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參數(shù)資料
| 型號： | LMV102 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | Fixed-Gain Amplifiers |
| 中文描述： | 固定增益放大器 |
| 文件頁數(shù)： | 9/12頁 |
| 文件大小： | 769K |
| 代理商： | LMV102 |
Application Notes
(Continued)
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
and the C
form a pole to increase
stability by adding more phase margin to the overall system.
The desired performance depends on the value of R
iso
. The
bigger the R
resistor value, the more stable V
will be.
Figure 2 is an output waveform of Figure 1 using 100
for
R
iso
and 1000pF for C
L
.
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, andAC input signal is
amplified by (R
/R
). Capacitor C
acts as an input AC cou-
pling capacitor to block DC potentials.Acapacitor 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
CC
(R
1
/R
2
).
It is recommended that a small-valued capacitor be used
across the feedback resistor (R
) to eliminate stability prob-
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
serves two important functions. First, it
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
and the 10k input resistor
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
= 10k
in LMV110. Output capacitor C
OUT
is used
to block the DC output from the next stage. R
bias
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.
L
www.national.com
9
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