OA-13 Current Feedback Loop Gain Analysis and Performance Enhancement

With the introduction of commercially available amplifiers using the current feedback topology by Comlinear Corporation in the early 1980’s, previously unattainable gain and bandwidths in a DC coupled amplifier became easily available to any design engineer. The basic achievement realized by the current feedback topology is to de-couple the signal gain from the loop gain part of the overall transfer function. Commonly available voltage feedback amplifiers offer a signal gain expression that appears identically in the loop gain expression, yielding a tight coupling between the desired gain and the resulting bandwidth. This historically has led to the gain-bandwidth product idea for voltage feedback amplifiers. The current feedback topology transcends this limitation to offer a signal bandwidth that is largely independent of gain. This application note develops the current feedback transfer function with an eye towards manipulating the loop gain.