SSM2211

REV. 0

–9–

Driving a speaker differentially from a bridged output offers an-

other advantage in that it eliminates the need for an output cou-

pling capacitor to the load. In a single supply application, the

quiescent voltage at the output is 1/2 of the supply voltage. If a

speaker were connected in a single ended configuration, a cou-

pling capacitor would be needed to prevent dc current from

flowing through the speaker. This capacitor would also need to

be large enough to prevent low frequency roll-off. The corner

frequency is given by:

f

RC

dB

LC

−

=

3

1

2

π

(4)

Where

For an 8

Ω speaker and a corner frequency of 20 Hz, a 1000 µF

capacitor would be needed, which is quite physically large and

costly. By connecting a speaker in a bridged output configura-

tion, the quiescent differential voltage across the speaker be-

comes nearly zero, eliminating the need for the coupling

capacitor.

Speaker Efficiency and Loudness

The effective loudness of 1 W of power delivered into an 8

Ω

speaker is a function of the efficiency of the speaker. The effi-

ciency of a speaker is typically rated as the sound pressure level

(SPL) at 1 meter in front of the speaker with 1 W of power

applied to the speaker. Most speakers are between 85 dB and

95 dB SPL at 1 meter at 1 W. Table I shows a comparison of

the relative loudness of different sounds.

Table I. Typical Sound Pressure Levels

Source of Sound

dB SPL

Threshold of Pain

120

Heavy Street Traffic

95

Cabin of Jet Aircraft

80

Average Conversation

65

Average Home at Night

50

Quiet Recording Studio

30

Threshold of Hearing

0

It can easily be seen that 1 W of power into a speaker can pro-

duce quite a bit of acoustic energy.

Power Dissipation

Another important advantage in using a bridged output configu-

ration is the fact that bridged output amplifiers are more effi-

cient than single ended amplifiers in delivering power to a load.

Efficiency is defined as the ratio of power from the power supply

to the power delivered to the load

η =









P

P

L

SY

. An amplifier

with a higher efficiency has less internal power dissipation,

which results in a lower die-to-case junction temperature, as

compared to an amplifier that is less efficient. This is important

when considering the amplifier device’s maximum power dissi-

pation rating versus ambient temperature. An internal power

dissipation versus output power equation can be derived to fully

understand this.

The internal power dissipation of the amplifier is the internal

voltage drop multiplied by the average value of the supply cur-

rent. An easier way to find internal power dissipation is to take

the difference between the power delivered by the supply voltage

source and the power delivered into the load. The waveform of

the supply current for a bridged output amplifier is shown in

Figure 40.

TIME

T

T

TIME

Figure 40. Bridged Amplifier Output Voltage and Supply

Current vs. Time

By integrating the supply current over a period T, then dividing

peak output voltage and load resistance:

I

V

R

DD AVG

PEAK

L

,

=

2

π

(5)

therefore power delivered by the supply, neglecting the bias cur-

rent for the device is,

P

VV

R

SY

DD

PEAK

L

=

2

π

(6)

Now, the power dissipated by the amplifier internally is simply

the difference between Equation 6 and Equation 3. The equa-

power delivered to the load and load resistance is:

P

V

R

PP

DISS

DD

L

LL

=

×

−

22

π

(7)

The graph of this equation is shown in Figure 41.