DC Analysis Of A Diode Circuit Using SPICE And Comparative M

DC Analysis of a Diode Circuit Using SPICE and Comparative Methods

The purpose of this experiment is to gain practical experience with the mechanics of a SPICE simulation, to compare its results with approximate hand calculations using different diode models, and to validate findings through laboratory measurement. The core objective is to analyze a diode circuit under varying assumptions and models, assessing the diode's state, voltage, and current in each scenario.

The experiment involves simulating the circuit with ideal and non-ideal diode models in SPICE, conducting hand calculations based on ideal and constant-voltage-drop (CVD) assumptions, and verifying outcomes with actual measurements. This comprehensive approach enhances understanding of diode behavior, SPICE simulation techniques, and the importance of model selection in circuit analysis.

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Introduction

Diodes are fundamental semiconductor devices with a unidirectional conduction property, critical in numerous electronic applications. Accurate analysis of diode circuits requires understanding both idealized models and more realistic representations that account for non-ideal behavior. SPICE (Simulation Program with Integrated Circuit Emphasis) is a widely used tool in circuit design, enabling precise simulations that incorporate complex device characteristics. This experiment explores diode circuit analysis through theoretical calculations, simulation, and experimental measurement, focusing on the 1N4148 small-signal silicon diode.

Optimal analysis of diode circuits involves considering different models. The ideal diode model simplifies analysis by assuming zero voltage drop when conducting (on state) and infinite resistance when off. However, this does not capture real-world behavior observed in physical devices. The constant-voltage-drop (CVD) model, which assumes a fixed forward voltage approximately equal to 0.7 V for silicon diodes, offers a more realistic approximation, especially at moderate voltage levels.

Preliminary calculations involve analyzing the circuit with these models, predicting the diode's operational state, voltage, and current. These predictions set expectations for subsequent simulations and measurements, facilitating comparison and validation.

Theoretical Analysis

Using the circuit configuration presented in Figure 1 (not shown here), the first step is to assume an ideal diode. When the diode is forward-biased, it conducts with zero voltage drop, and the current is limited only by resistor values and supply voltage. Conversely, when reverse-biased, it does not conduct, and the diode voltage is approximately zero.

Suppose the supply voltage is V_s, and the series resistor is R. Theoretical current, I, when the diode is on, can be calculated via Ohm’s Law: I = (V_s - V_D) / R. For an ideal diode, V_D = 0 V when conducting, thus I = V_s / R. When the diode is off, the current is zero, and the diode voltage is roughly equal to the supply voltage (if no other paths exist).

Second, using the CVD model where V_D ≈ 0.7 V, the diode current becomes I = (V_s - 0.7 V) / R, provided V_s > 0.7 V. If V_s

Simulation Methodology

Using PSpice software, the circuit is simulated twice. First, with the standard 1N4148 diode model from the component library, and second, with a custom diode model configured with specific parameters: IS = 10^-15 A and N = 0.01. The SPICE `.MODEL` statement for the second configuration is formatted as: .MODEL 1N4148 D( N=0.01 IS=1E-15 ).

The simulation involves a DC bias point analysis, which computes the steady-state operating point of the circuit. The resulting diode voltage and current are recorded from the simulation output, providing numerical data for comparison against calculations.

During the process, the diode model parameters are altered to observe the effects of more realistic conduction characteristics. The N factor influences the diode’s turn-on voltage and the shape of the I-V curve, thus affecting the simulated diode current and voltage.

Experimental Procedure

For laboratory validation, the actual circuit is assembled on a breadboard or test fixture with a 1N4148 diode, resistors, and the appropriate power supply. Using a digital multimeter, the diode voltage and current are measured directly. Voltage is measured across the diode, while current can be inferred by measuring voltage across a known resistor or directly if the meter permits.

These measurements provide empirical data for comparison with both theoretical predictions and simulation results, highlighting discrepancies attributable to model limitations, measurement tolerances, and real-world effects.

Results and Discussion

The first set of results involves theoretical calculations assuming an ideal diode. When the supply voltage V_s exceeds the voltage necessary to overcome the bias voltage and resistor limitations, the diode conducts, and the current is approximately V_s / R. For example, with V_s = 10 V and R = 1 kΩ, the ideal diode conducts with a current of roughly 10 mA and a diode voltage of nearly 0 V.

Switching to the CVD diode model, the forward voltage is set at 0.7 V. Under the same supply conditions, the diode is forward biased if V_s exceeds 0.7 V, and the current is (V_s - 0.7 V) / R, leading to a current of about 9.3 mA for 10 V supply voltage.

Simulations using PSpice with the standard library diode model produce similar results, with diode voltage around 0.7 V when conducting and current aligning with hand calculations. The custom diode model with N=0.01 and IS=10^-15 A introduces minor variations, slightly modifying the conduction behavior, especially near the threshold voltage.

Experimental measurements closely match simulation results when instruments are properly calibrated. Typically, diode voltage measurements around 0.7 V confirm the CVD assumption, and the measured current correlates with theoretical calculations. Minor deviations are due to real-world factors such as contact resistances, device tolerances, and measurement inaccuracies.

When the diode model’s N parameter is artificially set to zero, it results in a non-physical scenario. The diode behaves as a perfect conductor with zero voltage drop when on, producing unrealistically high currents, or as an insulator with no conduction. This highlights the importance of realistic parameters for meaningful simulation outcomes, closely reflecting actual device behavior.

Conclusion

This experiment demonstrates the significance of diode modeling in circuit analysis. The ideal diode approximation provides quick estimates but lacks accuracy under real conditions. The CVD model offers a better approximation, capturing the key characteristic of silicon diodes—a forward voltage around 0.7 V. SPICE simulations using realistic parameters yield results consistent with laboratory measurements, validating the model’s applicability.

The comparison reveals that precise diode parameters, like the N and IS values, influence the conduction characteristics. The experiment also underscores the limitations of overly simplistic models and emphasizes the need for realistic parameters for accurate simulations. Overall, integrating theoretical calculations, simulation, and measurement deepens the understanding of diode behavior in electronic circuits.

References

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