Posted on Jan 9, 2013

The traditional restnct1on of constant gainbandwidth products for a voltage amplifier can be overcome by employing feedback around a current amplifier. Two current mirrors, constructed from transistors in a CA3096AE array, effectively turn the LF351 op amp into a current amplifier. Feedback is then applied by using R2 and R1, turning the whole circuit into a feedback voltage amplifier with a noninverting gain of G of 1 + R112R2.

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Using the values shown, a constant bandwidth of 3.5 MHz is obtained for all voltage gains up to and beyond 100 at 10 V pk-pk output, equivalent to a gain-bandwidth product of 350 MHz from an op amp with an advertised unity gain-bandwidth of 10 MHz. An inverting gain configuration is also possible (see Fig. 2) where G ~ R1/ZR2. Slewing rates are significantly improved by this approach; even a 741 can manage 100 V p,s under these conditions since its output is a virtual earth. However, because the new configurations use current feedback to achieve bandwidth independence, an output buffer should be added for circuits where a significant output current is required.

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