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 1 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.
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| Figure 1. |
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 kΩ, 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/√Hz, consistent with the 6-pA/√Hz noninverting input current noise and the 3-kΩ resistance. To minimize board-space usage, select the MSOP-packaged version of the LT1396.
