To improve power supply ripple rejection of the above micro-

phone preamplifier, another capacitor and a pot can be con-

nected to pin 1 as shown in

Figure 6. The impedance of the

two capacitors at audio frequencies are low. The R

be adjusted so that the supply ripples injected through both

the inverting input and the non-inverting input cancel each

other at the output. If we ignore the impedance of the capaci-

tors, we can select the pot value based on the following

equation:

Z

the gain of the preamplifier in absolute value.

6.0 Adjustable-Gain Amplifier

The LMV101/102/105/110 not only provides fixed gain of −1,

−2, −5, and −10, it can also be configured for different gains

by adding only one external resistor.

You can decrease the gain by putting a resistor in series with

pin1(

Figure 7). You can increase the gain by connecting a

resistor from pin 1 to pin 3 (

Figure 8).

If you are using the LMV110 as a microphone preamplifier

for an electret microphone (

Figure 5), and the output imped-

ance of the microphone is 1k

Ω, then the gain of the pream-

plifier is

If we choose a small value for R, then we could get a pream-

plifier with a gain close to 100 (40dB), which is 10 times the

gain provided by LMV110.

DS101234-15

FIGURE 5. Microphone Preamplifier with 20dB Gain

DS101234-17

FIGURE 6. Improved Ripple Rejection

DS101234-18

FIGURE 7. Decreased Gain

DS101234-19

FIGURE 8. Increased Gain

www.national.com

10