Phys 586 Laboratory Lab11

Goal: In this lab you will make measurements with a photodiode. While not the same type of diode used in diode arrays for radiation planning or microstrip detectors in high energy physics, many of the basic device characteristics are the same. Additionally, photodiodes are used in x-ray imaging (along with a scintillator layer) and avalanche photodiodes find application in high energy physics.

Reading: See Photodiode Characteristics article in the Readings.

Lab:

1. Construct the LED circuit shown in Figure 1.

a) Is the LED forward or reverse biased?

The LED is forward biased.

b) What is the purpose of the resistor?

The value of the resistor is chosen so that the specified LED operating current results in the desired LED voltage drop. The value that this resistor should have can be determined using Kirchhoff’s loop rule and Ohm’s Law.

Vary the voltage and convince yourself the brightness of the LED changes. Note the longer leg on the LED is the anode.

1 2. Measure the current through the LED and the voltage across the LED for LED supply voltages of 2.5, 5, 8, 10, 13, 15, 18 and 20V.

LED LED LED Supply Voltage Current Voltage (V) (V) (mA) 2.5 1.67 2.46 5.01 1.76 4.91 8.01 1.82 7.8 10.05 1.85 9.9 13 1.9 12.83 15 1.92 14.9 18 1.96 17.82 20 1.99 20.1

Circuit 1: LED Current vs LED Voltage

25

20

15

10 LED Current (mA) 5

0 1.65 1.7 1.75 1.8 1.85 1.9 1.95 2 2.05 LED Voltage (V)

2 3. Look over the data sheet for the photodiodes we are using.

a) What is the maximum reverse voltage?

For OP505(A-D), the max reverse voltage is 30 V.

For PDB-C157, the maximum reverse voltage is 30 V.

b) What is the typical dark current?

For OP505(A-D), the typical dark current is 4 nA (at TA=25⁰C). For PDB-C157, the typical dark current is 2 nA.

c) What is the response time?

For OP505(A-D), the response time is 500 ns.

For PDB-C157, the response time is 50 ns.

4. Construct the photodiode circuit shown in Figure 2. The LED should be made to illuminate the photodiode.

a) What mode of operation does this circuit correspond to?

The circuit corresponds to operation in zero-bias or photovoltaic mode.

Note the longer leg on the photodiode is the anode.

3 5. a) Measure the voltage across R2 with the room lights on and room lights off with no voltage across the LED.

With room lights on, the voltage across R2: V2 = 26.5 mV.

With room lights off, the voltage across R2: V2=0 mV.

So the saturation current should be Isat=V2/R2=2.65 μA.

b) With the room lights off or using the dark box measure the voltage across R2 for LED supply voltages of 2.5, 5, 8, 10, 13, 15, 18 and 20V.

LED Supply Voltage across R2: Voltage (V) V2 (mV) 2.5 0.7 5 5.5 7.98 13.5 10 19.4 13.1 28.6 15.02 34.5 17.99 43 20 48.6

4 6. Plot the ILED versus the voltage across R2.

Plot of I_LED vs. R_2

16 14 12

A) 10 μ 8

I_LED ( 6 4 2 0 0 10 20 30 40 50 60 Voltage across R2: V2 (mV)

a) Comment on these results.

These results were not at all what I expected. I expected that the initial voltage that we measured with the lights on, V2=26.5 mV, and the corresponding current, I2 = 2.65 microA gave us the saturation current for the LED. For each of the subsequent trials performed in the dark, I expected that the LED supply voltage would be split between V2 and the voltage across the diode. Then I expected that the voltage across the diode could be inserted into the exponential current-voltage characteristic equation, which would give the current across the LED, I_LED, which would be plotted against V2. I expected it to show I_LED decreasing with increasing V2. I expected that I_LED would then be similar to the current across V2. The numbers acquired with the aforementioned method made no sense, and thus I concluded that there was a mistake in my data taking, or that I did not understand the phenomenon.

The plot above was produced by using V2 in the IV-characteristics curve.

5 7. Construct the circuit shown in Figure 3. The LED should be made to illuminate the photodiode.

a) What mode of operation does this circuit correspond to? Note the longer leg on the photodiode is the anode.

This circuit is operating in reverse bias mode.

8. With the lights off and with the voltage to the LED off, make a measurement of the voltage across R3 for photodiode supply voltages of 0, 0.1, 0.2, 0.5, 1, 2, 5, 10, and 15V. Also measure the voltage across the photodiode for each supply voltage setting. You should do this latter step separately.

Photodiode Supply Photodiode Photodiode V3 Voltage (V) Voltage (V) Current (μA) (mV) 15 13.4 -14.8 2.7 10 8.99 -9.9 2.2 5 4.42 -4.8 1.6 2 1.71 -1.9 1.5 1 0.79 -0.8 1.6 0.5 0.387 0.4 1.5 0.2 0.13 0.1 1.6 0.1 0.0000245 0 1.9 0 0 0 0.9

a) Does the photodiode current agree with your expectations? Explain.

