ECE2201 Lab Report 1

ECE 2201 – Microelectronic Circuits I

LAB 1 - DIODE CHARACTERISTICS

ABSTRACT:

This lab investigates the diode characteristics within a broad perspective. The experiments are designed to introduce students to the applications of the theory learned in the classroom. Each experiment is focused on a different subsection of diode characteristics; junction, signal, and rectifier diodes in different applications.

EQUIPMENT:

TDS 2004B Four Channel OscilloscopeAFG 3021 Single Channel Function Generator34405A 5 ½ Digit MultimeterGPS 3303 DC Power SupplyTransformer Power Supply1N4148 Diode (Part I & III)1k Ohm resistor (Part I & III)1M Ohm resistor (Part I)1N4004 Diode (Part II)1N5231B Zener Diode (Part II)51 Ohm Resistor (Part III & IV)

PART 1: DIODE V-I CHARACTERISTICS

INTRODUCTION:

The goal of the first experiment is to familiarize and explain the V-I Characteristics of the 1N4148 diode in forward and reverse bias.

Circuit Description:The V-I Characteristic for the 1N4148 diode was obtained by taking data samples of voltages in the simple circuit, adjusting the DC power supply between 0 and 10 volts.

The DC power supply voltage (V1) at 10 volts measured 9.43 volts over R1 and 0.608 volts over D1 (connected in Forward Bias), as displayed in the table below.

V1 (Measured) VR(Measured) VD(Measured) ID(Calc, mA)0.200 0.002 0.200 0.0020.410 0.055 0.356 0.0560.507 0.113 0.391 0.1150.593 0.177 0.415 0.1800.650 0.224 0.427 0.2270.690 0.260 0.434 0.2640.804 0.355 0.449 0.3601.000 0.530 0.468 0.5381.510 1.010 0.499 1.0242.990 2.450 0.543 2.4853.990 3.440 0.558 3.4895.030 4.460 0.572 4.5235.990 5.420 0.581 5.4977.000 6.410 0.589 6.5017.990 7.400 0.596 7.5059.010 8.410 0.603 8.529

10.010 9.430 0.608 9.564

As suspected, the data observed follows an exponential curve, as calculated by Shockley’s equation, which will be described after the reverse bias table and graph below.

Reverse Bias V1(Measured) VR(Measured) VD(Measured) ID(mA)

0 0 0 01.94 -1.768 0.004 -0.0044.09 -3.72 0.005 -0.0056.02 -5.48 0.006 -0.006

8.149 -7.418 0.007 -0.00710.02 -9.129 0.007 -0.00712.04 -10.967 0.007 -0.00714.08 -12.82 0.007 -0.00716.14 -14.69 0.007 -0.00718.01 -16.4 0.007 -0.00720.48 -18.65 0.008 -0.008

According to the data sampled, the reverse bias of the 1N4148 diode follows the expected linear format for voltages between -20V to 0V.

Shockley’s diode equation was used to obtain the value of saturation current (Is) and process parameter (n):

Two data points were chosen as VD1, ID1 (0.608V, 9.43mA) and VD2, ID2 (0.499V, 1.00mA). Assuming Vt is 0.026 and applying the following formula, Is drops out and we solve for n:

Solving for n leads to: n = (0.608 –

0.499) / ln(9.43/1)

* (0.026) n = 1.86

Now plugging n back into

Shockley’s equation to find Is … Is = 0.499 / (e (1 / (1.86 * 0.026) ) ) Is = 3.30 x 10-8 Data obtained via Shockley’s equation is graphed in the next figure, in logarithmic scale. Note the perfect R2 value (1) signifying a perfect exponential curve (linear in logarithmic scale).

Based on our data, a constant voltage drop model of a diode on a current of 5mA can be derived graphically or numerically. According to Figure 2 (Forward Bias Diode Characteristic), a current of 5mA will cause a drop in voltage of about 0.58 Volts.

Using Shockley’s equation; VD = n * VT * ln( iD/Is ) 1.86 * 0.026 * ln ( 0.005 / 3.3 x 10-8 ) 0.576 V. Another method would be to pick a data sample close to 5mA (5.49 mA), where the voltage drop is recorded as 0.581 V, which agrees with our other forms of analysis.

To create a piece-wise linear model, rd will be chosen using the equation rd = ΔVD/ΔID: where ΔVD/ΔID were found by drawing a tangent line to the bias point. rd = (0.62 – 0.53) / (10mA – 0mA) = 0.09 / 10mA 9 Ω. VD0 was found graphically, as the x-axis intercept with the piece-wise line.

Experimental Results:Each model has it’s own conveniences and disadvantages. The constant voltage drop model exhibits an easy to use method that is moderately accurate within the range of its operating characteristics (in this case, about ± 10% tolerance within 3-7mA). In addition to not being accurate around varying voltages, the constant voltage drop model does not make sense in certain situations (i.e. less than 0.57 V, or negative voltage near or around the breakdown voltage). The piecewise linear model is somewhat more accurate and the formula is simple: ID = (VD-VD0) / rD if VD < VD0. This “small signal” method works well for designing linear amplifiers using transistors, but becomes obsolete at small (or negative) and higher voltages.The exponential model is the most accurate, but also takes the most time. It is also, due to its non-linear nature, the most difficult to use.

