EE 2212

EXPERIMENT 4

3 October 2019

Diode Parameter Extraction and the Half Wave Rectifier

PURPOSE

Ø Use laboratory measurements to extract key diode model parameters including I_S, n (also called η or in SPICE, N) from the I_D-V_D measurements of the 1N4002. Don’t confuse “n”=N = η ideality factor with n as an electron concentration. Apparently not enough letters in the alphabet!

Ø Implement designs of the half wave rectifier circuit and measure time domain characteristics and the transfer characteristic, vo(t) vs. vs(t).

Ø Measure and compute ripple voltage as a percentage and as an rms value. You can use both the soft-keys on the oscilloscope or the multimeter. I will demonstrate this.

Ø Compare individual diode results and circuit results using SPICE simulations.

Ø Problem Set 4, due on Wednesday, 2 October, should be very helpful especially with respect to SPICE simulations.

COMPONENTS

Ø 1N4002 Diodes (Use the 1N4002 diode model in the SPICE library)

Ø 100 Ω and 1 kΩ resistors

Ø 0.1 μF, 1μF, and 10μF capacitors Actual values not critical since you are just showing the “filtering/smoothing” effect to minimize ripple voltage.

PROCEDURE

I_D-V_D Characteristics and Diode Model Parameter Extraction

Ø Using SPICE, simulate the circuit shown in Figure 1. Obtain the I_D-V_D characteristic curve for the 1N4002 in SPICE over a range at least of 0 to 0.8 volts for V_D and find the diode current value for the diode when V_D = 0.7 volts. For this, it might be useful to use a DC voltage sweep in conjunction with a VDC source. In addition, you will need to change the x-axis value to be the voltage across the diode (v+) – (v-) under Plot_Axis Settings…_Axis Variable…-

Ø Examine the model characteristics for the 1N4002 PSPICE, which can be

found by selecting the device and then Edit_Model…_Edit Instance Model (Text)… You will use this information for comparing to your measurements.

Ø Construct the Figure 1 circuit. Use the multimeter to measure I_D and the multimeter also to measure V_D. Note the I_D is measured by measuring the voltage across the series resistor and dividing by R, that is apply Ohm’s Law. Pay attention to the diode orientation. The banded side is the cathode end. Change the supply voltage V_S to adjust I_D to the desired current setting, then measure V_D. Take enough readings to accurately define the diode characteristic. You should measure out to I_D values of a few mA. Record your results in a data table in both your laboratory notebook and in your laboratory report. Use EXCEL for calculations and graphing. For example your data columns might look like:

VD

VR

ID=VR/R

LOG(ID)

Number of data points is at your discretion. You can then graph ID vs. VD and LOG (ID) vs VD for parameter extraction. It is then straight forward to get intercept and slope using a linear regression. Refer to Problem Set 4.

Ø Do not use of the multimeter for measuring current directly because very often the internal fuse in the mutimeter is “blown”, that is open-circuited, given the high laboratory equipment usage factor and it is a stinker to replace since we have to open the case-poor instrumentation design in my opinion! I will explain what not to do!!! Use Ohms law to obtain current by measuring the voltage across the series resistor. The power supply ammeter is not as accurate for this current measurement.

Ø Consider the equation which approximates to when the diode is forward biased. To facilitate graphing over a number of orders of magnitude we obtain and graph,

Note that log(_{base 10)} e = 0.434

Ø From this equation, determine and fit a straight line (linear regression) to your plotted log(I_D)-V_D semi-log graph. Your equation will be in the form y = mx + b.

Ø Use these data to find I_s and n. Compare to the SPICE model parameters.

Figure 1

Half-Wave Rectifier

Ø Refer to Figure 2. Change your signal source to a 10 volt zero-to-peak 100 Hz sinusoid. Perform a SPICE transient analysis simulation and observe the half-wave rectified output like we did in class last week. Refer to class notes. Also note the effect of the diode offset voltage when you compare the input and output waveforms. Observe and plot Vout(t) and the transfer characteristic, Vo vs Vinput.

Ø Experimentally observe the operation on the oscilloscope in both the time domain and as a transfer function.

Ø Now we want to “smooth out” the pulsating DC by using capacitors. Place a C across the 1 kΩ resistor. Now use all three values of C to illustrate the change in the ripple voltage by measuring Vout(t). Use the the oscilloscope to measure the rms voltage of the output using dc and ac coupling. Explain the differences in these measurements and explain what these measurements are illustrating. Use your diode model and check your lab measurements using SPICE. Observe that ripple voltage is defined as either the (DV/Vpeak) x 100% or as (Vrms or as Vrms of the output-voltage/Vpeak)x 100% )x 100%. Watch your polarity on the electrolytic capacitors or else Also, since electrolytic capacitors have a broad tolerance, their values must be checked on the capacitance meter to obtain accurate results.

Figure 2

(An added historical note: The background screen is a photo of a “cat whisker” diode used as an AM radio detector in the 1905-1915 era of early radio before the widespread use of vacuum tubes. You can purchase these setups on the WEB. A sharp “springy” wire (“cat whisker”) formed a pressure (point contact) junction with a galena crystal. Galena is PbS (lead sulfide) and has a bandgap of about 0.4 eV. Historically, of course, the underlying physics was unknown at the time. The physics to explain this phenomena was a couple of decades in the future. Primitive, but it did work-sort of. A reincarnation of this was used by soldiers in World War II in what was called a “foxhole radio”. The junction for detection of strong AM radio signals was a sharp wire contacting a “blue edge” razor blade to form a crude junction. The metallurgical “bluing” process to harden the steel cutting edge on the single edge razor blade of the time creates a difference in the work functions between the wire and the metal razor which results in a rectifying junction. I will talk more about AM radio generation and detection in lab.

This is the historically classic data sheet for a Write Only Memory produced by Signetics Engineers with too much time on their hands and probably written over a long liquid lunch. This data sheet actually slipped by the Signetics Quality Control managers and was published in a data book before the “joke” was discovered. It has become a classic in the semiconductor industry. I was never able to find out if there was a subsequent employment issue with the engineers involved but things were different in the industrial world in those days. Read it carefully including the footnotes for this “Write Only Memory”.

enjoy!