SPICE Model: Opamp_2

SCHEMATIC

 

• Behaviors Modelled
  • opamp_1
    • Gain
    • Bandwidth
    • Input / Output Resistance
  • opamp_2
    •  add Max Slew Rate

• LTSPICE files
  • Op Amp Schematic: Opamp_2.asc
  • Op Amp Symbol: Opamp_2.asy
  • Test Schematic: Test_Circuit_opamp_2.asc
  • Right Click on filename, select "Save link as...",

• Design Guide
  • Early in circuit develop, set device for a high bandwidth and slew rate, fu=1e9 and Slew=100 (V/us).
  • Later reduce to more realistic slew rates and bandwidths to determine which are critical.

• 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.
• Slew - Maximum rate of voltage change dV/dt achievable by the device.
• Ro - Output Resistance.

HOW IT WORKS

• DC Gain provided by current source B_G1 (gain of gm) and R1
  •  v(out) = i(B_G1)*R1 = (v(+) - v(-))*gm*R1
  • Aol = gm*R1
• Slew Rate defined by max B_G1 max current flowing into C1.
  • B_G1 current bounded using LIMIT function defined by I1max
  • I = limit( {gm}*(v(+) - v(-)), +I1max, -I1max )
  • Slew Rate = dV/dt = Imax / C1
• 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 I1max = 0.001   (choose G1's max current).
• .param C = I1max / (slew * 1e6)
• .param fp = fu/Aol
• .param C = 1/(2*pi*fp*R)
• .param gm = Aol/R

TEST IT

• Run a .TRAN simulation of Test_Circuit_opamp_2.asc
• Set Slew = 1 and Vs for an output near 5V.
• Plot v(vs) and v(vo).
• Does the output change at a constant slew rate of 1V/us?
• Change parameter Slew - what is the impact on dV/dt?

 

• Back to Design Series