OPA656 is a broadband, unity gain stable, field-effect transistor input operational amplifier 7

Putting high resistance values into formula 2 can quickly control the total equivalent input reference noise. The source impedance on the non rotating input 3k Ω will add a Johnson voltage noise term, which is equal to the voltage noise term of the amplifier itself (7nV/Hz). Although the JFET input of OPA656 is an ideal choice for high source impedance applications, thediode overall bandwidth and noise will be limited by the higher source impedance in the non vertical configuration shown in Figure 1.

frequency response control

Voltage feedback operational amplifiers, such as OPA656, exhibit a decrease in signal bandwidth with increasing signal gain. In theory, this relationship is described by the GBP shown in the electrical characteristics. Ideally, dividing GBP by the non rotational signal gain (also known as noise gain, or NG) will predict the closed-loop bandwidth. In fact, this only holds true when the phase margin approaches 90 °, just like in high gain configurations. At low gain (increasing feedback factor), most high-speed amplifiers will exhibit more complex responses and lower phase margins. Compensate OPA156 to provide the maximum flat second-order Butterworth closed-loop response at a non rotational gain of+2 (Figure 1). This resulted in a typical+2 bandwidth gain of 200MHz, far exceeding the prediction of 230MHz GBP divided by 2. Increasing the gain will result in a phase margin close to 90 ° and a bandwidth closer to the predicted value of GBP/NG. At a gain of+10, OPA656 will display a predicted 23MHz bandwidth and a typical GBP of 230MHz using a simple formula.diode

Operational amplifiers with stable unit gain, such as OPA656, can also be limited to feedback resistors using capacitors. For the non vertical configuration in Figure 1, the capacitor through the feedback resistor will decrease the gain as the frequency decreases to+1. For example, to limit the gain of the+2 design to 20MHz, a 32pF capacitor can be placed in parallel with a 250 ohm feedback resistor. However, this will only reduce the gain from 2 to 1. In the inverted configuration of Figure 2, using feedback capacitors to limit signal bandwidth is more effective. Adding the same capacitor to the feedback in Figure 2 will set a pole in the 20MHz signal frequency response, but in this case, it will continue to attenuate the signal gain to below 1. However, the output noise contribution caused by the input voltage noise of OPA656 will still only decrease to a gain of 1 with the addition of feedback capacitors.diode