HC5503

FN4344 Rev 5.00

Page 6 of 17

June 2004

Design Information

Line Feed Amplifiers

The line feed amplifiers are high power operational amplifiers

and are connected to the subscriber loop through 150

 of feed

resistance as shown in Figure 1. The feed resistors and

synthesized impedance via feedback provide a 600

 balanced

load for the 2-wire to 4-wire transmission.

The tip feed amplifier is configured as a unity gain noninverting

buffer. A -4V bias (derived from the negative battery (VBAT) in

the bias network) is applied to the input of the amplifier. Hence,

the tip feed DC level is at -4V. The principal reason for this

offset is to accommodate sourcing and sinking of longitudinal

noise currents up to 15mARMS without saturating the amplifier

output and to provide sufficient overhead for receive signals.

The tip feed amplifier also feeds the ring feed amplifier, which

is configured as a unity gain inverting amplifier as seen from

the tip feed amplifier. The noninverting input to the ring feed

amp is biased at a VBAT/2. Looking into this terminal the

amplifier has a noninverting gain of 2. Thus, the DC output at

ring feed is:

VRF(DC) = (4 + VBAT) Volts

For a -24V battery, VRF = -20V. Hence, the nominal battery

feed across the loop provided by the SLIC is 16V. When the

subscriber goes off-hook this DC feed causes current (metallic

current) to flow around the loop.

The received audio signal RX is fed into the tip feed amplifier

and appears at the tip feed terminal. It is also fed through the

ring feed amplifier and is inverted. Thus, a differential signal of

2VRX appears between tip feed and ring feed. The RX signal

causes AC audio currents to flow around the loop which are

then AC coupled to the earpiece of the telephone set.

2-Wire Impedance Matching

The HC5503 is optimized for operation with a -24V battery.

Impedance matching to a 600

 load, is achieved through the

combination of the feed resistors (RB1, RB2) and negative

feedback through resistor R2 (reference Figure 1). RB1 and

RB2 are sense resistors that detect loop current and provide

negative feedback to synthesize the remaining 300

 required

to match a 600

 line.

The impedance looking into the tip terminal is 150

 (RB1) plus

the synthesized impedance of the tip amplifier. The

synthesized tip impedance is equal to the tip feed voltage Va

divided by

IL. (Note, the tip feed amplifier is a voltage

follower. Thus, the tip feed voltage is equal to the receive input

voltage VRX, both are labeled Va.) The synthesized

impedance of the ring terminal is calculated the same way and

is the ring feed voltage divided by

IL. (Note, the ring feed

voltage is equal in magnitude to the tip feed voltage, but

opposite in phase as a result of the ring feed amplifier gain.)

TIP

150



TIP FEED

-

+

90k



-

+ 4VDC

(NOTE)

C3

R3

TO TRANSHYBRID OP-AMP

R1

INPUT

FROM

CODEC

-

+

+

-

-

+2

C4

4RS

IL

+

+4RS

IL

+

-

RING FEED

RB2

150



- IL +

RING

VBAT

2

(NOTE)

RR

HC5503

ZIN

RB1 = RB2 = RS = 150

-

+

VIN

R2

Va

RX

TX

RB1

IL

+

-

NOTE: Grounded for AC analysis.

FIGURE 1. IMPEDANCE MATCHING CIRCUITRY