Analog Circuit Simulation:Netlists.

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Netlists

Before simulating, a circuit must be coded into a netlist. Figure 13.1 shows the circuit for a simple differential pair. Circuit nodes are formed wherever two or more elements meet. This particular circuit has seven nodes, which are numbered zero to six. The ground or datum node is traditionally numbered as zero. The circuit elements (or branches) connect the nodes.

The netlist provides a description of the topography of a circuit and is simply a list of the branches (or elements) that make up the circuit. Typically, the elements may be entered in any order and each has a unique name, a list of nodes, and either a value or model identifier. For the differential amplifier of Figure 13.1, the netlist is shown in Figure 13.2.

The first line gives the title of the circuit (and is required in many simulators). The next three lines define the three voltage sources. The letter V at the beginning tells SPICE that this is a voltage source element. The list of nodes (two in this case) is next followed by the value in volts. The syntax for the resistor is similar to that of the voltage source; the starting letter R in the names of the resistors tells SPICE

Analog Circuit Simulation-0119

that these are resistors. SPICE also understands that the abbreviation “k” after a value means 1000. For the two transistors Q1 and Q2, the starting letter Q indicates a bipolar transistor. Q1 and Q2 each have three nodes and in SPICE, the convention for their ordering is collector, base, emitter. So, for Q1, the collector is connected to node 3, the base to node 4, and the emitter to node 2. The final entry “m2n2222” is a reference to the model for the bipolar transistor (note that both Q1 and Q2 reference the same model). The “.model” statement at the end of the listing defines this model. The model type is npn (for an npn bipolar junction transistor), and a list of “parameter = value” entries follow. These entries define the numerical values of constants in the mathematical models which are used for the bipolar transistor. (Models will be discused in more detail later on.) Most commercial circuit simulation packages come with “schematic capture” software that allows the user to draw the circuit by placing and connecting the elements with the mouse.

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