LM4894 National Semiconductor, LM4894 Datasheet - Page 11

• Manufacturer Part Number: LM4894
• Description: 1 Watt Fully Differential Audio Power Amplifier With Shutdown Select
• Manufacturer: National Semiconductor

Typical Performance Characteristics
Non-LD Specific Characteristics
Application Information
DIFFERENTIAL AMPLIFIER EXPLANATION
The LM4894 is a fully differential audio amplifier that fea-
tures differential input and output stages. Internally this is
accomplished by two circuits: a differential amplifier and a
common mode feedback amplifier that adjusts the output
voltages so that the average value remains V
setting the differential gain, the amplifier can be considered
to have "halves". Each half uses an input and feedback
resistor (R
(see Figure 1). With R
at -R
It is extremely important to match the input resistors to each
other, as well as the feedback resistors to each other for best
F
/R
i
for each half. This results in a differential gain of
i1
and R
DC Common-Mode Voltage
Common-Mode Voltage
F1
) to set its respective closed-loop gain
i1
A
= R
VD
PSRR vs
THD vs
i2
= -R
and R
F
/R
i
F1
= R
F2
, the gain is set
DD
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11
amplifier performance. See the Proper Selection of Exter-
nal Components section for more information. A differential
amplifier works in a manner where the difference between
the two input signals is amplified. In most applications, this
would require input signals that are 180˚ out of phase with
each other. The LM4894 can be used, however, as a single
ended input amplifier while still retaining its fully differential
benefits. In fact, completely unrelated signals may be placed
on the input pins. The LM4894 simply amplifies the differ-
ence between them. Figures 2 and 3 show single-ended
applications of the LM4894 that still take advantage of the
differential nature of the amplifier and the benefits to PSRR,
common-mode noise reduction, and "click and pop" reduc-
tion.
All of these applications, either single-ended or fully differ-
ential, provide what is known as a "bridged mode" output
(bridge-tied-load, BTL). This results in output signals at V
and V
o2
that are 180˚ out of phase with respect to each
DC Common-Mode Voltage
Common-Mode Voltage
PSRR vs
THD vs
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