使用宽带电压和电流反馈运算放大器时的应用基础

时间：2013-11-13来源：网络
DDING-TOP: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px">In the range of possible applications for wideband op amps, several types must use VFB devices. These circuits can sometimes be forced to work using CFB devices, but usually at the cost of complexity and poorer performance. Any circuit that requires flexibility in the feedback element and/or capacitors in the feedback will have stability problems with a CFB device. This instability is the result of the loop gain depending on the feedback impedance. Therefore, any circuit that needs a lot of flexibility in that impedance is going to interact with the achievable frequency response if a CFB amplifier is used.

The following example circuits should use a VFB device for implementation.

A. Transimpedance amplifiers. These circuits take a current source input, typically from a capacitive source, and turn it into a voltage at the output. The feedback resistor is the gain element and normally needs a compensation capacitor in parallel for correct operation.Figure 6shows an example using the OPA657, a very wideband JFET input device uniquely suited to the transimpedance application. This device is a non-unity gain stable VFB with relatively low input noise voltage and very high gain bandwidth product. For a given diode source capacitance, the amplifier gain bandwidth product (GBP) determines the achievable bandwidth and/or transimpedance gain (Reference 3).


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Figure 6. Example transimpedance design using the OPA657.


In this example, the 500 kΩ along with the 200 pF diode capacitance gives a noise gain zero at approximately 1.6 kHz. With the feedback capacitor set to achieve a maximally flat Butterworth response, the resulting F-3dBwill be at the geometric mean of this zero and the 1.6 GHz gain bandwidth product of the OPA657. (Reference 3 gives a detailed analysis for compensation and noise in a transimpedance design.)Figure 7shows the 1.6 MHz transimpedance bandwidth in a simulated frequency response of Figure 6.


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Figure 7. Simulated transimpedance frequency response of Figure 6.


B. Integrator based circuits. These applications are looking for a capacitive feedback to implement an integrator function. A good example would be the Multiple feedback (MFB) active filter.Figure 8shows an example using the OPA820, a low-noise, wideband voltage feedback op amp uniquely suited to this application.

Embedded within this filter circuit is an integrator configuration that arises from the 200 Ω and the 12.5 pF feedback capacitor.

At very high frequencies, that capacitor shorts out, giving a local unity-gain feedback for the amplifier. This result suggests that a unity gain stable VFB should be used in this type of circuit to avoid high frequency oscillations. This particular example is targeting a 10 MHz low-pass Butterworth response with an in-band gain of -4 V/V. (Reference 4describes how to ch

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关键词： 宽带电压 电流反馈 运算放大器

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