the article focuses on Varsha Agrawal Part I of this article exposed you to the world of photo sensors, covering the whole gamut of various types of sensors, their principle of operation and characteristic parameters. After reading this concluding part, you will have knowledge of the frontend circuit configurations for various types of sensors and also wisdom to choose and design the right configuration for your intended application. Fig. 29: Photoconductor circuit Fig. 30: Photoconductor circuit Fig. 31: The higher the value of R, the higher the output voltage and the poorer the relative responsivity Fig. 32: The higher the value of R, the lower the output voltage and the better the relative responsivity Fig. 33: Basic trans-impedance amplifier Fig. 34: AC-coupled amplifier The most commonly used photo sensors are photoconductors, photodiodes, phototransistors, thermistors, thermopiles and pyroelectric detectors. The front-end configurations for these sensors are discussed here. Photoconductors Photoconductors are used in a variety of applications ranging from consumer items like camera light meters, clock radios, security alarms, street lights and outdoor clocks to infrared astronomy and spectroscopy applications. Photoconductors are usually used for detection of IR radiation. As discussed in part-i of the article, photoconductors are semiconductor materials whose resistance decreases with increase in the amount of incident light. When a bias is applied to the photoconductor in the absence of radiation, a current is generated which can be referred to as the dark current. When light is incident on the photoconductor, its resistance decreases and the current through it increases. Photo signal is the increase in the current caused by radiation. Generally, it is much smaller (of the order of a few parts in thousand) than the dark current. Extracting this small signal from the dark current is the primary task of the front-end optoelectronic circuit. Figs 29 and 30 show the simplest possible circuits for photoconductors. However, using the photoconductors 132 m a r c h 2 0 08 electronics for you
Fig. 35: Voltage-mode amplifier Fig. 36: Photovoltaic-mode photodiode Fig. 37: Photovoltaic-mode photodiode Fig. 38: Equivalent resistance across the photodiode Fig. 39: Effect of reverse voltage on the cut-off frequency in this mode reduces the responsivity of the conductor as the relative change in the circuit resistance is smaller because of load resistance R. The choice of R and R sen also affects the output voltage from the circuit. The higher the value of R in Fig. 29, the higher the output voltage and the poorer the relative responsivity (R rel ) as shown in Fig. 31. From Fig. 32 you can see that the higher the value of R in Fig. 30, the lower the output voltage and the better the relative responsivity (R rel ). Photoconductors are used in conjunction with amplifiers to obtain both good responsivity and high output voltage. Two possible circuit configurations are voltage-mode amplifiers and current-mode/trans-impedance amplifiers. The basic trans-impedance amplifier is shown in Fig. 33. As can be seen from the figure, the non-inverting input of the op-amp is connected to ground through resistance R com to minimise the DC offset voltage. The gain of the trans-impedance amplifier should be set such that the amplifier does not saturate at the maximum expected radiation intensity. Also, if the bias voltage of the photoconductor is more than the maximum rated input voltage of the op-amp, a zener diode can be added at the inverting input of the op-amp in the circuit of Fig. 33. The most common method used to extract the signal is to modulate the incident radiation at a specific frequency, either by placing a mechanical chopper in front of the detector or by electrically modulating the radiation source. The signal generated due to radiation is now an AC signal while Design Fig. 40: Effect of reverse voltage on the linearity limits Fig. 41: Photodiode in photoconductive mode Fig. 42: Photodiode in the dark current is a DC signal. The AC signal can be separated from the DC background signal using an AC-coupled amplifier (Fig. 34). As is clear from the figure, the output of the first stage is AC-coupled to the second stage. T h e v o l t a g e - mode amp l i f i e r i s shown in Fig. 35. The v o l t a g e across resistor R is AC-coupled to the operational As you can see from the figure, in this case only a single ope r a t i o n a l amplifier is required as opposed to two required in the case of Fig. 43: Photodiode in trans-imp e d a n c e However, two amplifier electronics for you m a r c h 2008 133
