SPICE Model: Opamp_1

SCHEMATIC

 

• Behaviors Modelled
  • Gain
  • Bandwidth
  • Input / Output Resistance

• LTSPICE files
  • Op Amp Schematic: Opamp_1.asc
  • Op Amp Symbol: Opamp_1.asy
  • Test Schematic: Test_Circuit_opamp_1.asc
  • Right Click on filename, select "Save link as...",

• 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.

• Back to Design Series

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?

 

• Back to Design Series