Hi,

Let me use LTspice to illustrate some of the features of the measuring the open loop gain of an op-amp. I have used the universal op-amp in LTspice so that I have a known answer. I have configured the op-amp to have an open loop gain of 1E6 and and a GBW product of 500K.

**AC domain Model**In SPICE I can have a floating source so I do not need to include the transformer. The open loop gain is obtained by plotting V(a)/V(b).

The results show that I have an amplifier with a single pole slope, a gain of 1E6 and a GBW of 500kHz. This is the correct answer for the universal op-amp used in the model.

The gain is 94dB at 10Hz.

**Time Domain Model**Switching to the time domain, transient analysis, and using a 10Hz 2V p-p signal the signals at nodes A and B can be examined.

The voltage between nodes A and B is 2V p-p, as defined by the source. The voltage from A to ground is 2V p-p. The voltage on node B with respect to ground is only 40uV p-p. This is the inject signal divided by the loop gain at the injection frequency.

The signal on Node B is very small. It is -94dB, 50000 times smaller than the injection signal.

**Coupling Transformer LF bandwidth**The coupling transformer LF -3dB point is given by:

F = (Rsource // Rload) / 2 x Pi x Lmag

I have chosen Lmag = 68mH

This gives a LF (-3dB) point at 58.5 Hz

**Adding the Transformer to the open loop measurement**Adding the transformer with a -3dB point of 60Hz does not impact the open loop response at all as shown in this model. It does reduce the signal amplitude at 10Hz by 16dB.

It is the signal to noise ratio and the noise floor of the FRA that impacts the measurement, not the bandwidth of the isolation transformer directly.

Regards,

Jay_Diddy_B