Voltage-feedback op amps exhibit decreasing closed-loop bandwidth as the signal gain is increased. In theory, this relationship is described by the gain bandwidth product (GBP) shown in Section 6.5 and Section 6.7. Ideally, dividing GBP by the noninverting signal gain (also called the noise gain, or NG) predicts the closed-loop bandwidth. In practice, this relationship only holds true when the phase margin approaches 90°, as in high-gain configurations. At low gains (increased feedback factors), most amplifiers exhibit a more complex response with lower phase margin. The OPA690 is compensated to give a slightly peaked response in a noninverting gain of 2 V/V (see Figure 7-1). This compensation results in a typical gain of 2 bandwidth of 220 MHz, far exceeding that predicted by dividing the 300 MHz GBP by 2. Increasing the gain causes the phase margin to approach 90° and the bandwidth to more closely approach the predicted value of (GBP/NG). At a gain of 10, the 30-MHz bandwidth shown in the ±5-V Electrical Characteristics agrees with that predicted using the simple formula and the typical GBP of 300 MHz.

The frequency response in a gain of 2 V/V can be modified to achieve exceptional flatness by simply increasing the noise gain to 2.5. One method, without affecting the two signal gain, is to add an 804-Ω resistor across the two inputs in the circuit of Figure 7-1. A similar technique can be used to reduce peaking in unity-gain (voltage-follower) applications. For example, by using a 402-Ω feedback resistor along with a 402-Ω resistor across the two op amp inputs, the voltage follower response is similar to the gain of 2 response of Figure 7-2. Reduce the value of the resistor across the op amp inputs to further limit the frequency response due to increased noise gain.