Alternate approaches to LG simulation
Some sources suggest a similar approach to Figure 3 but with the loop broken at the output(s). An example using the same circuit of Figure 1 is shown in Figure 4. Here, the loop is broken by the inductors at the output pins and a signal injected into the feedback network developing a path through the feedback dividers then forward through the amplifier’s open loop gain and phase being measured at the device output pins.
LG simulation with the loop broken at the output pins.
Click here for larger image
This simulation is also showing a phase margin problem but at 207Mhz LG=0dB crossover – not matching the resonance of Figure 1. Breaking the loop as shown in Figure 5 is not including the effect of the open loop output impedance combined with the feedback load – it does see the 500Ω load, but for simulation accuracy, the feedback load should also be included. Where the device has a low open loop output impedance, or the load is dominant over the feedback impedance, this approach can work well. Note the approach of Figure 4 directly includes the differential input impedance internal to the THS4551 model (Reference 4) - so that is not split out separately.
To make this approach more accurate, placing the inductor directly at the outputs and then adding a model for the open loop output impedance driving both the load and feedback impedance would be required. That would then require an RLC model for the impedance of Figure 2 (Reference 5). While those RLC elements are included in the THS4551 TINA (Reference 4) model, no explicit listing of that appears in the data sheet. The differential input impedance is detailed in the input specifications for the THS4551 - easing the LG simulation task using the approach of Figure 3.
Assessing phase margin in modern high-performance op amp’s and FDA’s can be easily accomplished using the technique shown here. This is a critical step in the design review process where reliably producing this simulation is 1/2 the battle. The vendor models do need to include all the requisite elements to be accurate. The new RR output stages introduce more phase margin problems in simple circuits than might be anticipated. Quickly assessing that will ease the resolution process. Going on to the next step of resolving the phase margin problem in the circuit of Figure 1 will be taken up in a subsequent discussion.
References for Loop Gain Simulation article.
- “Feedback Plots Define Op Amp AC Performance” Burr-Brown Application Note, Jerald Graeme, 1989
- TINA simulator available from DesignSoft for <$350 for the Basic Plus edition. Includes a wide range of vendor op amps and is the standard platform for TI op amp models.
- TI, THS4551 , “Low Noise, Precision, 150-MHz, Fully Differential Amplifier”,
- TI, THS4551 TINA simulation model
- “Modeling the output impedance of an op amp for stability analysis”, TI Analog Applications Journal, 3Q2016, Collin Wells, Miro Oljaca