Power Supply

Rejection Ratio

Open Loop

Frequency Response

20001321

20001322

Supply Current vs

Supply Voltage

20001323

External Components Description

Figure 1

Components

Functional Description

1.

R

i

Inverting input resistance which sets the closed-loop gain in conjunction with R

high pass filter with C

πR

2.

C

i

Input coupling capacitor which blocks the DC voltage at the amplifier’s input terminals. Also creates a

highpass filter with R

πR

for an explanation of how to determine the value of C

3.

R

f

Feedback resistance which sets the closed-loop gain in conjunction with R

4.

C

s

Supply bypass capacitor which provides power supply filtering. Refer to the Power Supply Bypassing

section for information concerning proper placement and selection of the supply bypass capacitor.

5.

C

B

Bypass pin capacitor which provides half-supply filtering. Refer to the section, Proper Selection of

External Components, for information concerning proper placement and selection of C

Application Information

ELIMINATING OUTPUT COUPLING CAPACITORS

Typical single-supply audio amplifiers that can switch be-

tween driving bridge-tied-load (BTL) speakers and single-

ended (SE) headphones use a coupling capacitor on each

SE output. This capacitor blocks the half-supply voltage to

which the output amplifiers are typically biased and couples

the audio signal to the headphones. The signal return to

circuit ground is through the headphone jack’s sleeve.

The LM4867 eliminates these coupling capacitors. Amp2A is

internally configured to apply V

jack’s sleeve. This voltage matches the quiescent voltage

present on the Amp1A and Amp1B outputs that drive the

headphones. The headphones operate in a manner very

similar to a bridge-tied-load (BTL). The same DC voltage is

www.national.com

11