Operating Point Analysis #

Operating point analysis is a fundamental simulation tool in Breadpad that calculates the DC voltage and current values at each node in your circuit under steady-state conditions. This is essential for understanding how your circuit behaves with no time-varying signals present.

What is Operating Point Analysis? #

Operating point analysis (sometimes called DC analysis or bias point analysis) solves for the steady-state condition of your circuit. It:

• Calculates the DC voltage at every node
• Determines the current through every component
• Provides the baseline around which small-signal AC analysis is performed
• Is the first step in most other analyses (like transient or AC analysis)

When to Use Operating Point Analysis #

• Verifying bias conditions in amplifier circuits
• Checking that transistors are operating in the correct region (e.g., saturation vs. linear)
• Determining DC power consumption
• Confirming voltage divider outputs
• Troubleshooting DC-coupled circuits
• Validating that components are within their specified operating ranges

Using Operating Point Analysis in Breadpad #

Setting Up the Analysis #

1. Build your circuit on the breadboard
2. Place voltage probes at points of interest
3. Open the oscilloscope and select “Operating Point” from the oscilloscope menu

Running the Analysis #

1. Click “Run Simulation” to calculate the operating point
2. Results are displayed as a bar chart showing voltage levels at each probe point
3. Additional data is shown in a table format, including:
   • Node voltages
   • Branch currents
   • Component power dissipation

Interpreting Results #

• All voltage values are in reference to ground (0V)
• Positive current flows from higher potential to lower potential
• Power values indicate heat dissipation in each component

Advanced Features #

• Search: Filter results to find specific nodes or components
• Export: Save the operating point data as CSV for further analysis
• Temperature Analysis: See how operating points shift with temperature (Premium feature)
• Component Parameters: View the operating conditions of active components like transistors

Example: Transistor Bias Circuit #

For a simple transistor amplifier:

1. Build a common-emitter amplifier circuit on the breadboard
2. Place probes at the base, collector, and emitter of the transistor
3. Run operating point analysis
4. Verify that:
   • Base-emitter voltage is ~0.7V (for silicon BJTs)
   • Collector voltage is between supply and ground
   • Collector current matches your design calculations

Tips for Effective Operating Point Analysis #

• Always check operating points before running more complex simulations
• Verify that active components are biased in their intended regions of operation
• Compare simulation results with hand calculations to verify circuit understanding
• Use operating point results to detect potential problems like:
  • Transistors in cutoff or saturation when linear operation is desired
  • Excessive voltage drops across components
  • Components operating outside their safe operating area

Troubleshooting with Operating Point Analysis #

If operating point analysis fails to converge or shows unexpected values:

1. Check for disconnected nodes (floating inputs)
2. Verify component values and connections
3. Ensure your circuit has a complete path to ground
4. Look for components operating outside their models’ valid ranges
5. Add small resistance values (e.g., 1 GΩ) to floating nodes to help convergence

Operating point analysis is often the first step in identifying issues with your circuit design before moving on to more complex simulations.