Electronic Design

Current-Feedback Op Amps Save Power In Photodiode Apps

Current-feedback amplifiers offer high bandwidth with minimal power-supply current draw. However, they're rarely used as transimpedance amplifiers in photodiode applications due to the high current noise of their inverting inputs. The figure shows how to take advantage of the 400-MHz (unity-gain) bandwidth of the LT1396 current-feedback op amp in photodiode applications while avoiding inverting-input current noise.

The photodiode is connected to the noninverting input of amplifier a of the dual LT1396 in a gain of 2. Amplifier a's output drives the inverting input of amplifier b through R1, which is selected for optimal time-domain response. Feedback resistor R2 sets the transimpedance gain at 3.01 kĦ, and C1 compensates the photodiode capacitance.

While risetime was measured at 6 ns (10% to 90%), bandwidth was modeled in Pspice at 75 MHz, assuming a 3-pF photodiode. For an 8-dB noise figure, output noise spectral density was 18 nV/ãHz, consistent with the 6-pA/ãHz noninverting input current noise and the 3-kƒ¶ resistance. To minimize board-space usage, select the MSOP-packaged version of the LT1396.

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