Diodes 1 Prelab Assignment
- Write the diode equation, ID(VD), identifying the key parameters: saturation current and ideality factor. You will be extracting the values of these parameters from the data obtained in the experiments.
Draw an equivalent circuit of a real diode that includes the symbol of a diode, series
resistance and a shunt resistance.
Using PSPICE or MULTISIM simulate ID(VD) curves for a real diode in the forward and reverse bias. Use the diode equivalent circuit from with an ideal diode, and two resistors, one in 5Ω – 15Ω range, and one in 10 GΩ – 20 GΩ range. Plot ID(VD) forward bias curves on a linear and a semi-log graphs in 0 – 1 V voltage range and the reverse bias curve on a linear graph in 1 – 20 V range.
- Diodes in switching circuits are rapidly turned on and off. Draw a schematic of a simple circuit consisting of a resistor and a diode connected in series with one terminal of the diode at ground. If a square wave ± 2 V is applied between the end of the resistor (the one not connected to the diode) and the ground, the diode will be switched on and off. Draw this waveform and the waveform of the voltage across the diode. Indicate when the dioce is on and off. Do you expect a delay in the diode switching as the waveform frequency increases? Will the delay mainly occur when the diode is switched on or off? Why?
Reference: Jasprit Singh Semiconductor Devices, John Wiley & Sons 2001. pp. 174 – 207.
Experiments with Semiconductor Diodes
Measure the forward bias I(V) characteristic of a silicon diode (1N4148) with digital meters.
In the forward bias, the diode current increases rapidly with small increase of the voltage across the diode so a safer and more convenient measuring method is using the power supply in the current source mode. Increase the current in small intervals while measuring the voltage with the benchtop DVM; do not relay on the less accurate voltage meter on the power supply. The rated power of N4148 diode is 500 mW, the rated current is 300 mA (400 mA for a short time). While recording the diode voltage VD and the corresponding current ID, plot these values on ID (VD) graph using Excel or other graphing program. This will help you to decide how many data points (and where) are needed to obtain a smooth curve of the diode characteristic.
To measure reverse bias current you will need a very sensitive ammeter. Connect it in series with the diode. Connecting to the power supply make sure that the diode is reverse bias! Putting a 10 k to 100k resistor in series may be a good idea. Increase the reverse bias voltage form 0 to 20 V and observe the current. You may not have zero current at zero V as the sensitive meter may be difficult to balance but note the difference between 0 V and 20 V and calculate the resistance based on these two points. Note: If you use a voltmeter to measure the voltage, be careful how you connect it so that you do not disturb the measurement of ID.
The goal of these measurements is determining the saturation current, the ideality factor (using a semi-log graph) and possibly estimating the series resistance from the forward bias I-V curve and the shunt resistance from the reverse bias curve (on a linear graph).
Hint:The diode equation on the semi-log graph is a straight line (except near the origin). Its slope and the intercept are defined by the diode parameters. If all data points do not follow a straight line on a semi-log plot select those that do and fit to them an exponential trend line, displaying its equation on the graph. If points corresponding to high current divert from the exponential trandline, they may be plotted on a linear graph to estimate the diode series resistance from a linear trend-line equation. You may not reach high enough forward current to find this resistance precisely but at least you may estimate its lower limit.
Measurements with Agilent U2722A source measure unit.
This instrument is a computer controlled voltage or current power supply which also measures both current and voltage supplied to the circuit. To measure a diode characteristic you can program the instrument to step up the voltage in increments. You can then obtain an I-V curve after exporting the data to a graphing program like Excel. In setting up the instrument, you should select the proper range of voltage and current to obtain precise measurements. Just like using a digital multimeter, you do not select a 10 A range to measure a current of 1.5 mA because you will not get precise results. When your measurements cover a wide range of values you may have to switch ranges for low and high values.
- Measure the forward bias ID (VD) characteristics of the same diode used in (1). Use the maximum current range (120 mA). Compare graphically with the results in (1)
- Using Agilent source measure unit obtain forward bias characteristics of another diode, for example 1N4001.
- Use Agilent U2722A source measure unit for testing a Zener diode (such as 1N4733A). Obtain both forward and reverse bias curve but set up the instrument to obtain enough points in the “interesting” voltage range, which will be different for the two biasing directions. Plot data on a linear I-V graph and fit the linear trend line only to the points on the straight part of the reverse bias curve. Its slope will give you Zener resistance Rz and its intercept with the voltage axis Zener voltage Vz for this particular diode.
- Diode switching. Supply a square wave signal to a 1N4001 diode through a series resistor (1k to 10k). Use two properly adjusted oscilloscope probes, one across the input terminals (waveform generator), and the other across the diode. The oscilloscope inputs should be set to DC. Save the oscilloscope screen images on a computer, preferably showing the measurements with cursors. Include the images in the report.
- Adjust the square wave amplitude (± 2V, measured on the oscilloscope) and the frequency of 1 kHz to 5 kHz. Record signals from the two oscilloscope channels simultaneously (on one screen). Identify parts of the signal that correspond to the diode forward and revers bias and measure their amplitude.
- Without changing the generator signal amplitude, increase the frequency to between 50 kHz and 100 kHz. Measure the time relationship between the input and output signals. Do you see a delay in the diode switching ON or OFF?
Report and Analysis
- Present clearly all schematics of the experimental circuits. Show all relevant clearly labeled graphs. Include data tables for part 1 (they can be put in an appendix). The data tables must have titles indicating to which measurements they refer. Do not include large data spreadsheets for part 2. Your instructor may ask you to upload them, in which case make sure that you label them indicating which measurements they represent.
- Extract parameters of the diodes: saturation current, ideality factor, and when possible equivalent series and shunt resistances from measurements 1 and 2. Determine parameters Rz and Vz from measurements 2 b. The values of the parameters must be clearly related to the graphs of the data and trend lines form which they were derived.
- For part 3 include schematic of the diode and the resistor showing which terminals were connected to the oscilloscope, the waveform generator, and ground.
- Comment if the values of the derived parameters are reasonable, in agreement with your understanding of the measured devices.
- Explain the observed amplitude difference between the input and output waveforms in 3a and between the input and output waveforms timing for the two frequencies in 3b.