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OPA1641: OPA1641 SPICE model question

Part Number: OPA1641
Other Parts Discussed in Thread: TL081, TL072, TL082,


I'm modeling the input impedance of a simple non-inverting stage and finding that the impedance is very different from other TI JFET input opamps.  Here is the simulation:

Running an ac sim on the above gives me:

The input impedance as shown in the bottom trace drops to 1.1Meg at 20kHz.

This all makes sense given an input capacitance of 6pF but similar simulations with TL072 and TL081 show very different behaviour with the impedance holding at 2Meg beyond 10MHz.

So why the difference? Is it that the TL072 and TL082 models do not model input capacitance?



  • Hello AC,

    The OPA1641 is a modern CMOS op amp that uses out latest op amp model that includes many of the op amp electrical characteristics. Common-mode and differential mode capacitances are among them.

    The TL072 and TL082 are legacy JFET input op amps developed in the 70s and 80s. Their Pspice models use the Boyle op amp model and are dated around 1990. The Boyle model is much simpler and has less electrical characteristics modeled compared to modern op amp simulation models. 

    If you are working on a new design and want accurate simulation results it is best to consider using modern op amps and their models.

    Regards, Thomas

    Precision Amplifiers Applications Engineering