參數(shù)資料
型號(hào)： LM2876MWC
廠商： NATIONAL SEMICONDUCTOR CORP
元件分類： 音頻／視頻放大
英文描述： 40 W, 1 CHANNEL, AUDIO AMPLIFIER, UUC7
封裝： WAFER
文件頁(yè)數(shù)： 8/22頁(yè)
文件大小： 611K
代理商： LM2876MWC
Definition of Terms (Continued)
ciently short that the amplifier power supply does not sag
during the measurement, or when high quality external
power supplies are used. This measurement (an IHF stan-
dard) assumes that with normal music program material the
amplifier power supplies will sag insignificantly.
Peak Power: Most commonly referred to as the power out-
put capability of an amplifier that can be delivered to the
load; specified by the part’s maximum voltage swing.
Headroom: The margin between an actual signal operating
level (usually the power rating of the amplifier with particular
supply voltages, a rated load value, and a rated THD + N fig-
ure) and the level just before clipping distortion occurs, ex-
pressed in decibels.
Large Signal Voltage Gain: The ratio of the output voltage
swing to the differential input voltage required to drive the
output from zero to either swing limit. The output swing limit
is the supply voltage less a specified quasi-saturation volt-
age. A pulse of short enough duration to minimize thermal ef-
fects is used as a measurement signal.
Output-Current Limit: The output current with a fixed out-
put voltage and a large input overdrive. The limiting current
drops with time once SPiKe protection circuitry is activated.
Output Saturation Threshold (Clipping Point): The output
swing limit for a specified input drive beyond that required for
zero output. It is measured with respect to the supply to
which the output is swinging.
Output Resistance: The ratio of the change in output volt-
age to the change in output current with the output around
zero.
Power Dissipation Rating: The power that can be dissi-
pated for a specified time interval without activating the pro-
tection circuitry. For time intervals in excess of 100 ms, dis-
sipation capability is determined by heat sinking of the IC
package rather than by the IC itself.
Thermal Resistance: The peak, junction-temperature rise,
per unit of internal power dissipation (units in C/W), above
the case temperature as measured at the center of the pack-
age bottom.
The DC thermal resistance applies when one output transis-
tor is operating continuously. The AC thermal resistance ap-
plies with the output transistors conducting alternately at a
high enough frequency that the peak capability of neither
transistor is exceeded.
Power Bandwidth: The power bandwidth of an audio ampli-
fier is the frequency range over which the amplifier voltage
gain does not fall below 0.707 of the flat band voltage gain
specified for a given load and output power.
Power bandwidth also can be measured by the frequencies
at which a specified level of distortion is obtained while the
amplifier delivers a power output 3 dB below the rated out-
put. For example, an amplifier rated at 60W with
≤ 0.25%
THD + N, would make its power bandwidth measured as the
difference between the upper and lower frequencies at which
0.25% distortion was obtained while the amplifier was deliv-
ering 30W.
Gain-Bandwidth Product: The Gain-Bandwidth Product is
a way of predicting the high-frequency usefulness of an op
amp. The Gain-Bandwidth Product is sometimes called the
unity-gain frequency or unity-gain cross frequency because
the open-loop gain characteristic passes through or crosses
unity gain at this frequency. Simply, we have the following re-
lationship: A
CL1 xf1 =ACL2 xf2
Assuming that at unity-gain (A
CL1 = 1 or (0 dB)) fu = fi =
GBWP, then we have the following: GBWP = A
CL2 xf2
This says that once fu (GBWP) is known for an amplifier,
then the open-loop gain can be found at any frequency. This
is also an excellent equation to determine the 3 dB point of a
closed-loop gain, assuming that you know the GBWP of the
device. Refer to the diagram on the following page.
Biamplification: The technique of splitting the audio fre-
quency spectrum into two sections and using individual
power amplifiers to drive a separate woofer and tweeter.
Crossover frequencies for the amplifiers usually vary be-
tween 500 Hz and 1600 Hz. “Biamping” has the advantages
of allowing smaller power amps to produce a given sound
pressure level and reducing distortion effects prodused by
overdrive in one part of the frequency spectrum affecting the
other part.
C.C.I.R./A.R.M.:
Literally:
International Radio Consultative Committee
Average Responding Meter
This refers to a weighted noise measurement for a Dolby B
type noise reduction system. A filter characteristic is used
that gives a closer correlation of the measurement with the
subjective annoyance of noise to the ear. Measurements
made with this filter cannot necessarily be related to un-
weighted noise measurements by some fixed conversion
factor since the answers obtained will depend on the spec-
trum of the noise source.
S.P.L.: Sound Pressure Level — usually measured with a
microphone/meter combination calibrated to a pressure level
of 0.0002 Bars (approximately the threshold hearing level).
S.P.L. = 20 Log 10P/0.0002 dB
where P is the R.M.S. sound pressure in microbars.
(1 Bar = 1 atmosphere = 14.5 lb/in2 = 194 dB S.P.L.).
LM2876
www.national.com
16
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