TL074: How constant is TL074's output resistance?

Hi Stephen,

The TL074 is an older device and the output impedance wasn't directly specified in the product datasheet.  That said, a functional block diagram for the device is provided in Section 8.2 which shows that the device features a classic common emitter output stage.  Similar devices, such as the OPA627 and OPA827 feature flat resistive output impedances over frequency.  Therefore, I would expect something similar for the TL074.  Since there look to be a passive 128Ohm impedance directly in series with the output and also 64Ohm resistances in series with the transistor impedances, I would expect an open-loop output impedance between 160 and 250 Ohms.

Hi Stephen,

The graph in Figure 6 can be used to calculate the on resistance of the output transistors while in a saturated condition when the output is slammed to the rail of the supply. The output impedance when operating in a linear operating condition may be slightly different, but will likely be close to the 200Ohms calculated in the post you've linked to as suggested in my post above.

Since we don't have any data to back up the claim our recommendation was to use an overly large deviation of -50% so your system is robust even it if changes by much less than that.

I'm sure what you're asking for with all the current limit and extra resistor questions, but I'll add some additional information that may be helpful.  When the output is saturated you'll see the 200 Ohms of output impedance that was calculated in the posts you linked to originally.  

When the op amp is operating in a linear condition the open-loop gain (Aol) of the amplifier is very large at low frequencies.  In a linear operating condition the closed-loop output impedance (Zout) presented to the load will equal the open-loop output impedance (Zo) divided by 1 + Aol*B, where B (Beta) is the feedback factor and Aol*B is the "Loop-Gain".  So for frequencies where the loop-gain is large the closed loop output impedance (Zout) will be small.  Lets take an example at 10Hz assuming the open-loop output impedance (Zo) is roughly 200 Ohms and the op-amp is configured in a unity-gain configuration so the loop-gain (Aol*B) is equal to Aol. Since Zout = Zo / 1+Aol*B we can solve for the closed-loop output impedance at 10Hz as shown:

Zout = Zo / (1+Aol*B) = 200 / (1 + 200,000V/V) = 999uOhms (9.99E-4 Ohms). 

Yes, the resistor(s) are inside the package.