Laboratory sessions on light emitting diode (LED) and photodiode Testing the constancy of gain‐bandwidth product Parts 
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2 Red LEDs Photodiode FDS100 General purpose transistor NPN (2N3904 or equivalent) General purpose Op‐amp with 1MHz unity gain bandwidth (LF411 or equivalent) Function generator that can reach 1MHz Power supply +10V Science goals 1. Characterization of LED a. Determine the I‐V curve: What is the threshold voltage and breakdown voltage? b. What determines the brightness of the LED? Voltage or current? c. How fast is the response of a LED? 2. Characterization of photodetector a. Build an ultrasensitive photodetector that is more sensitive than your eyes! b. What determines the response time of your photodetector? c. Test the prediction of Gain‐bandwidth product Basic information Transistor Red LED Polarity of FPS 100 1a. I‐V curve A. Make the following connection with V=10 Volt. Remark: If your ammeter is not sensitive enough to perform the measurement, you may also measure the voltage across the resistor R or the diode with a voltmeter, but remember your voltmeter has an impedance of 10M Ohm (or 10G Ohm if you have a high end multimeter). B. Use several resistors in the range of R=80 to 1M and measure/calculate the current I and the voltage across the diode. Remark: Recommended values: 100, 1k, 10k, 100k, 1M. You may try R<100 , but the resistor will get hot. With even lower R, LED may not last very long. C. Plot i vs Vdiode. The result should be similar to the follow plot. D. Now reverse the diode and measure the I‐V curve in the reverse bias regime. Remark: you might need a different strategy to measure the very small current. Do not go below ‐40V, or the diode will be damaged. The result should be something like this. E. Does your measurement agree with the model we discussed in the class: 
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V
I  I 0 exp( diode )  1 ? V0
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Remark: A typical response of a diode is shown below. 1b. Brightness of LED A. To measure the brightness of LED, first build the following bare bones photodetector. Remark: Reverse bias improves the sensitivity and speed of the photodiode, but do not go beyond 20V. Choose RL=1 k. If you do not see much noise, you can remove the noise filter section. B. Fix the photodiode as close to the LED as possible. Use 1 k for the LED resistor. Do you see the output voltage V0 depend on the LED emission? If yes, this is your first photodetector. C. Record V0 for different RL and show that the voltage is linearly proportional to resistance. Higher resistance gives you a more sensitive photodetector. The catch is that the output impedance is also higher. Remark: Recommended range for RL = 100 , 1 k, 10 k, 100 k. D. Now keep RL = 1 k and measure the illumination power of LED when you vary the current through the LED. Plot the photodetector signal as a function of the LED current. Remark: You may repeat 1a.B and try resistor R=100, 1k, 10k, 100k and 1M in the LED circuit, and record the photodiode output V0. You should get an almost linear dependence like below. Remark: At very low light level, make sure the photodetector signal is not polluted by room light or your force field. E. Explain why the light emission intensity of a LED should linearly depend on its current I, instead of its voltage V or power P=IV? 1c. How fast is the response of a LED? A. Build the following transistor circuit to switch on and off a LED. Use Vdd=10 Volt. Start with function generator at 1Hz square wave and make sure you can see the LED is blinking at 1Hz. Remark: adjust the amplitude and offset of the function generator so you see a good contrast between LED on and LED off. B. Connect the photodiode output V0 to an oscilloscope and make sure you can see the 1Hz signal on the scope. Optimize the contrast on the scope and record the peak‐to‐peak voltage. This is the “DC response”. C. Now switch to the triangular wave /\/\/…. Is the photodetector signal also shows a triangular wave /\/\/…? If not, choose the proper offset and amplitude on function generator so the response is as close to a triangular wave as possible. 2a Build a high gain photodetector using the following circuit. 1.
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Remark: If you do not have a 10 M resistor, 1M would be fine too. Using the golden rules in the class, show that the output is V=–IoRF. So ‐RF is the gain. The 2pF is used to suppress feedback instability. If you still see oscillations at the output, put in a larger capacitor or smaller resistor. The above circuit is extremely sensitive to spurious light that enters the photodiode. Isolate the photodiode from other light sources. With this ultrasensitive detector, you should be able to detect pico‐Watt light intensity. A. Turn on your LED circuit at the lowest level of current that your eye can barely see the light. Can your photo detector see it? If the lowest level of detectable signal is 1mV on the photodetector, how many times is your detector more sensitive than your eyes? B. Go back to a reasonable brightness of LED such that your photodetector gives you a signal of V = 1V, and apply a sin wave to the LED transistor circuit. Adjust the sine wave offset and amplitude such that the photodetector output is approximately a sine wave with a peak‐to‐
peak voltage of 1 V. C. Now increase the modulation frequency and determine the bandwidth of your photodetector from the reduction of the amplitude. Remark: 1. Increase frequency logarithmically: 10 Hz, 30 Hz, 100 Hz, 300 Hz, 1 kHz… to the highest frequency that the signal is completely diminished or swarmed by noise. 2. Normalize your Pk‐Pk voltages measured at different frequencies to that at 1Hz and plot the ratio as a function of frequency in the log‐log scale. An example is shown below. Definition of amplitude dB (decibel) Gain  10  log10
AmplitudeAC
(dB) AmplitudeDC
Example 1: Amplitude reduces by 2. Gain = 10  log10
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 3 dB 2
Example 2: Amplitude reduces by 10. Gain = 10  log10
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 10 dB 10
3. Determine the bandwidth, which is defined as the frequency when the signal reduces by 3dB. This is the formal definition of the Bandwidth of the photodetector. 2b. An important prediction for the performance of an amplifier is the gain‐bandwidth product, which states Gain*Bandwidth = Constant (GBW). The gain of a photodetector is its sensitivity to external light, which is given by Rf. The above predicts that in order to improve the response time of the detector, we need to sacrifice the sensitivity. We will check if this is really so. (GBW is also called unity gain bandwidth in technical documents). A. From 2a, we have determined the bandwidth with Rf=1M or whatever value you used. Report your GBW in the unit of Hz. B. Now reduce Rf by a factor of 10 each time to 100 k, 10 k, 1 k… and measure the bandwidth again. Make a plot of the Bandwidth vs. Rf. in the log‐log scale. Do you see GBW is a constant? Remark: 1. Every time you reduce Rf, adjust the modulation offset and amplitude so the signal is still around 1V. Otherwise the signal would be 10x weaker and eventually so weak to be measured precisely. 2. If you do it properly, GBW should be between 109 and 1010  Hz and is determined by the quality of the photodiode and the op‐amp you are using. C. (option) if you indeed reach the limit of the op‐amp with GBW > 109  Hz, you can ask TA to find you a different op‐amp or photodiode. You may have a nice circuit to distinguish fast op‐
amps/photodiodes from so so ones. Congratulations, you have finished the lab sessions.