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When OPA197 is used as a general DC non-inverting amplifier with a power supply voltage of 24 V (single power supply)
What is the minimum output impedance?
Konno-san,
The OPA197 open-loop output impedance Zo is approximately 3 kΩ, at very low frequencies and DC according to the datasheet Fig. 29 graph shown below. That is a typical value and we don't provide minimum or maximum Zo values, but even if it varies somewhat there is little effect on the closed-loop output impedance Zout.
When the op amp loop is closed in the non-inverting amplifier you propose Zout becomes very, very low compared to the Zo value. Zout is related to Zo by the following relationship:
Zout = Zo / (1 + AOL∙ β)
where: AOL is the open-loop gain, and β = Ri /(Ri + Rf)
Ri = input resistance, Rf = feedback resistance
For a unity gain buffer, Rf = 0 and Ri = ∞ and β = 1, and using Zo = 3 kΩ at DC,and Aol = 120 dB (106 V/V) from the datasheet Electrical Characteristics table:
Zout = 3000 Ω / ( 1 + 106 V/V) = 0.003 Ω
Thus, the closed-loop output impedance Zout is very, very low. Changing Zo or Aol still results in similarly low Zout.
The supply voltage level has minimal effect on Zo.
Regards, Thomas
Precision Amplifiers Applications Engineering
