ECE1201- Electronic Measurements and Circuits Laboratory
Experiment #4 -- Diode Characteristics and Circuit Models
The silicon diode is a fundamental component in modern electronics. You have or will be studying the theory of these devices in ECE 1247 and their applications in ECE 0257. Refer to Sedra & Smith, Chap. 3 (4th or 5th edition), for a detailed discussion. In this lab we will be concerned with measuring the diodes’I-V characteristics and developing useful models of the diodes from these measured characteristics. The circuit symbol for a diode is:
When VD is positive we say the diode is forward biased and refer to the appropriate ID-VD characteristic as the diode’s forward i-v characteristic. This characteristic is described by the following non-linear relationship:
(1)
where IS, n, VT are constants related to the diode’s fabrication and its ambient temperature. (See Section 3.2 of Sedra) For many applications we may assume that when VD≤0 (reverse biased region) then ID = 0. (If VD is negative and –VD/nVT > 4 then ID -Is where Is is small and is termed the reverse saturation current.)
In the forward biased regime the diode is non-linear. We would like some reasonable piece-wise linear models for the diode so that we can analyze circuits which have diodes with linear circuit analysis. The simplest model is shown below:
This models the diode as a short circuit when VD > 0 and an open circuit when VD<0.
A somewhat more accurate model, often used in digital circuit analysis is shown on the next page.
In this model the diode does not conduct for VDV′ volts.
For silicon diodes, V′ ≈ 0.6 –0.7 volts. This model corresponds to a piecewise linear circuit and can be described by the diode depicted in Model 1 in series with a voltage source V′.
The most accurate model typically used for the forward region is characterized below:
Experiment:
1.First you will measure the i-v characteristic for a regular silicon diode with the circuit below:
Measure ID for a range of Vi values in the range [-10 V, + 10 V]. Keep ID below 100 mA in these measurements. If your power supply won’t give you a low enough voltage to get points over the full range of interest for VD, then consider inserting a voltage divider at point A to get a reduced voltage. Take enough points to get a good plot of the v-i characteristic. You won’t need too many points in the reverse region, but will want to take a lot of points as ID begins to rise in the forward region. Measure ID and VD by taking voltage measures at points A and B with respect to the circuit ground. (This is preferable to using the ammeter feature of the DMM since the insertion of the ammeter would affect the circuit more than the placing of the DMM voltmeter across the stated points in the circuit.)
2. Find the value of IS and nVT. There are two good ways to do this. A) Plot the data in the region where VD > VT. In that case and a plot of ln(ID) versus V will be a straight line of slope 1/(nVT) and intercept ln(IS). b) Alternately if you plot the ID versus V in same strongly forward biased region and use Excel to establish an exponential trend line, y = A exp(Bx), the value of A will be IS and B will be 1/(nVT).
3.Next estimate the parameters VDO and RD from Model 3 using the measured v-i characteristic.
Build the circuits below and measure the currents through D1 and D2. (Do this directly with the ammeter feature of your DMM) The batteries shown in the diagrams are realized with DC power supplies.
Use the piecewise linear diode model with the parameters VDO and RD as found in Step 3, to calculate the currents through D1 and D2. Compare these theoretical predictions with the measured data in this step. (Note: the +5 volt sources are realized with your dc power supply – not a 5v battery.) The analysis of these circuits should be completed as part of your pre-lab.
5.Next measure the i-v characteristic for a light emitting diode (LED) using the methods in step 1. Then estimate the parameters VDO and RD for the LED. How do these differ from a standard silicon diode? How does the light output vary with forward diode current? How would you design a circuit to drive such a diode with a voltage source and with a particular forward current?
- Finally you will be considering a variation on the silicon diode termed a Zener diode. (See section 3.6, Sedra 4th edition or section 3.4 Sedra 5th edition.) Such diodes are designed to be operated in the reverse biased region and have relatively low “breakdown” voltages. The breakdown voltage is the reverse voltage where the diode begins to conduct heavily. A typical model for the Zener diode in its reverse region is shown below:
Figure A. The V-I characteristic for the Zener diode.
Measure the Zener diode's reverse i-v characteristic with the following circuit:
Figure B. The current ID shown in this figure is the negative of current IZ shown in Figure A. The voltage VD in this figure is the negative of the voltage VZ shown in Figure A.
With R=150 use the same methods you have used for the other i-v characteristic measurements. Then estimate RZ and VZO for the Zener diode model given above. Develop a linear circuit model for the reversed-biased Zener diode. Discuss how a Zener characteristic might be useful in “clamping” a voltage at some point in a circuit (i.e. making sure it doesn’t exceed some maximum known voltage) especially if a wide variety of Zener breakdown voltages is available. Zener diodes in series yield a higher threshold voltage.
7.For the above circuit, make a “load-line” plot i.e. plot ID = Vps/R – VD/R. The slope of this plot is
-1/RD and the “y” or ID intercept will be Vps/R. The plot is known as a load-line. Draw the Zener VD-ID device characteristic on the same graph. The intersection of the two curves determines the circuit operating point for fixed Vps and fixed R. The use of a load-line to find the operating point for a diode is explained in Sedra and Smith (5th edition page154, 4th edition page 157). Although the Sedra discussion deals with a normal (not Zener) diode, the same principle can be applied.
- At any time before or during the performance of this lab learn to use the curve tracer instrument in the lab. The TA will instruct in its use. Measure the i-v characteristics of the three diodes (silicon, LED and Zener) with the curve tracer. Compare these results to the VD-ID characteristics measured directly.
Lab updatedJanuary 24, 2006
Minor change September 12, 2006
Item 7 updated September 18, 2006
Wording changes to item 8 on September 18, 2006
Review and minor changes September 17, 2007
Minor changes on January 28, 2008
Wording changes to Item 2 on September 21, 2011
Revised August 21, 2017