Qualitatively, the photodiode current does agree with my expectations. That is, since the photodiode is operating in reverse bias mode, the photodiode voltage is negative, which means that at some value of that photodiode voltage, breakdown occurs, and the photodiode current becomes large and negative for more negative photodiode voltages. However, earlier in the lab, I noted that the breakdown voltage is expected to be 30V for these photodiodes, and the data does not agree with that expectation.

6 9. Repeat this procedure using 5V and 10V for the LED supply voltage.

LED Supply Voltage: 5V Photodiode Supply Photodiode Photodiode V3 (V) Voltage (V) Voltage (V) Current (μA) 15 13.12 14.5 0.308 10 8.67 9.6 0.306 5 4.17 4.6 0.302 2 1.437 1.6 0.3 1 0.586 0.6 0.298 0.5 0.098 0.1 0.296 0.2 -0.159 -0.2 0.276 0.1 -0.171 -0.2 0.201 0 -0.18 -0.3 0.176

LED Supply Voltage: 10 V Photodiode Supply Photodiode Photodiode V3 (V) Voltage (V) Voltage (V) Current (μA) 15 12.33 13.6 1.085 10 7.91 8.7 1.079 5 3.39 3.7 1.063 2 0.672 0.7 1.048 1 -0.142 -0.2 1.02 0.5 -0.214 -0.7 0.624 0.2 -0.229 -1 0.308 0.1 -0.232 -1.1 0.275 0 -0.233 -1.1 0.228

7 a) Plot the photodiode current (y) versus the voltage across the photodiode (x) for all three data sets on the same plot.

Photodiode Current vs. Voltage Across the Photodiode 5

0 -15 -10 -5 0 5 -5 LED Supply Voltage = 0

-10 LED Supply Voltage = 5 V LED Supply Voltage = 10 V -15

-20 Photodiode Voltage (V)

Photodiode Current (microA) b) Comment on these results.

The above plot likely shows an error in the data taking process, as it makes very little sense with what I expect to see. What I expect to see are three classic photodector I-V characteristic curves, which are shifted vertically based on the photocurrent induced by the light from the LED. That is, as the LED supply voltage is increased, the the I-V curve is shifted downward by induced photocurrent in the photodetector, I_p. So we should see three “classic” photodetector I-V curves separated by the increase in photocurrent induced by the increase in the LED supply current.

8

10. Set the photodiode voltage to be 10V. Make measurements of the voltage across R3 for LED voltages of 2.5, 5, 8, 10, 13, 15, 18 and 20V.

Photodiode Supply Voltage: 10V LED supply Voltage (V) Photodiode Current (μA) V3 (V) 2.5 9.8 0.0422 5 9.5 0.321 8 9 0.767 10 8.7 1.112 13 8.2 1.633 15 7.8 1.96 18 7.3 2.47 20 6.9 2.78 a) Plot the photodiode current (y) versus the LED current (x).

Photodiode Current vs. LED Current 12

10

8

6

4

2

0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 Photodiode Current (microA) LED Current (microA)

9

b) Comment on your results.

Again, I am not sure what to make of these results. I think that we either did not perform the correct measurements, or we did not correctly assemble the circuit. The slope of this curve should reflect how the photocurrent I_p induced in the photodetector by the light generated from the LED depends on the LED supply voltage. That is, the photodiode supply voltage is kept constant at 10V, so the photodiode current will just be shifted down by the induced photocurrent I_d. As the LED supply voltage increases, the LED current increases, and thus the intensity of the LED light shining on the photodetector increases, and thus the induced photocurrent increases, which shifts the photodiode current to lower values. This plot does actually show the photodiode current decreasing with increasing LED current, which agrees with the explanation above. But I must again note that I am unsure of my calculation of the LED current. *** We did not perform the part of the lab with the op amp. (Parts 11-13)

In your lab writeup, please include:

1. Answers to the questions embedded in the lab procedure above.

See above

2. Plot of the voltage across R2 versus the LED current for circuit 2 See above

3. Plot of the V-I curve of the photodiode using circuit 3.

See above

4. Plot of the photodiode current versus LED current for circuit 3.

See above

5. Printout of your pulse on the digital oscilloscope.

Did not perform this part.

10 6. Circuit 4 forms the basis of a pulse oximeter. Briefly what does a pulse oximeter measure and what is the principle of the measurement. An application of photodiodes!

A pulse oximeter measures O2 saturation by detecting the absorption of certain wavelength pulses. It determines the absorbances due to to pulsed arterial blood alone, while excluding extraneous sources of “signal” such as venous blood, skin, bone, muscle, and fat. Source: http://en.wikipedia.org/wiki/Pulse_oximetry

11