LM6211MF datasheet(16/19 Pages) NSC

16 / 19 page

Typical Applications (Continued)

AUDIO PREAMPLIFIER

With low input referred voltage noise, low supply voltage and

low supply current, and low harmonic distortion, the LM6211

is ideal for audio applications. Its wide unity gain bandwidth

allows it to provide large gain over a wide frequency range

and it can be used to design a preamplifier to drive a load of

as low as 600

Ω with less than 0.001% distortion. Two am-

plifier circuits are shown in Figure 10 and Figure 11. Figure

10 is an inverting amplifier, with a 10 k

Ω feedback resistor,

2,anda1k

Ω input resistor, R

1, and hence provides a gain

of −10. Figure 11 is a non-inverting amplifier, using the same

values for R

1 and R2, and provides a gain of 11. In either of

these circuits, the coupling capacitor C

C1 decides the lower

frequency at which the circuit starts providing gain, while the

feedback capacitor C

F decides the frequency at which the

gain starts dropping off. Figure 12 shows the frequency

response of the circuit in Figure 10 with different values of

TRANSIMPEDANCE AMPLIFIER

A transimpedance amplifier converts a small input current

into a voltage. This current is usually generated by a photo-

diode. The transimpedance gain, measured as the ratio of

the output voltage to the input current, is expected to be

large and wide-band. Since the circuit deals with currents in

the range of a few nA, low noise performance is essential.

The LM6211, being a CMOS input op amp, provides a wide

bandwidth and low noise performance while drawing very

low input bias current, and is hence ideal for transimpedance

applications.

A transimpedance amplifier is designed on the basis of the

current source driving the input. A photodiode is a very

common capacitive current source, which requires transim-

pedance gain for transforming its miniscule current into eas-

ily detectable voltages. The photodiode and amplifier’s gain

are selected with respect to the speed and accuracy re-

quired of the circuit. A faster circuit would require a photo-

diode with lesser capacitance and a faster amplifier. A more

sensitive circuit would require a sensitive photodiode and a

high gain. A typical transimpedance amplifier is shown in

Figure 13. The output voltage of the amplifier is given by the

equation V

OUT =−IINRF. Since the output swing of the am-

plifier is limited, R

F should be selected such that all possible

values of I

IN can be detected.

The LM6211 has a large gain-bandwidth product (20 MHz),

which enables high gains at wide bandwidths. A rail-to-rail

output swing at 24V supply allows detection and amplifica-

tion of a wide range of input currents. A CMOS input stage

with negligible input current noise and low input voltage

noise allows the LM6211 to provide high fidelity amplification

for wide bandwidths. These properties make the LM6211

ideal for systems requiring wide-band transimpedance am-

plification.

20120341

FIGURE 10. Inverting Audio Amplifier

20120342

FIGURE 11. Non-Inverting Audio Preamplifier

20120343

FIGURE 12. Frequency Response of the Non-Inverting

Preamplifier

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