Application Information
(Continued)
The minimum gain from
Equation (8)
is:
A
V
≥
12.6
We select a gain of 13 (Non-Inverting Amplifier); resulting in
a sensitivity of 973 mV.
Letting R
IN
equal 100 k
Ω
gives the required input imped-
ance, however, this would eliminate the “volume control”
unless an additional input impedance was placed in series
with the 10 k
Ω
potentiometer that is depicted in
Figure 1
.
Adding the additional 100 k
Ω
resistor would ensure the
minumum required input impedance.
For low DC offsets at the output we let R
f1
= 100 k
Ω
. Solving
for Ri (Non-Inverting Amplifier) gives the following:
Ri = R
f1
/(A
V
− 1) = 100k/(13 − 1) = 8.3 k
Ω
; use 8.2 k
Ω
The bandwidth requirement must be stated as a pole, i.e.,
the 3 dB frequency. Five times away from a pole gives
0.17 dB down, which is better than the required 0.25 dB.
Therefore:
f
L
= 20 Hz/5 = 4 Hz
f
H
= 20 kHz x 5 = 100 kHz
At this point, it is a good idea to ensure that the Gain-
Bandwidth Product for the part will provide the designed gain
out to the upper 3 dB point of 100 kHz. This is why the
minimum GBWP of the LM3886 is important.
GBWP
≥
A
V
x f3 dB = 13 x 100 kHz = 1.3 MHz
GBWP = 2.0 MHz (min) for the LM3886
Solving for the low frequency roll-off capacitor, Ci, we have:
Ci
≥
1/(2
π
Ri f
L
) = 4.85 µF; use 4.7 µF.
Definition of Terms
Input Offset Voltage:
The absolute value of the voltage
which must be applied between the input terminals through
two equal resistances to obtain zero output voltage and
current.
Input Bias Current:
The absolute value of the average of
the two input currents with the output voltage and current at
zero.
Input Offset Current:
The absolute value of the difference
in the two input currents with the output voltage and current
at zero.
Input Common-Mode Voltage Range (or Input Voltage
Range):
The range of voltages on the input terminals for
which the amplifier is operational. Note that the specifica-
tions are not guaranteed over the full common-mode voltage
range unless specifically stated.
Common-Mode Rejection:
The ratio of the input common-
mode voltage range to the peak-to-peak change in input
offset voltage over this range.
Power Supply Rejection:
The ratio of the change in input
offset voltage to the change in power supply voltages pro-
ducing it.
Quiescent Supply Current:
The current required from the
power supply to operate the amplifier with no load and the
output voltage and current at zero.
Slew Rate:
The internally limited rate of change in output
voltage with a large amplitude step function applied to the
input.
Class B Amplifier:
The most common type of audio power
amplifier that consists of two output devices each of which
conducts for 180˚ of the input cycle. The LM3886 is a
Quasi-AB type amplifier.
Crossover Distortion:
Distortion caused in the output stage
of a class B amplifier. It can result from inadequate bias
current providing a dead zone where the output does not
respond to the input as the input cycle goes through its zero
crossing point. Also for ICs an inadequate frequency re-
sponse of the output PNP device can cause a turn-on delay
giving crossover distortion on the negative going transition
through zero crossing at the higher audio frequencies.
THD + N:
Total Harmonic Distortion plus Noise refers to the
measurement technique in which the fundamental compo-
nent is removed by a bandreject (notch) filter and all remain-
ing energy is measured including harmonics and noise.
Signal-to-Noise Ratio:
The ratio of a system’s output signal
level to the system’s output noise level obtained in the
absence of a signal. The output reference signal is either
specified or measured at a specified distortion level.
Continuous Average Output Power:
The minimum sine
wave continuous average power output in watts (or dBW)
that can be delivered into the rated load, over the rated
bandwidth, at the rated maximum total harmonic distortion.
Music Power:
A measurement of the peak output power
capability of an amplifier with either a signal duration suffi-
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
figure) and the level just before clipping distortion occurs,
expressed 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
effects 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
protection circuitry. For time intervals in excess of 100 ms,
dissipation 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
package bottom.
The DC thermal resistance applies when one output transis-
tor is operating continuously. The AC thermal resistance
LM3886
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