使用寬帶電壓和電流反饋運(yùn)算放大器時(shí)的應(yīng)用基礎(chǔ)
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Figure 9. Wideband differencing amplifier using an OPA690.
A series resistor into the non-inverting input is then added to achieve bias current cancellation, which would only work to improve output offset voltage in a VFB implementation. This is assuming 0 Ω sources for each source and two independent sources.
This same circuit can be built using the CFB OPA691.Figure 10shows a CMRR simulation where both inputs are tied together and driven.
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Figure 10. CMRR simulation for OPA690 and OPA691.
The resulting small output gain (large negative dB gain) is then input-referred and the negative taken to get a typical CMRR plot. The OPA690 shows slightly higher CMRR than the OPA691. In this case, the resistors of Figure 9 have been used in both simulations, and no adjustment for improved CMRR made in the OPA691 simulation.
Differential input/output circuits
An emerging class of amplifiers called fully differential amplifiers (FDA) easily can take a single or differential input signal. and produce a differential output centered on a user-selected common-mode operation point. An alternative approach in going differential-in to differential-out has been to use standard dual op amps. A brief review of that approach helps to set the background for the FDA. These approaches are useful also because once they are understood, they open up a large range of dual op amps to the designer for possible application.
Application A
Differential I/O circuits can be easily implemented using either a VFB or CFB. There is, however, some difference between a non-inverting or inverting input implementation with regard to how the common-mode voltage is treated. In the non-inverting input case, the two inputs show a high input impedance to the differential source (allowing filters or other passive circuits to be easily inserted up to these inputs). The common-mode gain from the non-inverting inputs to the differential outputs will be one.
Figure 11shows an example of this design where the wideband, high output current, dual OPA2614 is used to implement a DSL driver.
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Figure 11. Non-inverting differential I/O circuit using the OPA2614.
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