LMH6611_14 Datasheet, PDF(31/42 Page) National Semiconductor (TI) – LMH6611/LMH6612 Single Supply 345 MHz Rail-to-Rail Output Amplifiers

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LMH6611_14 Datasheet, PDF (31/42 Pages) National Semiconductor (TI) – LMH6611/LMH6612 Single Supply 345 MHz Rail-to-Rail Output Amplifiers

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LMH6611, LMH6612

www.ti.com

SNOSB00K â NOVEMBER 2007 â REVISED OCTOBER 2013

TRANSIMPEDANCE AMPLIFIER

By definition, a photodiode produces either a current or voltage output from exposure to a light source. A

Transimpedance Amplifier (TIA) is utilized to convert this low-level current to a usable voltage signal. The TIA

often will need to be compensated to insure proper operation.

LMH6611

CPD

CIN

Figure 77. Photodiode Modeled with Capacitance Elements

Figure 77 shows the LMH6611 modeled with photodiode and the internal op amp capacitances. The LMH6611

allows circuit operation of a low intensity light due to its low input bias current by using larger values of gain (RF).

The total capacitance (CT) on the inverting terminal of the op amp includes the photodiode capacitance (CPD) and

the input capacitance of the op amp (CIN). This total capacitance (CT) plays an important role in the stability of

the circuit. The noise gain of this circuit determines the stability and is defined by:

1 + sRF (CT + CF)

NG =

1 + sCFRF

(5)

Where,

25RFCT

and

25RFCF

(6)

OP AMP OPEN

LOOP GAIN

I-V GAIN (:)

NOISE GAIN (NG)

1 + sRF (CT + CF)

1 + sRFCF

1 + CIN

0 dB

FREQUENCY fz # 1

2SRFCT

GBWP

fP = 2SRFCF

Figure 78. Bode Plot of Noise Gain Intersecting with Op Amp Open Loop Gain

Figure 78 shows the bode plot of the noise gain intersecting the op amp open loop gain. With larger values of

gain, CT and RF create a zero in the transfer function. At higher frequencies the circuit can become unstable due

to excess phase shift around the loop.

A pole at fP in the noise gain function is created by placing a feedback capacitor (CF) across RF. The noise gain

slope is flattened by choosing an appropriate value of CF for optimum performance.

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