Lab5_Fall2023

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Lab Assignment 5: Diode Characterization and Applications Revision February 08, 2023 1 © Washington State University School of EECS Summary In this lab, various diodes (general purpose switching, rectifier, and Zener diodes) are studied for their characteristics in forward and reverse biased modes. These diodes are used in circuit application such as voltage regulator, voltage clipper, peak detector and voltage doublers. The following tasks will be performed: • Obtain I D vs V D characteristics for forward biased and reverse biased diodes using either a curve tracer (TEK571) or Digilent Discovery III. • Estimate I S and n from the data obtained in the lab assignment. • Voltage Regulator using a Zener diode. • Voltage Clipper. • Voltage Doubler circuit Learning Outcomes: After completing this laboratory assignment, you should be able to: • Obtain diode characteristics and estimate diode equation parameters. • Simulate diode SPICE characteristics using obtained diode model parameter and compare them. • Design a voltage regulator circuit using a Zener diode. • Analyze a voltage limiter/clipping circuit. • Analyze a voltage doubler circuit and observe the effect of load on its performance in terms of the output voltage and ripple voltage. Required Equipment • EE352 Analog parts kit • Breadboard • Function Generator • Oscilloscope • DMM (2) • DC Power Supply • TEK 571 Curve tracer or Discovery III

Lab Assignment 5: Diode Characterization and Applications 2 © Washington State University School of EECS I. Diode Forward Characteristics: In this part of the lab assignment, you will obtain the characteristics curves for forward biased diodes (1N4001 and 1N914) and calculate the critical diode parameters such as I S and n . Pre-Lab: (a) A diode equation is given as 𝐼𝐼 𝐷𝐷 = 𝐼𝐼 𝑠𝑠 ( 𝑒𝑒 𝑉𝑉 𝐷𝐷 𝑛𝑛𝑉𝑉 𝑇𝑇 − 1) where 1 being very small compared to 𝑒𝑒 𝑉𝑉 𝐷𝐷 𝑛𝑛𝑉𝑉 𝑇𝑇 term, it can be ignored to derive diode parameters I S and n . This diode equation can be expressed in a linear form by taking natural log on both sides, i.e. ln( 𝐼𝐼 𝐷𝐷 ) = ln( 𝐼𝐼 𝑠𝑠 ) + 𝑉𝑉 𝐷𝐷 𝑛𝑛𝑉𝑉 𝑇𝑇 . Two variables n and I s can be calculated by taking two sets of (I D , V D ) data points. (b) Derive an expression for the dynamic resistance of a diode, di dv r D = Plot dynamic diode resistance r D as a function of diode voltage. Indicate the point of maximum resistance. Is it consistent with the plot of a diode characteristic? (c) Obtain LTSPICE diode curve by defining a custom diode in a schematic diagram and plotting I D vs V D . In LTSPICE define a diode model as .model my_diode D( I s =1E-12, n=1.7) and select V D as x-axis and I D as and y-axis . These parameters will be updated later with the calculated values for diodes 1N914 and 1N4001. Lab Procedures: 1. Using the circuit below obtain 10 data points (I D , V D ) for diodes IN4001, and 1N914. 2. Plot I D vs. V D for diodes 1N914 and 1N4001 and demonstrate it to the TA Note: (i) Make sure DMM1 is configured to measure current, and DMM2 to measure voltage. (ii) You will find that there is very small current for voltage less than 0.5V, and at higher voltage the current increases very rapidly. Therefore, be careful while incrementing voltage V. Figure 1. Diode voltage and current measurement 2. Use Curve Tracer TEK571 to obtain the diode characteristics. You may use Digilent Discovery III board if Curve Tracer is not available. Take the picture of plots for both diodes 1N914 and 1N4001. Make sure that you have at least 30mA as maximum currents for these plots. DMM2 Voltmeter 270 Ω 0-10V DMM1 Ammeter

