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LT6301

sn6301 6301f

of n. To analyze this circuit, first ground the input. As RBT =

RL/n, and assuming RP2>>RL we require that:

VA = VO (1 – 1/n) to increase the effective value of

RBT by n.

VP = VO (1 – 1/n)/(1 + RF/RG)

VO = VP (1 + RP2/RP1)

Eliminating VP, we get the following:

(1 + RP2/RP1) = (1 + RF/RG)/(1 – 1/n)

For example, reducing RBT by a factor of n = 4, and with an

amplifer gain of (1 + RF/RG) = 10 requires that RP2/RP1

= 12.3.

Note that the overall gain is increased:

nR R

()

() +

()

[]−+

()

[]

A simpler method of using positive feedback to reduce the

back-termination is shown in Figure 14. In this case, the

drivers are driven differentially and provide complemen-

tary outputs. Grounding the inputs, we see there is invert-

ing gain of –RF/RP from –VO to VA

VA = VO (RF/RP)

and assuming RP >> RL, we require

VA = VO (1 – 1/n)

solving

RF/RP = 1 – 1/n

So to reduce the back-termination by a factor of 3 choose

RF/RP = 2/3. Note that the overall gain is increased to:

VO/VI = (1 + RF/RG + RF/RP)/[2(1 – RF/RP)]

Using positive feedback is often referred to as active

termination.

Figure 16 shows a full-rate ADSL line driver incorporating

positive feedback to reduce the power lost in the back

termination resistors by 40% yet still maintains the proper

impedance match to the100

Ω characteristic line imped-

ance. This circuit also reduces the transformer turns ratio

over the standard line driving approach resulting in lower

peak current requirements. With lower current and less

power loss in the back termination resistors, this driver

dissipates only 1W of power, a 30% reduction.

While the power savings of positive feedback are attractive

there is one important system consideration to be ad-

dressed, received signal sensitivity. The signal received

APPLICATIO S I FOR ATIO

Figure 14. Back Termination Using Differential Postive Feedback

Figure 13. Back Termination Using Postive Feedback

6301 F13

RBT

RP2

RP1

1 +

= 1 –

–

FOR RBT =

()

1 +

()

RP1

RP1 + RP2

RP1

RP2 + RP1

RP2/(RP2 + RP1)

()

1 + 1/n

–

RBT

–VI

–VA

–VO

RBT

6301 F14

n =

1 –

FOR RBT =

1 +

1 –

()

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