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# SPICE Model: Opamp_2

## SCHEMATIC

• Behaviors Modelled
• opamp_1
• Gain
• Bandwidth
• Input / Output Resistance
• opamp_2
• 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.

## 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?