Analysis Guide

What is Circuit Analysis?

Circuit analysis is the process of determining voltages, currents, and other important parameters in an electrical circuit. GEEC provides multiple analysis engines to explore circuit behavior under different conditions.

There are two fundamental approaches to circuit analysis:

Numerical Analysis

Numerical analysis computes approximate numerical solutions using simulation engines:

• Engine: NGSpice (industry-standard SPICE simulator)
• Output: Numbers and graphs showing circuit behavior
• Speed: Fast computation
• Best for: Complex circuits, non-linear elements, time-domain behavior
• Examples: DC operating point, transient response, frequency response

Symbolic Analysis

Symbolic analysis provides exact mathematical formulas for circuit behavior:

• Engines: Python, SymCirc
• Output: Mathematical expressions and formulas
• Speed: Slower for complex circuits, but provides insight
• Best for: Linear circuits, transfer functions, design equations
• Examples: Symbolic transfer functions, frequency response equations

Choosing an Analysis Type

Analysis Decision Tree

Question 1: Is your circuit linear?

• No (contains diodes, transistors) - Use NGSpice numerical analysis
• Yes (only R, L, C, and sources) - Consider both approaches

Question 2: Do you need exact formulas or approximate numbers?

• Formulas - Use Symbolic analysis
• Numbers/graphs - Use NGSpice numerical analysis

Question 3: What do you need to find?

Analysis Workflow

Every analysis in GEEC follows this workflow:

Step 1: Design Your Circuit

Before running analysis, your circuit must be:

• Fully connected (no floating components)
• Have defined component values
• Preferably include a ground node (0V reference)

See Circuit Basics for help.

Step 2: Switch to Analysis Tab

1. Click the Analysis tab in the top menu bar
2. The analysis tab shows five sub-tabs:

Step 3: Select Analysis Type

Click on the desired sub-tab, then click an analysis button to add or reopen a dialog:

• Result analyses (OP, AC, TF, Tran): reuse the same dialog if already created; run automatically when opened
• Graph analyses (DC graph, AC graph, Tran graph, PZ graph): create a new graph window each time you click

Step 4: Configure Parameters

Each analysis type has specific settings:

• Frequency range for AC analysis (start freq, stop freq, number of points)
• Time range for transient analysis (0 to end time, time step)
• Sweep parameters for parameter studies (which component to vary, range)
• Output variables to calculate (for result-based analyses optional)

See detailed sections below for each analysis type.

Step 5: Run Analysis

• Single analysis: Click the Run button for that specific analysis
• All analyses: Press F9 or click Run All (Right panel)
• Wait for completion (progress indicator shows status)
• Results will be displayed in the dialog as graphs or tables

See Results & Export for details.

Analysis Types Overview

GEEC provides three analysis engines, each available in the Analysis tab:

Numeric (NGSpice)

Symbolic

Full symbolic analysis - every result is an exact algebraic expression.

Symcirc (Python)

Other

NGSpice Numerical Analysis

NGSpice is the default numerical analysis engine, ideal for most circuits.

Understanding NGSpice

NGSpice is the open-source successor to SPICE, the industry standard circuit simulator used for:

• IC design verification
• PCB circuit prototyping
• Educational circuit simulation
• General-purpose circuit analysis

Capabilities:

• Handles linear and non-linear circuits
• Supports semiconductor models
• Computes exact numerical solutions (within numerical precision)
• Fast computation even for complex circuits

NGSpice Analysis Types

GEEC offers the following NGSpice analyses:

Quick Start: Operating Point Analysis

The simplest analysis - find DC voltages and currents:

1. Create a circuit with DC voltage source and resistor (see Getting Started)
2. Switch to Analysis tab ? Numeric sub-tab
3. Click the OP button
4. The analysis runs immediately; results show voltages at each node and currents through components

Example: A 10V source with 1k? resistor:

Node 1 (positive terminal): 10V
Node 2 (negative terminal): 0V (ground)
Current through resistor: 10mA (by Ohm's law)

For detailed coverage of every NGSpice analysis type, see Basic Analyses.

Symbolic Analysis

For linear circuits only, symbolic analysis provides exact mathematical solutions.

When to Use Symbolic Analysis

Symbolic analysis is most useful for:

• Educational purposes - See the mathematical structure of the circuit
• Deriving transfer functions - Get equations for H(s) or H(jw)
• Parameter sensitivity - Understand how each component affects the circuit
• Design optimization - Express performance in terms of component values

Symbolic Analysis Limitations

• Only linear circuits - No diodes, transistors, or non-linear elements
• Smaller circuits - Complex circuits may take longer to analyze

Symbolic Analysis Types

For detailed information, see Basic Analyses and Other Analyses.

Analysis Dialog Menus

Every analysis dialog (both result and graph) has a menu bar with four menus:

Result Format Options (non-graph analyses)

For result-type analyses (OP, AC, TF, Tran), you can switch the number format from the Results menu:

AC Result View Types

For AC analyses, the Results menu lets you choose which quantities to display:

Result Representation

From the Results menu, toggle between:

• Plain - plain text display
• LaTeX - render results as LaTeX-formatted equations (useful for paper-ready output)

Favorites System

GEEC lets you mark frequently-used analyses as Favorites for quick access:

1. For results click on a star icon next to a given result
2. For graphs click File -> Save to Favorites in the graph's menu bar
3. The result/graph appears in the Favorites section in the right panel

Tip: Press F9 to run all currently open analysis dialogs plus all items in Favorites.