EL1503

FN7038 Rev 0.00

Page 14 of 16

July 17, 2001

Applications Information

The EL1503 consists of two high-power line driver amplifiers

that can be connected for full duplex differential line

transmission. The amplifiers are designed to be used with

signals up to 4MHz and produce low distortion levels. A typical

interface circuit is shown in Figure 40 below.

The amplifiers are wired with one in positive gain and the other

in a negative gain configuration to generate a differential output

for a single-ended input. They will exhibit very similar

frequency responses for gains of three or greater and thus

generate very small common-mode outputs over frequency,

but for low gains the two drivers RF's need to be adjusted to

give similar frequency responses. The positive-gain driver will

generally exhibit more bandwidth and peaking than the

negative-gain driver. If a differential signal is available to the

drive amplifiers, they may be wired so:

Each amplifier has identical positive gain connections, and

optimum common-mode rejection occurs. Further, DC input

errors are duplicated and create common-mode rather than

differential line errors.

Input Connections

The EL1503 amplifiers are somewhat sensitive to source

impedance. In particular, they do not like being driven by

inductive sources. More than 100nH of source impedance can

cause ringing or even oscillations. This inductance is

equivalent to about 4” of unshielded wiring, or 6” of

unterminated transmission line. Normal high-frequency

construction obviates any such problem.

Power Supplies & Dissipation

Due to the high power drive capability of the EL1503, much

attention needs to be paid to power dissipation. The power that

needs to be dissipated in the EL1503 has two main

contributors. The first is the quiescent current dissipation. The

second is the dissipation of the output stage.

The quiescent power in the EL1503 is not constant with

varying outputs. In reality, 7mA of the 12.5mA needed to power

each driver is converted in to output current. Therefore, in the

equation below we should subtract the average output current,

IO, or 7mA, whichever is the lowest. We’ll call this term IX.

Therefore, we can determine a quiescent current with the

equation:

where:

VS is the supply voltage (VS+ to VS-)

IS is the maximum quiescent supply current (IS+ + IS-)

IX is the lesser of IO or 7mA (generally IX = 7mA)

The dissipation in the output stage has two main contributors.

Firstly, we have the average voltage drop across the output

transistor and secondly, the average output current. For

minimal power dissipation, the user should select the supply

voltage and the line transformer ratio accordingly. The supply

voltage should be kept as low as possible, while the

transformer ratio should be selected so that the peak voltage

required from the EL1503 is close to the maximum available

output swing. There is a trade of however with the selection of

transformer ratio. As the ratio is increased, the receive signal

available to the receivers is reduced.

Once the user has selected the transformer ratio, the

dissipation in the output stages can be selected with the

following equation:

where:

VS is the supply voltage (VS+ to VS-)

VO is the average output voltage per channel

IO is the average output current per channel

The overall power dissipation (PDISS) is obtained by adding

PDquiescent and PDtransistor.

FIGURE 40. TYPICAL LINE INTERFACE CONNECTION

-

+

-

+

-

+

-

+

RECEIVE

OUT -

RECEIVE

OUT +

DRIVER

INPUT

RG

RF

RF

RF

R

RIN

R

RIN

RF

ROUT

ROUT

LINE +

RECEIVE

AMPLIFIERS

ZLINE

LINE -

FIGURE 41. DRIVERS WIRED FOR DIFFERENTIAL INPUT

-

+

-

+

2RG

RF

RF

PDquiescent

VS IS 2IX

–





=

PDtransistors

2IO

VS

2

-------









VO

–





=