LMV792 datasheet(17/21 Pages) NSC

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Application Notes (Continued)

As mentioned earlier, the following parameters are used to

design a transimpedance amplifier: the amplifier gain-

bandwidth product, A

0; the amplifier input capacitance, CCM;

the photodiode capacitance, C

D; the transimpedance gain

required, R

F; and the amplifier output swing. Once a feasible

F is selected using the amplifier output swing, these num-

bers can be used to design an amplifier with the desired

transimpedance gain and a maximally flat frequency re-

sponse.

An essential component for obtaining a maximally flat re-

sponse is the feedback capacitor, C

F. The capacitance seen

at the input of the amplifier, C

IN, combined with the feedback

capacitor, R

F, generate a phase lag which causes gain-

peaking and can destabilize the circuit. C

IN is usually just the

sum of C

D and CCM. The feedback capacitor CF creates a

pole, f

P in the noise gain of the circuit, which neutralizes the

zero in the noise gain, f

Z, created by the combination of RF

and C

IN. If properly positioned, the noise gain pole created

by C

F can ensure that the slope of the gain remains at 20

dB/decade till the unity gain frequency of the amplifier is

reached, thus ensuring stability. As shown in Figure 9,f

P is

positioned such that it coincides with the point where the

noise gain intersects the op amp’s open loop gain. In this

case, f

P is also the overall 3 dB frequency of the transim-

pedance amplifier. The value of C

F needed to make it so is

given by Equation (3). A larger value of C

F causes excessive

reduction of bandwidth, while a smaller value fails to prevent

gain peaking and instability.

(3)

Calculating C

F from Equation (3) can sometimes return un-

reasonably small values (<1 pF), especially for high speed

applications. In these cases, its often more practical to use

the circuit shown in Figure 10 in order to allow more reason-

able values. In this circuit, the capacitance C

F' is (1+ RB/RA)

time the effective feedback capacitance, C

F. A larger capaci-

tor can now be used in this circuit to obtain a smaller effec-

tive capacitance.

For example, if a C

F of 0.5 pF is needed, while onlya5pF

capacitor is available, R

B and RA can be selected such that

B/RA = 9. This would convert a CF'of5pFintoaCF of

0.5 pF. This relationship holds as long as R

A << RF.

LMV791 AS A TRANSIMPEDANCE AMPLIFIER

The LMV791 was used to design a number of amplifiers with

varying transimpedance gains and source capacitances.

The gains, bandwidths and feedback capacitances of the

circuits created are summarized in Table 1. The frequency

responses are presented in Figure 11 and Figure 12. The

feedback capacitances are slightly different from the formula

in Equation (3), since the parasitic capacitance of the board

and the feedback resistor R

F had to be accounted for.

20116869

FIGURE 8. Photodiode Transimpedance Amplifier

20116884

FIGURE 9. C

F Selection for Stability

20116871

FIGURE 10. Obtaining Small C

F from large CF'

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