stages offer better noise reduction and better performance if optimally designed. Photodiodes As discussed in the first part of the article, photodiodes can be operated in two modes, namely, photovoltaic and photoconductive. The photovoltaic mode is generally used for low-speed applications or for detecting low-light levels. The two possible circuits in the photovoltaic mode are shown in Figs 36 and 37. The output voltages of these circuits are given by I det R and I det R f. respectively, where I det is the current through the photodiode. The circuit in Fig. 37 offers better linearity than the circuit in Fig. 36 as the equivalent input resistance for the photodiode in this case is R f /A, where A is the open-loop gain of the operational It is obvious that the value of R f /A is much lower than R in the case of Fig. 36. For a better linear response, the equivalent resistance across the photodiode should be as small as possible, as is evident from Fig. 38. Frequency response and linearity can be improved by operating the photodiode in. Figs 39 and 40 show the effect of reverse voltage on the cut-off frequency and linearity limits, respectively. The curves are representative. For exact values, you should look into the datasheet of the chosen photodiode. Reverse bias, however, increases dark current and noise levels of the photodiode. Figs 41 through 44 show four possible circuits using photodiodes in. In Fig. 42 the operational amplifier is used as a voltage amplifier, whereas in Figs 43 and 44 the operational amplifier is used in trans-impedance mode. For the circuit in Fig. 42, the output voltage and the effective resistance across the photodiode is I D R and R, respectively. The output voltage and effective resistance across the photodiode in Figs 43 and 44 are I D R f and R f /A, respectively, where I D is the photodiode current and A is the open-loop gain of the Fig. 44: Photodiode in Fig. 45: Load line for the photodiodes operating in Fig. 46: Avalanche photodiode Fig. 47: Common-emitter phototransistor operational The load line for the photodiodes o p e r a t i n g in photoconductive m o d e i s shown in Fig. 45. As you can see, c i r c u i t s with lowerresistance load line offer better linearity. A v a - l a n c h e p h o t o - d i o d e s (APDs) are also connected in a similar m a n n e r as normal Fig. 48: Commoncollector phototransistor photodiodes, except that a much higher reverse-bias voltage is required. Also, the power consumption of APDs during operation is much higher than of PIN photodiodes and is given by the product of input signal, sensitivity and reverse-bias voltage. Hence a protective resistor is added to the bias circuit (Fig. 46) or a currentlimiting circuit is used. An excessive input voltage higher than the supply voltage of the stage following the photodiode would damage it, so a protective circuit should be connected to divert the excessive voltages at the input to the power-supply voltage line. The gain of APDs changes with temperature. So if they are operated over a wide temperature range, some temperatureoffset circuit has to be added which changes the reverse-bias voltage in accordance with the temperature. As an alternative, a temperature controller has to be added to keep the temperature of APD constant. For detecting low signal levels, shot noise from the background light should be limited by using optical filters, bet- Fig. 49: A thermistor used for light measurement Fig. 50: A thermistor used for light measurement Fig. 51: Measurement circuit using a Wheatstone bridge and inverting amplifier 136 m a r c h 2 0 08 electronics for you
ter laser modulation and restricted field-of-view. Phototransistors Phototransistors can be used in two configurations, namely, common-emitter (Fig. 47) and common-collector (Fig. 48). In common-emitter configuration Fig. 52: Two-stage amplifier used with thermopile detector for gain of more than 1000 Fig. 53: A single-op-amp thermopile configuration to compensate for the temperature variations the output is high and goes low when light is incident on the phototransistor, whereas in common-collector configuration the output goes from low to high. The transistor in both these configurations can act in two modes, namely, the active mode and the s w i t c h e d m o d e. I n t h e switched mode, transistor is switched between cut-off and saturation and the output is either high or low. In the active mode, the transistor operates in the active region and the output is proportional to the light intensity. The modes are controlled by the value of resistor R. Thermistors Thermistors are photosensors whose resistance changes as their temperature changes with incident light. The simplest possible configurations in which a thermistor can be used for light measurement are shown in Figs 49 and 50. Fig. 51 shows the measurement circuit using a Wheatstone bridge and an inverting It is used for precise light control application. If the amplifier in the figure is replaced with a comparator, the Fig. 54: Four op-amp thermopile configuration to compensate for the temperature variations circuit could be used for light on-off control. Thermopiles Thermopiles are used as heat sensors to measure thermal radiation as they respond to a broad infrared spectrum. They do not require a bias voltage or current and have maximum response at DC, which falls off rapidly with increase in frequency. This is so because the material on which the junctions are mounted has