Learning to Design Circuits by Using Multisim

We're currently covering the electricity section in physics class. I asked my dad if there was any software that could help me design circuits, and today he taught me how to use a software called Multisim. Not only can it draw circuits, but it also allows you to simulate them using virtual power sources and testing instruments—it's really amazing!

The basic interface of Multisim

An example

This is one of my physics electricity problems—I used Multisim to run an experiment and test it. It's so intuitive! You can perform circuit experiments without even building a physical circuit board.

In the circuit above, at first I was confused why the current through R2 isn't 9V/100Ω = 90mA, but instead 82.2mA. Turns out it's because the diode has a forward voltage drop (typically ~0.7V). I tried measuring the voltage across diode D2 with a multimeter—and sure enough, there was a voltage.

Study on the Forward Conduction Voltage Drop of Diodes

The forward conduction voltage drop of a diode refers to the voltage drop across its terminals when it is forward-biased and conducting current. Below is a detailed explanation:

Principle: When a diode is forward-biased (i.e., the anode is at a higher potential than the cathode), majority carriers can flow through the PN junction, creating a forward current. During this process, a voltage drop occurs due to the characteristics of the PN junction and the resistance of the semiconductor material. This voltage drop is termed the forward conduction voltage drop.

Typical Values:

• Silicon diodes: Generally around 0.6–0.7V.

• Germanium diodes: Smaller, typically 0.2–0.3V.

Affecting factors

1. Material Properties: Diodes made from different semiconductor materials exhibit variations in their forward conduction voltage drop due to differences in physical characteristics such as bandgap. For example, silicon has a larger bandgap than germanium, resulting in a higher forward voltage drop for silicon diodes compared to germanium diodes.

2. Temperature: Temperature influences the forward conduction voltage drop. Generally, as temperature increases, the voltage drop decreases slightly. This occurs because higher temperatures enhance carrier concentration in the semiconductor material, improving conductivity and reducing the voltage required for conduction.

3. Current Magnitude: Within a certain range, an increase in forward current leads to a slight rise in the voltage drop. However, when the current exceeds a critical level, the increase in voltage drop slows due to saturation effects in carrier mobility and junction behavior.

Reverse Conduction of Diodes

There is another type of diode that can conduct in reverse bias, and its voltage can stabilize within a specific range after conduction. This diode is called a Zener diode. I conducted experiments on this.

Usage of Signal Generator and Oscilloscope

I designed an inverter using a transistor, which involved the use of a signal generator and an oscilloscope. The results were very intuitive.

Multisim Component Library

Multisim has an extensive component library that can basically meet the needs of common circuit design. There are many components I'm not yet familiar with, so I'll need to learn them gradually. However, with Multisim for simulation experiments, my learning process will become much easier.