LM6211 datasheet(17/19 Pages) NSC

17 / 19 page

Typical Applications (Continued)

The following parameters are used to design a transimped-

ance amplifier: the amplifier gain-bandwidth product, A

0; the

amplifier input capacitance, C

CM; the photodiode capaci-

tance, C

D; the transimpedance gain required, RF; and the

amplifier output swing. Once a feasible R

F is selected using

the amplifier output swing, these numbers can be used to

design an amplifier with the desired transimpedance gain

and a maximally flat frequency response. The input

common-mode capacitance with respect to V

for the

LM6211 is give in Figure 14.

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

resistor, R

F, generates 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 16,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 (2). A larger value of C

F causes excessive

reduction of bandwidth, while a smaller value fails to prevent

gain peaking and maintain stability.

(2)

Calculating C

F from Equation (2) can sometimes return un-

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

applications. In these cases, it is often more practical to use

the circuit shown in Figure 15 in order to allow more reason-

able values. In this circuit, the capacitance C

F’ is (1+ RB/RA)

times the effective feedback capacitance, C

F. A larger ca-

pacitor can now be used in this circuit to obtain a smaller

effective 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 C

F’

of 5 pF into a

F of 0.5 pF. This relationship holds as long as RA << RF

20120344

FIGURE 13. Photodiode Transimpedance Amplifier

20120354

FIGURE 14. Input Common-Mode Capacitance vs. V

20120347

FIGURE 15. Modifying C

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