Application Information

BRIDGE CONFIGURATION EXPLANATION

As shown in Figure 1, the LM4817 consists of two pairs of

operational amplifiers, forming a two-channel (channel A and

channel B) stereo amplifier. (Though the following discusses

channel A, it applies equally to channel B.) External resistors

R

internal 20k

Ω resistors set Amp2A’s gain at -1. The LM4817

drives a load, such as a speaker, connected between the two

amplifier outputs, -OUTA and +OUTA.

Figure 1 shows that Amp1A’s output serves as Amp2A’s

input. This results in both amplifiers producing signals iden-

tical in magnitude, but 180˚ out of phase. Taking advantage

of this phase difference, a load is placed between -OUTA

and +OUTA and driven differentially (commonly referred to

as "bridge mode"). This results in a differential gain of

A

(1)

Bridge mode amplifiers are different from single-ended am-

plifiers that drive loads connected between a single amplifi-

er’s output and ground. For a given supply voltage, bridge

mode has a distinct advantage over the single-ended con-

figuration: its differential output doubles the voltage swing

across the load. This produces four times the output power

when compared to a single-ended amplifier under the same

conditions. This increase in attainable output power as-

sumes that the amplifier is not current limited or that the

output signal is not clipped. To ensure minimum output sig-

nal clipping when choosing an amplifier’s closed-loop gain,

refer to the Audio Power Amplifier Design section.

Another advantage of the differential bridge output is no net

DC voltage across the load. This is accomplished by biasing

channel A’s and channel B’s outputs at half-supply. This

eliminates the coupling capacitor that single supply, single-

ended amplifiers require. Eliminating an output coupling ca-

pacitor in a single-ended configuration forces a single-supply

amplifier’s half-supply bias voltage across the load. This

increases internal IC power dissipation and may perma-

nently damage loads such as speakers.

POWER DISSIPATION

Power dissipation is a major concern when designing a

successful single-ended or bridged amplifier. Equation (2)

states the maximum power dissipation point for a single-

ended amplifier operating at a given supply voltage and

driving a specified output load

P

(2)

However, a direct consequence of the increased power de-

livered to the load by a bridge amplifier is higher internal

power dissipation for the same conditions.

20078101

FIGURE 1. Typical Audio Amplifier Application Circuit

Pin out shown for the LLP package. Refer to the Connection Diagrams for the pinout of the TSSOP package.

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