3.0 Capacitive Load Tolerance

The LMV101/102/105/110 can directly drive 200pF capaci-

tive load with Vs = 5V at −1 gain configuration without oscil-

lation. Direct capacitive loading reduces the phase margin of

amplifiers. The combination of the amplifier’s output imped-

ance and the capacitive load induces phase lag. This results

in either an underdamped pulse or oscillation. To drive a

heavier capacitive load, a resistive isolation can be used as

shown in

Figure 1.

The isolation resistor R

stability by adding more phase margin to the overall system.

The desired performance depends on the value of R

bigger the R

Figure 2 is an output waveform of Figure 1 using 100

Ω for

R

4.0 Phase Inverting AC Amplifier

A single supply phase inverting AC amplifier can be easily

built with the LMV101/102/105/110 series (

Figure 3). The

output voltage is biased at mid-supply, and AC input signal is

amplified by (R

pling capacitor to block DC potentials. A capacitor of 0.1µF or

larger can be used. The output can swing rail-to-rail. To avoid

output distortion, the peak-to-peak amplitude of the input AC

signal should be less than V

It is recommended that a small-valued capacitor be used

across the feedback resistor (R

lems, prevent peaking of the response, and limit the band-

width of the circuit. This can also help to reduce high fre-

quency noise and some other interference. (See

Figure 4)

5.0 Microphone preamplifier

Most microphones have a low output voltage level. This out-

put signal needs to be amplified so that it can feed the next

stage with optimal level.

Figure 5 shows a microphone

preamplifier circuit with the LMV110. This microphone

preamplifier can provide 20dB gain. It can be implemented in

PCs, PDAs, and mobile phones.

Input capacitor C

blocks any DC voltage from the previous stage to prevent

the output from shifting to some unwanted DC level. This

could cause the output to saturate when audio signal is ap-

plied at the input. Second, the C

form a low pass filter to block any low frequency noise. The

cut-off frequency of this low pass filter is given by,

where R

to block the DC output from the next stage. R

according to the microphone requirement.

DS101234-13

FIGURE 1. Resistive Isolation of a Heavy Capacitive

Load

DS101234-12

FIGURE 2. Pulse Response of LMV101 in

Figure 1

DS101234-10

FIGURE 3. Phase Inverting AC Amplifier

DS101234-11

FIGURE 4.

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