Lab Assignment 5: Diode Characterization and Applications 3 © Washington State University School of EECS 3. Estimate diode equation variables I S and n using data in part 1. 4. Use these values for I s and n to plot diode characteristics in LTSPICE. Are these plots similar to the pictures taken in part 2. Comment on the similarity or dissimilarity between the picture taken and LTSPICE plots. Also comment on the I S and n values for the two diodes used. 5. Determine I s and n using a least-squares fit of a curve to the data obtained in (1). Use any software available to you for data fitting. Assume room temperature to be 27 Ο C or 300 Ο K to calculate thermal voltage V T. II . Diode Reverse Characteristics In this lab assignment, the reverse characteristics of a switching diode and a Zener diode are studied. Pre-lab: None Lab Procedures: 1. Construct the circuit shown in Figure 2. Make sure that the OP27 is connected to +15V and - 15V power supplies and its ground is connected to the ground of the rest of the circuit. Use 1N914 switching diode in the circuit. Measure the amplifier resistors R 1 and R 2 and compute the expected amplifier gain G a = 1 + (R 2 /R 1 ). 2. You may not have three power supplies (0-10V or higher) available on your bench. You may use function generator output as PS power supply when it is operating with minimum of ac voltage @100 Hz and variable offset voltage as dc power supply. This offset voltage is the value for PS dc output. 3. Reduce PS voltage to zero and measure output voltage, V out at pin 6 of the OP27as shown in Figure 2. This voltage (V os1 ) is due to op-amp offset voltage and bias currents. You may have to use a 100K Ω, 47K Ω, or 22K Ω resistor for R in the circuit. Recommend starting value of R = 100K Ω . If R is too large, the op-amp can go unstable when the input is 0 volts, due to feedback of the offset currents through R. To check stability, use a x1 scope probe to measure V out with DC coupling on 5 volts/division scale, when the input is set to 0 volts. If there are large oscillations or noise in V out (≥ 1V p -p), reduce R to the next smallest value and re-check. Use the largest value of R that gives you a stable V out , as larger values of R give you larger readings on the DMM and therefore increase your measurement resolution. Make sure you measure and record the resistor R. 4. Vary PS voltage from 0 to 10V (by changing offset voltage of the function generator) in 10 steps and measure and record the op amp output voltage, V out , using a DMM set on volts DC connected between V out and ground. Make sure that the op-amp output voltage is not saturated, i.e. close to (within 1 volt of) power supply voltage for op-amp, when increasing the PS voltage. If the amplifier saturates, use a lower value for R and continue your measurements from the point you left off; note in your lab notebook when you changed the value of R and also what the measured R was. Note that the measurement in step 5 below must also be done for every one of the 10 steps in the measurement of V out . The reverse diode current is given as 𝐼𝐼 𝑠𝑠 = (V out − V os1 ) / (G a × R).

Lab Assignment 5: Diode Characterization and Applications 4 © Washington State University School of EECS For each measured V out , compute I S using the above formula with the values of resistor R and gain G a that were actually used to take the measurement. 5. For every step on the measurement of V out in part 4, also measure the voltage V D across the diode, using a second DMM set on volts DC with the positive lead hooked to the cathode side of the diode and the negative lead hooked to the other side of the diode. Very important: you cannot measure V out accurately while the second DMM is measuring V D . You must disconnect both leads of the V D DMM before taking any V out measurement. You may however leave the V out DMM connected throughout the experiment, as it will not affect the V D measurement. 6. Plot reverse diode characteristic I S vs V D . Is the value for 𝐼𝐼 𝑠𝑠 at the highest value of V D in agreement with the value calculated earlier? Demonstrate your results and get it checked by the TA. 7. Repeat steps 1-6 using Zener diode 1N4735. You might have to reduce resistor R to 10K or even 1K and amplifier gain to a smaller value such as gain of 11 to keep op-amp output voltage V out from saturation. When you are varying PS voltage beyond 4V, you should vary it in smaller increments such as 0.1 or 0.2 volts or so. (Note: once PS gets close to 6.0 V, recommend reducing R to 1K and amplifier gain to 11. Make a note on your lab notebook measurements whenever you switch either R or the amplifier gain, and record measured value of R and value of gain consistent with measured values of the amplifier resistors.) 8. Demonstrate the Zener diode results to the TA. Figure 2. Experimental set up to measure diode reverse characteristics III. Voltage Regulator In this lab assignment, we will design a voltage regulator to supply 6.2 V dc voltage. We will use Zener diode 1N4735. Its voltage regulation will be checked at 5mA load. Pre-lab: Using the Zener diode (1N4735), design a voltage regulator which can supply current 0-10mA. Note the value of resistor R used. The Zener diode is rated for 6.2V at 1mA. Use 10V power supply for calculations. Diode 4 + 3 - 2 7 6 OP27 A B Vout +15V -15V 1K 100K PS, 0-10V R R 1 R 2 −

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