TS615 datasheet(25/27 Pages) STMICROELECTRONICS

25 / 27 page

TS615

25/27

INCREASING THE LINE LEVEL BY USING AN

ACTIVE IMPEDANCE MATCHING

With a passive matching, the output signal ampli-

tude of the driver must be twice the amplitude on

the load. To go beyond this limitation an active

matching impedance can be used. With this tech-

nique, it is possible to keep a good impedance

matching with an amplitude on the load higher

than the half of the output driver amplitude. This

concept is shown in figure 74 for a differential line.

Figure 74 : TS615 as a differential line driver with

an active impedance matching

Component Calculation

Let us consider the equivalent circuit for a single

ended configuration, Figure75.

Figure 75 : Single ended equivalent circuit

Let us consider the unloaded system. Assuming

the currents through R1, R2 and R3

as respectively:

As Vo° equals Vo without load, the gain in this

case becomes :

The gain, for the loaded system will be (eq1):

As shown in figure76, this system is an ideal gen-

erator with a synthesized impedance as the inter-

nal impedance of the system. From this, the out-

put voltage becomes:

with Ro the synthesized impedance and Iout the

output current. On the other hand Vo can be ex-

pressed as:

By identification of both equations (eq2) and

(eq3),

the

synthesized

impedance

is,

with

Rs1=Rs2=Rs:

Figure 76 : Equivalent schematic. Ro is the

synthesized impedance

100

Ω

1:n

Hybrid

Transformer

GND

Vcc+

100n

Rs1

Rs2

10n

Vo°

GND

Vcc+

GND

1/2

Vcc/2

1/2RL

1/2R1

Rs1

-1

Vo°

1/2RL

1/2R1

Rs1

-1

Vo°

2Vi

---------

–

()

--------------------------- and

()

------------------------

Vo noload

()

---------------------------------

2R2

-----------

--------

–

------------------------------------

Vo with load

()

--------------------------------------

---

2R2

-----------

--------

–

------------------------------------ eq1

()

ViG

()

RoIout

()

–

eq2

()

Vi 1

2R2

-----------

--------





--------

–

------------------------------------------------

Rs1Iout

--------

–

----------------------- eq3

()

–

--------

–

----------------- eq4

()

Vi.Gi

Iout

1/2

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