# DC Analysis of Linear and Nonlinear Circuits in Schematic Design

November 15, 2019

These circuits can always use DC analysis

If you’re working with a new design that involves some custom circuitry, it always helps to understand how the circuit behaves in DC. There is some merit to this, particularly if you are designing a custom power conditioning system, amplifier, equalizer, or other circuit. These circuits and many more require a range of analyses to evaluate their functionality, but the most fundamental of these is a DC analysis.

## What is DC Analysis?

You can thank George Ohm and Gustav Kirchoff for giving the world the primary tools for DC analysis (and AC analysis). Although this is one of the simplest analyses, it tells you some valuable information about the behavior of your circuits. DC analysis effectively examines the steady state of a circuit when driven with a DC voltage, i.e., after all transients have decayed to zero and the system has reached a stable operating point. This analysis is applicable to time-invariant circuits (linear or nonlinear) and is performed directly from your schematic with a SPICE simulator.

In DC analysis, you are only examining the output from your circuit for a given input from a DC power supply. This involves sweeping through different input voltage values and monitoring the output voltage/current from the circuit. You can also examine the voltage and current for particular components in your circuit at each input voltage value. With linear time-invariant circuits, the output from DC analysis will simply be a straight line. If you are measuring an output current, the slope of this line is related to the inverse of the circuit’s Thevenin resistance.

In circuits with nonlinear components, such as diodes or transistors, the output from a DC analysis will not be a straight line. Although the curve could have an odd shape, this allows you to examine a number of properties of your circuit. One point to identify is the presence of saturation or exponential growth in the output from the circuit. These properties could help you explain the circuit’s stability when driven with different input signals.

Different portions of this DC-DC converter can be examined using DC analysis

It is important to note that, while DC analysis results form an important benchmark for further circuit design, there are many other behaviors that you cannot determine from a DC analysis alone, especially with nonlinear circuits. Some important examples include resonance and harmonic generation, modulation, how feedback affects signal behavior, and other important aspects of more advanced circuits. However, this allows you to quickly determine the appropriate operating limits for a circuit and make some important design decisions.

## Going Beyond DC Analysis and Making Design Decisions

As part of analog circuit design, DC analysis provides a starting point for further analog simulations. The output from a DC analysis for a linear circuit yields a functional relationship between the input voltage and the output current. When nonlinear elements are present in the circuit, you can place probes around these elements in order to determine how the current changes when the circuit’s supply voltage changes. This can help you decide the particular operating point for your circuit.

Consider an example from amplifier circuits. Transistors in amplifier circuits will eventually saturate, and you will need to determine the input voltage level in the circuit that produces saturation. If you want to work in either the linear or saturated regimes, you can determine the appropriate range of voltages that should be input into your particular circuit.

After you’ve determined an appropriate operating point for your circuit, you have a starting point for small signal analysis, sensitivity analysis, and other analyses in your circuits. If your circuit will switch between different operating points, you should perform a transient signal analysis or pole-zero analysis in order to examine the behavior of any transient signals as the circuit transitions between operating points. Which other simulations may need to be performed will depend on the exact functionality of your particular circuit.

You can perform DC analysis directly from your schematic with the right simulation tools.

## DC Analysis in Your PDN

Running a DC analysis in order to understand power distribution can be a difficult process when working in your schematic. There are a number of reasons for this. First, a schematic does not consider the geometry of your board or your traces. This means it cannot account for the inherent DC resistance of your traces, which leads to IR drop. It also cannot account for the DC resistance of vias and plane layers.

Because of the inherent DC resistance of conductors in your PCB, and the fact that the resistance depends on the geometry of various conductors in your board, you’ll need to use a post-layout simulation tool to examine power distribution in your PCB. This is where a PDN Analyzer tool can show you problems like hot spots, excessive voltage drop along vias and power rails, and even the potential for ground loops in your PDN.

The powerful PCB design and analysis tools in Altium Designer are ideal for running a variety of circuit simulations directly from your schematic, including DC analysis. This allows you to examine the performance of your circuits before you create your PCB layout. You’ll also have a complete set of tools for analyzing signal integrity and preparing deliverables for your manufacturer.