eCircuit  Center
SOLVE OP AMP DESIGN ISSUES
LTSPICE SERIES

About SPICE | SPICE Basics | Running SPICE | CIRCUIT COLLECTION

# SPICE Model: Opamp_1

## SCHEMATIC

• Behaviors Modelled
• Gain
• Bandwidth
• Input / Output Resistance
• Design Guide
• Develop early topology / circuit ideas using super wide bandwidth device, fu=1e9.
• Later reduce fu to more realistic bandwidths.
• Finally, begin adding other real-world behaviors (slew rate, current limit) with higher level models.

## INPUT PARAMETERS

• Aol - Open-Loop DC Gain
• fu - Unity-Gain Frequency.
•  Also called Gain-Bandwidth-Product (GBP) in most op amps.
• Ro - Output Resistance.

## HOW IT WORKS

• DC Gain provided by current source B_G1 (unity gain) and R1
:  v(out) = i(B_G1)*R1 = (v(+) - v(-)) * R1
= (v(+) - v(-)) * Aol
• 1st Pole formed by R1, C1
: fp1 = 1/(2*pi*R1*C1)
• Unity-Gain Frequency formed by DC Gain and 1st pole.
:  fu = Aol*fp1

## INTERNAL CALCULATIONS

• .param R = Aol
• .param fp = fu/Aol
• .param C=1/(2*pi*fp*R)

## TEST IT

• Run a .TRAN simulation of Test_Circuit_opamp_1.asc
• Plot v(vs) and v(vo).
• Change fu - what is the impact on rise time?
• Change Aol - what is the impact on final value?