Hello, and welcome to Part 6 of the TI Precision Labs on Op Amp Stability. This lecture will describe the RISO with dual feedback stability compensation method. In the previous video, we learned about the RISO compensation method. While the RISO method is simple, the voltage drop across the RISO element can make it impractical for some precision applications. A solution to the voltage drop from the RISO circuit is to implement the RISO plus dual feedback circuit shown here.

The operation of the RISO plus dual feedback circuit can be analyzed using the equivalent DC and AC representations of the circuit. At DC, the feedback capacitor, CF, acts as an open circuit, and RF closes the feedback loop around RISO. Since RISO is now in the op amp feedback loop, the op amp output will increase to overcome the RISO voltage drop such that the load voltage, VLoad, is equal to Vn.

At AC frequencies, CF acts as a short. When this happens, RF can be thought of as an open circuit because the impedance of CF, xCF, will be much smaller than the impedance of RF. Therefore at AC, this circuit looks effectively the same as the standard RISO circuit. The first design step in this circuit is to select RISO.

The same method that was used to select RISO in the previous video is used again, and RISO is selected to produce a zero in the AOL curve at the frequency where the AOL equals 20 dB. Then RF can be selected to any value greater than 100 times RISO in order to prevent interactions with RISO.

The last step is to select a value of CF based on the range shown here. Using this range ensures that the two feedback paths never create a resonance that would cause instability. Smaller values for CF will result in faster settling times at the expense of overshoot for certain load ranges.

The circuit results show that the output and load voltage arrive at the final level without excessive overshoot or ringing, indicating a stable system. The increase in VO to overcome the voltage drop from RISO can be clearly seen here. While the RISO plus dual feedback circuit solves the DC accuracy issue with the RISO circuit, it has some disadvantages as well.

As shown here, an RISO circuit will generally remain stable with reasonable variation in the transient response over a wide range of capacitive loads. The RISO plus dual feedback circuit is not as tolerant to changes in the output capacitance and can quickly become unstable. Therefore, the RISO plus dual feedback circuit is best for situations where the output capacitance is known and will not vary significantly. Also, the RISO plus dual feedback circuit generally results in slower settling times in the RISO circuit.

Part 3 showed how to perform open loop analysis on many common op amp circuits. However, those circuits all had one feedback loop. If we want to perform a simulated open loop analysis on multiple feedback circuits, like the RISO plus dual feedback compensation, a different method is required, which we'll now discuss.

Opening either feedback path still leaves a closed loop feedback path around the other loop. If FB1 is opened, FB2 remains a closed loop feedback path. If FB2 is opened, FB1 remains as a closed loop feedback path. The circuit will not properly report open loop curves unless both feedback loops are open.

Breaking the loop directly at the output will remove the connections between the output and both feedback loops, resulting in an open loop circuit. However, breaking the loop in this location disconnects the output capacitive load, CL, from the op amp output. Therefore, CL will not interact with the open loop output impedance, RO, preventing the simulation from identifying possible stability issues caused by the capacitive load, as discussed in the previous video.

The recommended method for this circuit and other similar circuits with multiple feedback loops is to break the loop directly at the inverting input of the amplifier. Breaking the loop in this location also disconnects both feedback loops, but now the output impedance of the op amp can interact with the output loading and feedback network. However, by breaking the loop at the input, the inherent input capacitance of the amplifier no longer interacts with the feedback network. Therefore, it is required to place a representation of the amplifier input capacitance, CN, on the other side of the inductor to match the amplifier input capacitance.

The differential and common mode input capacitances are typically specified in op amp data sheets. This information can be used to develop a simple model of the input capacitance of the amplifier as shown. In this circuit, the non-inverting input is grounded, so the negative common mode capacitor is shorted out, and the positive common mode capacitor and differential input capacitor are in parallel with each other. The parallel sum of the two capacitors is 8 picofarads, and should be added to the circuit above the inductor as shown.

Breaking the loop at the input requires different equations to obtain the open loop results. The equations for generating the desired curves are as follows. Loaded AOL equals Vo. 1 over beta equals Vo divided by Vfb. And loop gain equals Vfb. The procedure for determining the rate of closure and measuring the phase margin is the same as before.

In summary, this video described the RISO plus dual feedback method for stability compensation and showed its advantage of DC accuracy compared to the RISO method. A new method for performing open loop analysis on multiple feedback circuits was also shown. While both the RISO and RISO plus dual feedback compensation methods are effective, there are many other methods that can also be used to compensate stability issues.

Stay tuned for future videos which will detail more compensation methods that are better suited for certain applications. Thanks for your time. Please try the quiz to check your understanding of this video's content.