EE 2212

EXPERIMENT 4

28 February and 7 March 2013

ADDITIONAL DIODE CIRCUITS

This is a two week experiment. Work to be done 28 February and 7 March with a report due Thursday, 14 March. The report is to be double length, that is a maximum of 6 pages plus a cover page, which includes an abstract, in the usual format. The report will be evaluated on a 40-point scale, rather than a 20-point scale.

PURPOSE

Experimentally study the following circuits

Ø Double-diode clipper; both time domain and transfer (vo versus vs) characteristics

Ø AND Gate

Ø OR Gate

Ø Precision Rectifier, time domain and transfer characteristics

Ø Obtain C_j(V_R) for the 1N4001 by constructing an adjustable corner frequency analog passive low-pass filter and compare to the data sheet. This is the only circuit that requires some computations.

BACKGROUND

In addition to rectification related to power supply applications as demonstrated in Experiment 3, diode circuits are used to obtain a variety of important signal processing functions. Among them is the clipper, precision rectifier, LC network electronic tuning, and diode logic. You will have an opportunity to demonstrate these applications both experimentally and using SPICE simulations.

Ø For example, inherent in many ICs is the use of diodes to limit input voltage transients to levels that do not damage the IC. We will observe this necessary diode protection function when we study MOS and CMOS IC technology in a couple of weeks. Virtually all MOS ICs have this integral to their design.

Ø Diode logic is a good way to illustrate Boolean functions using simple hardware realizations and useful for power switching applications. Refer to your 25 and 27 February class notes on the AND and OR gates implemented with diodes. To a degree, diode logic is part of more complex digital IC families.

Ø Precision rectification is used in DSP (Digital Signal Processing) applications where the “switch” and absolute value function needs to be implemented but there must be a minimization of the effect of the diode forward voltage. Can we design a circuit that negates the 0.7 volt forward voltage drop? Of course the answer is yes or why would we spend the time in the lab!

Ø We will also study “electronic tuning” of a circuit. We will also measure C_j(V_R) for the 1N4001 by constructing an adjustable corner frequency analog passive low-pass filter. The C_j(V_R) is useful for electronic tuning of communications systems. Refer to the 1N4001 data sheet distributed on the class WEB page as well as the specialty devices on the Motorola data sheets, also distributed on the class WEB page. Also refer to Problem Set 5. It is important to review the passive LPF circuits we discussed in class and measured in lab the first two weeks of the semester.

COMPONENTS

Ø 1N4001 silicon diodes

Ø mA741 operational amplifiers

Ø A selection of resistors between 1 kW and 100 kW

PROCEDURE

Ø Construct the circuit shown in Figure 1. This circuit is called a Double-Diode Clipper. Initially, set v_s(t) = 7 sin (2p x 100t). Slowly adjust the amplitude of v_s(t) and observe and record the effect on v_o(t) for various positive and negative values of V1 and V2. Also look at the transfer characteristics and compare your results to the handout distributed in class. A triangular wave with a 7 volt peak amplitude will also work.

Figure 1 Double-Diode Clipper

Ø Construct the circuits shown in Figures 2 and 3 , an AND gate and OR gate respectively. Set various combinations of VA and VB voltage levels to verify the appropriate logic gate operation. Use a square wave on one of the inputs recognizing that you will need to DC off set the square wave voltage such that the minimum voltage is 0. Suggest R on the order of 5 kW. Define the voltage ranges for the LOGIC ZERO and ONE logic levels

.

Figure 2 Diode Logic AND Gate

Figure 3 Diode Logic OR Gate

Ø Measure the transfer characteristic of the circuit shown in Figure 4a. Pay particular attention to the effect of the diode offset voltage. Now construct the circuit shown in Figure 4b. Use ±12 volts for the mA 741 operational amplifier. Measure the transfer characteristic and compare to the results in Figure 4b. Justify the term “precision rectification” when applied to the circuit in Figure 4b. Refer to Section 12.8 of the text, page 760.

Figure 4(a) Figure 4(b)

Ø This portion of the experiment will allow you to measure C_j(V_R) for the 1N4001 by constructing an adjustable corner frequency analog passive low-pass filter . These measured results will then be compared to the data curves distributed on the class WEB page and a SPICE simulation. Construct the circuit shown Figure 5(a). You will need to determine the effective value of R2 and C_Fixed. The best approach for determining R2, which is the input resistance of the oscilloscope is by using a DC voltage divider with R1. It will either be 1 Megohm or 10 Megohms depending upon the input resistance of your oscilloscope. It should be marked near the oscilloscope input connectors. Observe that the signal generator allows you to include a DC offset. The best approach to determine C_Fixed which includes the effective capacitance of your leads, the oscilloscope, and the terminal strip with your wiring is by measuring the -3 dB corner frequency and back calculating to obtain a value for CFixed. Basically, sweep the frequency of Vs with VDC = 0 to obtain the basic low-pass filter dB amplitude plot. Now connect the two 1N4001 diodes in the circuit as shown in Figure 5(b). Starting with VDC=0, measure the resultant -3 dB corner frequency and back calculate to obtain C_j for the 1N4001. Recognize that you must subtract the C_Fixed and there are two diodes in parallel. We assume both diodes are identical and the reason two diodes are used is to improve accuracy within the ranges of our instruments. Continue with several other values of VDC so that you have several values of C_j to compare with the 1N4001 data curves and to generate a CJO for a SPICE simulation. You will also compare against the SPICE 1N4002 library model. Also note that a data sheet for CJ for the 1N4XXX diode family was distributed on the class WEB page. To minimize the value of C_Fixed , be neat with your wiring dress. Also note that the scope cables will add about 30 pF/foot. Check this using the capacitance meter.

Figure 5(a) Baseline Circuit

Figure 5(b) Diode Capacitance Measurement Circuit, Tunable Low-Pass Passive Filter

For all the circuits, compare your experimental results with SPICE simulations and support your discussions from circuit analysis. Use the .TRAN SPICE analysis for the first four circuits and the AC analysis for Circuit 5.

TO THINK ABOUT AND INCORPORATE IN YOUR REPORT

Ø Did the circuits operate as expected? Justify analytically and using SPICE.

Ø How did the diode offset voltage effect the results?

Ø Suggest system applications for all the circuits.

This a historically classic data sheet for a Write Only Memory produced by Signetics Engineers with too much time on their hands. It actually slipped by the Signetics QC managers and was published in a data book before the “joke” was discovered. It has become a classic in the semiconductor industry. Read it carefully and enjoy!