PART 2: DIODE V-I CHARACTERISTICS (SWEEP METHOD)

INTRODUCTION:

This part of the lab introduces the sweep method by using the oscilloscope as a curve tracer. 1N4004 diode is places in the circuit and then replaced by the Zener diode for the second part of the experiment. The difference between the two diodes is the way they operate. The 1N4004 diode has a reverse bias which stays zero while the zener diode has a significantly smaller break down voltage.

Circuit Description:

A transformer, a 120 Vrms power source, two different types of diodes are used to apply the sweep method. The V-I characteristic for both diodes are shown on the oscilloscope. The first channel of the oscilloscope was set to 200 mV and Channel 2 to 5V.

1N4004G Diode is connected in a forward bias configuration on Channel A while Channel B is connected to the negative side of the transformer. The V-I characteristic is displayed in the simulation and on the oscilloscope in the following figures.

Experimental Results:

The graphs on the simulation and the ones gathered in the lab are very similar. Same scale is used both on the simulation and in the lab. V-I Characteristic for the two diodes are significantly different. The forward bias for the 1N4004G diode has a curve with a smaller slope while the forward bias for Zener diode is steeper. Their reverse biases are also different; the zener diode has a steep curve which means its break down voltage is a lot smaller. The zener diode has more utility for circuits such as linear regulator whereas the 1N4004G diode is used mostly for blocking negative voltage and current. For this reason, we tried these two diodes which have different uses to observe how the sweep method applies in different areas.

PART 3: DIODE SIGNAL PROCESSING

INTRODUCTION:The purpose of this experiment is to observe and quantify the voltage and current of the 1N4148 diode used in a “clipping circuit.” The waveform of the input and output voltage (Vdiode) were measured and compared to inspect the basic theory behind the clipping circuit; where the diode should “cut” the input voltage to a fraction of its peak, which is commonly used to provide a safe voltage range for sensitive circuitry.

Circuit Description:

A function generator is used to provide a 5Vpk sine wave at 1kHz, in addition to a DC power supply at 2V.

The observed physical representation on the oscilloscope exactly matches the simulated environment. The diode clips (near 1V) while blocking negative current.

Below, the Voltage Transfer Characteristic is shown, with Vin displayed on the horizontal axis, and Vout on the vertical axis (Vout as a function of Vin) at 2V per division (horizontally and vertically). As expected, our physical observations match the simulation.

Design Challenge: this circuit was designed to clip the signal at ±0.7 volts. As displayed, the diode clips the voltage at about 0.7 volts (on a 1 V/div scale).

Experimental Results:In conclusion of this particular experiment, we learned that our experimental methods contained a small amount of error (due to the identical simulations and observations) which prove that the 1N4148 diode can be used in a “clipping circuit” to prevent large voltage spikes, and keep a regulated voltage. However, a second diode connected reverse in parallel would help to counter negative voltage spikes, as indicated on the Voltage Transfer Characteristic; the slope continues linearly when negative (apart from the exponential-like curve on positive voltages) which may be harder to predict.

PART 4: DIODE SWITCHING FREQUENCY LIMITATIONS

In this part of the experiment, diodes were evaluated at varying frequencies to determine their limits as rectifiers.

Figure 1: 100 HzAt 100 cycles per second, the diode acts normally as we would expect.

Figure 2: 1 kHzOne kilohertz is also a normal operating frequency for this diode, according to the simulation and observation

Figure 3: 10 kHzThere are now perturbations in the sine wave of the diode. The ability for the diode to block negative voltages is being affected by a higher frequency. This is officially where the diode stops operating as normal.

Figure 4: 100 kHzAt 100kHz, the diode’s operation is severely affected. The voltage reaction within the diode does not take place fast enough to keep up with the 100,000 cycles per second, as is in the following figure.

Figure 5: 1 MHz

The diode in this circuit, operated at 1MHz, is almost unnoticeable. The higher the frequency, the closer to the actual sine wave the diode gets, except for the voltage drop that takes place during the forward bias.

Conclusion:The first of four experiments in this lab involved measuring, graphing and plotting the V-I characteristics of the 1N4148 signal diode in forward/reverse bias. We determined that the signal diode operates in an exponential form in forward bias, linearly in reverse bias (within our voltage range). The second experiment was to graph and examine the models of Shockley’s equation, the constant voltage drop model, and the piece-wise linear model while explaining the advantages and disadvantages (listed in detail on page 7). The third experiment used the sweep method (or curve tracer) to examine the voltage transfer characteristics of the 1N4004 and 1N5231 diodes, which indicated very different transfer curves proving the wide utility of different applications. The final experiment inspected the utility of the 1N4148 diode in a clipping circuit, which proved useful in preventing damage to sensitive circuitry, whereas the frequency limitations of the 1N4004 rectifier diode proved that beyond 1kHz, the diode does not function properly.