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Output impedance matching with fully differential operational amplifiers

Attila Zsida
Prodigy 10 points
Other Parts Discussed in Thread: OPA171
Dear Mdm./Sir!
Attila Zsida. In your publication: www.ti.com/.../slyt326.pdf "Output impedance matching with fully differential operational amplifiers" page3 in "Figure 4. Simplified view for analysis of adding RP" you explain and caculate the one sided FDA.
My question is; can I use this calculation  to find the output impedance on a noninverting amplifier?
I thank you in advance,
  • 0
    TI__Guru* 93105 points

    Hi Attila,

    The approach provided in the article for finding the output impedance may only be applicable to a differential output op amp because as shown in the simplified diagram of Figure 4 shows Rp being driven by Vo+, which wouldn't be present in a standard single-ended op amp circuit. 

    You indicate you are interested in calculating the output impedance of a non-inverting amplifier. When contemplating op amp output impedance there are actually two output impedances to consider; the open-loop output impedance Zo, and the closed-loop output impedance Zout. The first, Zo is very important for stability analysis of an op amp circuit, while Zout is the impedance a load sees looking back into the preceding stage's output. That has other implications from a circuit performance standpoint.

    It is important to note that Zo can change dramatically with frequency and may include reactive components in addition to a resistive component (R +/- jX). Therefore, even if Zo is provided at one frequency it may not be valid at others. For this reason it is important that the op amp datasheet provide a plot of Zo vs. frequency. All new op amps from our Precision Analog Op amp group have Zo characterized and a plot is provided in the product datasheet. An example is seen in Figure 22 of the OPA171 datsheet:

    We take this one step further by including the complex Zo in our PA op amp simulation models. That allows a user to accurately assess stability questions using simulation.

    Regards, Thomas

    PA - Linear Applications Engineering