LMV792MM datasheet(16/21 Pages) NSC

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Application Notes (Continued)

AUDIO PREAMPLIFIER WITH BANDPASS FILTERING

With low input referred voltage noise, low supply voltage and

low supply current, and a low harmonic distortion, the

LMV791 family is ideal for audio applications. Its wide unity

gain bandwidth allows it to provide large gain for a wide

range of frequencies and it can be used to design a pream-

plifier to drive a load of as low as 600

Ω with less than 0.01%

distortion. Two amplifier circuits are shown in Figure 5 and

Figure 6. Figure 5 is an inverting amplifier, with a 10 k

Ω

feedback resistor, R

2,anda1k

Ω input resistor, R

1, and

hence provides a gain of −10. Figure 6 is a non-inverting

amplifier, using the same values of R

1and R2, and provides

a gain of 11. In either of these circuits, the coupling capacitor

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 7

shows the frequency response of the inverting amplifier with

different values of C

TRANSIMPEDANCE AMPLIFIER

CMOS input op amps are often used in transimpedance

applications as they have an extremely high input imped-

ance. A transimpedance amplifier converts a small input

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

photodiode. 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 LMV791/LMV792 are CMOS input op amps providing

wide bandwidth and low noise performance, and are hence

ideal for transimpedance applications.

Usually, 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 8. 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 LMV791/LMV792 have a large gain-bandwidth product

(17 MHz), which enables high gains at wide bandwidths. A

rail-to-rail output swing at 5.5V supply allows detection and

amplification of a wide range of input currents. A CMOS input

stage with negligible input current noise and low input volt-

age noise allows the LMV791/LMV792 to provide high fidel-

ity amplification for wide bandwidths. These properties make

the LMV791/LMV792 ideal for systems requiring wide-band

transimpedance amplification.

20116865

FIGURE 5. Inverting Audio Preamplifier

20116866

FIGURE 6. Non-inverting Audio Preamplifier

20116858

FIGURE 7. Frequency Response of the Inverting Audio

Preamplifier

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