a fairly substantial thermal mass and a low thermal conductivity path of the heat-sink. Hence, thermopiles are used to detect unmodulated radiation in applications like non-contact temperature measurements in industrial applications and process control, thermal imaging, thermal positioning and targeting, aircraft flame and fire detection, and so on. The responsivity of thermopiles is of the order of 10-100V/W and the typical signal output varies from a few tens of microvolts to a few millivolts. Thus these need low-noise and very low-offset operational amplifier for providing the gain. The gain required varies from as less as 10 to as large as 10,000 or more. Generally, for gains of less than 1000, a single-stage amplifier is used. For gains of more than 1000, two stages are used (Fig. 52). The thermopile signal (V sen ) would be positive or negative depending upon whether the temperature of the object filling the thermopile s field-of-view is greater than or less than that of the thermopile. Also, the output of the circuit varies with variation in the ambient temperature. It is therefore necessary to compensate for these ambient temperature-dependent variations. Many thermopile modules have an inbuilt thermistor to compensate for the ambient temperature variations. Two possible circuit configurations employing an inbuilt thermistor to compensate for the temperature variations are shown in Figs 53 and 54. The circuit in Fig. 53 uses a single op-amp, whereas that electronics for you m a r c h 2008 137
in Fig. 54 e m p l o y s four opamps. In Fig. 54, a single instrumentation opamp can Fig. 58: Voltage-mode pyroelectric detector b e u s e d instead of three op-amps A2, A3 and A4. Designers can choose the circuit taking into consideration the accuracy required, budget and the space constraints. In case the detector does not have a thermistor, the circuit shown in Fig. 55 can be used to eliminate the effect of variations in ambient Fig. 55: Alternative circuit to eliminate the effect of variations in ambient temperature temperature. The dependence of the output signal on the ambient temperature can be eliminated by alternately exposing the thermopile to the radiation to be measured and shielding it from radiation. The capacitive or AC coupling between the two amplifier stages allows only the difference signal between the radiant power from the source and the ambient temperature to pass to the second-stage Fig. 56: Voltage-mode detection circuit Fig. 57: A second stage capacitively coupled to the first stage of voltage detector Pyroelectric detectors Pyroelectric detectors are thermal detectors having a wide wavelength response. As discussed in the first part, they respond only to change in their temperature. Hence they respond to a modulated light input. They operate in two modes, namely, voltage mode and current mode. In the voltage mode, the voltage generated across the entire pyroelectric crystal is detected. In the currentmode operation, current Fig. 59: Current-mode pyroelectric detector flowing on and off the electrode on the exposed face of the crystal is detected. The voltage mode is more commonly used than the current mode. The circuit for voltage-mode detection is shown in Fig. 56. The operational amplifier chosen should have a very high input impedance of the order of 10 12 to 10 14 ohms. But the circuit suffers from the flaw that it is sensitive to ambient temperature variations. The problem can be avoided by keeping the detector at a constant temperature which is often not feasible. The more effective solution is to add a second stage that is capacitively coupled to the first stage (Fig. 57). The values of R5 and C1 are chosen such that the modulated light signal is passed to the second stage while the effect of ambient temperature drifts is blocked. Ambient temperature variations can also be removed by adding a compensation crystal in opposition, either in series or parallel. One crystal is exposed to radiation and the other is shielded from radiation. As the ambient temperature changes, the surface charge generated on one crystal is cancelled by the equal and opposite charge generated on the other crystal. The incident radiation, however, generates charge only on one crystal and is not cancelled. Voltage-mode pyroelectric detectors are generally integrated with a field-effect transistor. The shunt resistor (R S ) in the range of 10 10 to 10 11 ohms is added to provide thermal stabilisation. External connections include a power supply and load resistor R L (Fig. 58). The output voltage appears across R L. The circuit for current-mode operation is shown in Fig. 59. In current-mode operation, the modulation frequency can be much larger than in the case of voltage-mode operation. Hence, it is much easier to separate the signal generated from the ambient temperature drift. Concluded The author is a Scientist C at Laser Science & Technology Centre, Defence Research & Development Organisation (DRDO), Delhi, ministry of Defence, government of India 138 m a r c h 2 0 08 electronics for you