Application Information

BRIDGE CONFIGURATION EXPLANATION

As shown in

Figure 1 , the LM4865 has two operational am-

plifiers internally, allowing for a few different amplifier con-

figurations. The first amplifier’s gain is DC voltage controlled,

while the second amplifier is internally fixed in a unity-gain,

inverting configuration. The closed-loop gain of the first am-

plifier is set by an external DC voltage (refer to (

Figure 1),

while the second amplifier’s gain is fixed by the two internal

20 k

Ω resistors.

Figure 1 shows that the output of amplifier one serves as the

input to amplifier two which results in both amplifiers produc-

ing signals identical in magnitude, but out of phase 180˚.

By driving the load differentially through outputs V

V

“bridged mode” is established. Bridged mode operation is

different from the classical single-ended amplifier configura-

tion where one side of its load is connected to ground.

A bridge amplifier design has a few distinct advantages over

the single-ended configuration, as it provides differential

drive to the load, thus doubling output swing for a specified

supply voltage. Four times the output power is possible as

compared to a single-ended amplifier under the same condi-

tions. This increase in attainable output assumes that the

amplifier is not current limited or clipped. In order to choose

an amplifier’s closed-loop gain without causing excessive

clipping, please refer to the Audio Power Amplifier Design

section.

THD+N vs Output Power

DS101025-23

THD+N vs Output Power

DS101025-24

THD+N vs Output Power

DS101025-28

Output Power vs Load Resistance

DS101025-29

Clipping Voltage vs Supply Voltage

DS101025-30

Output Power vs Supply Voltage

DS101025-31

Output Power vs Supply Voltage

DS101025-32

Supply Current vs Supply Voltage

DS101025-33

www.national.com

5