Solve 10 Op Amp Design Issues w/ Custom SPICE Models

Why dive into these short design topics?

• Solve real-world design issues.
• Develop intuition with hands-on simulation.
• Refresh the theory underlying the circuit.

 

WHY CUSTOM MODELS?

Custom SPICE models provide powerful insight.

• Easily change any device parameter (Gain, Bandwidth, Slew Rate, etc.)
  • quickly isolate and understand impact of each on circuit behavior.
  • not possible with manufacturer's models.
• Use ideal op amps to test early circuit ideas.
• Add real-world behaviors progressively - which are most critical?
• Avoid SPICE convergence problems with many manufacturer’s models.
  (custom models have simpler internal blocks.)

 

DESIGN TOPICS

Topic	Design Issue	Op Amp Behavior
1	DC Accuracy	Open Loop Gain (Aol)
2	Bandwidth	Unity-Gain Freq (fu)
3	Signal Shape	Slew Rate Max
4	Voltage Clipping	Voltage Limit
4	Current Clipping	Current Limit
6	Output Offset Error	Input Offset Voltage
7	Output Offset Error	Input Bias Currents
9	Overshoot and Ringing	Cstray (and Ccomp)
8	Overshoot and Ringing	Ro (and Cload)  New!
10	Overshoot and Ringing	2nd,3rd Poles

 

DESIGN STRATEGY

A simple apprach helps you solve design Issues!

• Encounter a design issue and verify with a simulation.
• Adjust the op amp parameters of the custom SPICE model to solve the design issue.
• Choose an alternate op amp based on your adjusted model parameters.

 

OP AMP MODELS

Op Amp Model	Behaviors Modelled
opamp_1	Open-Loop Gain (Aol), Unity-Gain Freq (fu), Ro
opamp_2	add - Slew Rate Max
opamp_3	add - Voltage Limit, Current Limit, Floating Reference
opamp_4	add - 2nd and 3rd poles

 

• Use opamp_1 early in the design cycle to develop the overall topology.
• Later in the design, add higher level models to learn which parameters are most critical.
• Copy to new file and develop your own models.
• NOTE: Input Offset Voltage and Input Bias Currents can be added externally to any model.

 

LEARNING STRATEGY

• Best way to learn skills and develop intuition?
• Play - Experiment - Ask Why - Theorize
• Practice these often – even in short sessions!

 

GET STARTED

• Pick a topic above and dive right in!
• Modify circuit or copy to a new file - explore circuits on your own.
• Get LTSPICE link here.