HC5503

FN4344 Rev 5.00

Page 9 of 17

June 2004

The Transversal Amplifier (TA)

Whereas the feed amplifiers perform the 4-wire to 2-wire

transmission function, the transversal amplifier acts as the

2-wire to 4-wire hybrid. The TA is a summing amplifier

configured to reject common mode signals. It will reject 2-wire

common mode signals. RB1 and RB2 act as loop current sense

resistors. The voice signal output of the amplifier is a function

of the differential voltages appearing across RB1 and RB2.

The transversal amplifier also has a DC output proportional to

the metallic current in the loop. The output voltage is given

by:

VTX = 2(ITIP + IRING) (RB1 + RB2)

This DC level is used as an input to a comparator whose

output feeds into the logic circuitry as SH. This signal is used to

gate SHD output.

Voice signals on the loop are transformed by the TA into

ground referenced signals. Since the TA output has a DC

offset it is necessary to AC couple the output to any external

circuitry. Note, that during 4-wire to 2-wire transmission, the

transversal amplifier will have an audio signal at its output

proportional to the 4-wire audio receive signal and the loop’s

equivalent AC impedance. This is called the transhybrid return,

and must be cancelled (or balanced) out to prevent an echo

effect. Reference the Transhybrid Circuit section for more

information.

Loop Current Limiting

The maximum loop length for this application is a 533

 load

across the feed amplifiers (24VSUPPLY - 8VOVERHEAD)/

30mAMAX loop current). However, on a short loop the line

resistance often approaches zero. Thus, a need exists to

control the maximum DC loop current that can flow around the

loop to prevent an excessive current drain from the system

battery. This limit is internally set to 30mA on the HC5503.

Figure 3 depicts the feedback network that modifies the VRF

voltage as a function of metallic current. Figure 4 illustrates the

loop current characteristics as a function of line resistance.

As indicated above, the TA has a DC voltage output directly

proportional to the loop current. This voltage level is scaled

by R19 and R18. The scaled level forms the ‘Metallic’ input to

one side of a Transconductance Amplifier.

The reference input to this amplifier is generated in the bias

network, and is equivalent to 30mA. When the metallic input

exceeds the set reference level, the transconductance amplifier

sources current. This current will charge C1 in positive direction

causing the VRF (Ring Feed) voltage to approach the VTF (Tip

Feed), effectively reducing the battery feed across the loop

which will limit the DC loop current. C1 will continue to charge

until an equilibrium level is attained at ILOOP = ILOOPmA (Max).

The time constant of this feedback loop is set by R21 (90k) and

C1 which is nominally 0.33F.

The VRF voltage level is also modified to reduce or control loop

current during ring line faults (e.g., ground or power line

crosses), and thermal overload. Figure 8 illustrates this. The

thermal and fault current circuitry works in parallel with the

transconductance amplifier.

Longitudinal Amplifier

The longitudinal amplifier is an operational amplifier configured

as a closed loop differential amplifier with a nominal gain of

0.1. The output is a measure of any imbalance between ITIP

and IRING. The transfer function of this amplifier is given by:

VLONG = 0.1(ITIP - IRING) 150.

The gain factor is much less than one since ring voltage (up to

150VPEAK) can appear at the Ring or Ring Feed Sense

terminals and are attenuated to avoid exceeding the common

mode range of the longitudinal amplifier’s input.

C1

VRF

VTIP

RB2

RB1

VRING

VTF

TRANSVERSAL

AMP

VTX = -600 ILOOP

VB5

R19

R18

90K

-4V

VB/2

KVTX

FOR KVTX < VB5

IGM > 0,

RING

90K

AVCL = 2

1.8K

R21

FEED

+

-

+

-

+-

VRF

VTX

FIGURE 3. DC LOOP CURRENT CHARACTERISTICS