### QUESTION:

**The stability of my fully differential and voltage feedback amplifiers seems highly impacted by the value of my feedback resistors—the RF/RG ratio is always correct, so what’s happening?**

### Answer:

When a signal needs gain, an amplifier is the component of choice. The ratio of the feedback and gain resistors, R_{F}/R_{G}, for a voltage feedback and a fully differential amplifier, determines the gain. Once the ratio is set, the next step is to select a value for either R_{F} or R_{G}. The choice of R_{F} can impact the stability of the amplifier.

An amplifier’s internal input capacitance, found in the specification table of the data sheet, interacts with R_{F} to form a pole in the transfer function. If R_{F} is exceedingly large, this pole will affect stability. If the pole occurs at a frequency much larger than the crossover frequency, it will not affect stability. However, if the location of the pole as determined by f = 1/(2πR_{F}C_{in,amp}) occurs near the crossover frequency, the phase margin will be reduced leading to potential instability.

The example of Figure 1 shows the lab results of the small signal closed-loop gain vs. frequency response for the ADA4807-1 voltage feedback amplifier in a noninverting gain of 2 configuration with feedback resistors 499 Ω, 1 kΩ, and 10 kΩ. The data sheet recommended R_{F} value is 499 Ω.

The degree of peaking in the small signal frequency response indicates instability. Increasing R_{F} from 499 Ω to 1 kΩ marginally increases peaking. This would imply the amplifier has sufficient phase margin with an R_{F} of 1 kΩ and is stable. This is not the case for the R_{F} of 10 kΩ. The high level of peaking present implies instability (oscillation) and it is not recommended.

Validating a circuit in the lab is not a mandatory step for verifying potential instabilities. Figure 3 shows the simulation results using the SPICE model with the same R_{F} values of 499 Ω, 1 kΩ, and 10 kΩ. The results are consistent with Figure 1. Figure 3 shows the instability in the time domain. Adding a zero to the transfer function by placing a feedback capacitor across R_{F} will remove the instability as shown in Figure 4.

There are trade-offs in the selection of R_{F}, which are power dissipation, bandwidth, and stability. If power dissipation is critical, and the data sheet recommended feedback value cannot be used or a much higher R_{F} value is necessary, placing a feedback capacitor in parallel with R_{F} is an option. This choice results in lower bandwidth.

When selecting the R_{F} for a voltage feedback and a fully differential amplifier, consideration needs to be given to the system requirements. If speed is not critical, a feedback capacitor will help to stabilize a large R_{F} value. If speed is critical, the recommended data sheet R_{F} value is advised. Ignoring the relationship of R_{F} with respect to stability, bandwidth, and power, can hinder a system or, worse yet, prevent the system from achieving its full performance.