Typical Performance Characteristics

CMRR vs Frequency

V

Ω, 200mV

pp

CMRR vs Frequency

V

Ω, 200mV

pp

200737E8

200737F0

PSRR vs Common Mode Voltage

V

PSRR vs Common Mode Voltage

V

Ω, 217Hz, 200mV

pp

200737F2

200737F4

Application Information

DIFFERENTIAL AMPLIFIER EXPLANATION

The LM4898 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 two "halves". Each half uses an input and feedback

resistor (R

(see Figure 1). With R

at -R

A

(1)

It is extremely important to match the input resistors to each

other, as well as the feedback resistors to each other for best

amplifier performance. See the Proper Selection of External

Components section for more information. A differential am-

plifier 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 LM4898 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 LM4898 simply amplifies the differ-

ence between them.

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 Vo1

and Vo2 that are 180˚ out of phase with respect to each

other. Bridged mode operation is different from the single-

ended amplifier configuration that connects the load be-

tween the amplifier output and ground. A bridged amplifier

design has distinct advantages over the single-ended con-

figuration: it provides differential drive to the load, thus dou-

bling maximum possible output swing for a specific supply

voltage. Four times the output power is possible compared

with a single-ended amplifier under the same conditions.

This increase in attainable output power assumes that the

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