TABLE OF CONTENTS Sergio Franco
| Note | Again, Sergio Franco is another good explainer. What are the real differences between a conventional op-amp and a current-feedback Amplifier? Here's why and how they are not obvious! / rap |
In their effort to approximate the ideal op-amp, manufacturers strive not only to maximize the open-loop gain and minimize input-referred errors such as offset voltage, bias current, and noise, but also to ensure adequate bandwidth and settling-time characteristics. Amplifier dynamics are particularly important in high-speed applications such as bipolar DAC buffers, subranging ADCs, S/H circuits, ATE pin drivers, and video and IF drivers. Being voltage-processing devices, conventional op-amps are subject to the speed limitations inherent to voltage-mode operation, stemming primarily from the stray capacitances of nodes and the cutoff frequencies of transistors. Particularly severe is the effect of the stray capacitance between the input and output nodes of high-gain inverting stages because of the Miller effect, which multiplies this capacitance by the voltage gain of the stage. By contrast, current-mode operation has long been recognized as inherently faster than voltage-mode operation. The effect of stray inductances in an integrated circuit is usually less severe than that of its stray capacitances, and BJTs switch currents more rapidly than voltages. These technological reasons are at the basis of emitter coupled logic, bipolar DACs, current conveyors, and the high-speed amplifier topology known as current-feedback. [1]
For true current-mode operation, all nodes in the circuit should ideally be kept at fixed potentials to avoid...
