EDN Access -- 01.06.94 Line driver economically synthesizes impedanc

-January 06, 1994

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Design Ideas:January 6, 1994

Line driver economically synthesizes impedance

Victor Koren,
Orckit Communication Ltd, Ramat-Gan, Israel

The line-driver circuit in Fig 1 uses a second op amp to synthesize the output impedance. The conventional line-driver circuit employs a single op amp having a resistor whose resistance is equal to the desired impedance in series with the op amp's output. The principal drawback of the conventional circuit is that the single op amp must deliver twice the needed output voltage because of the large voltage drop across the output resistor. The circuit in Fig 1is similar to the famous Howland current pump but has different resistor values. To calculate the output resistance, RO, short the input to ground. Then


where VOZ is the output voltage with the input shorted.

IR5=(VO-VOZ)/R5=(VO/R5)x (1-(R1+R2)/(R1+R3))=(VO/R5)x(R3-R2)/(R1+R3)
RO=VO/IR5=RSx(R1+R3/(R3-R2) (1).

After fixing the values of R1, R2, and RS, to calculate R3 for a given RO, use


You can dispense with unity-gain buffer IC2 by connecting R3 directly to the output. The line driver's output impedance in this case is the value that Eq 1 gives in parallel with the series connection of the resistors R1 and R3. For this scheme to work, all resistors must have very close, matching tolerances. One way to ensure close tolerances and, at the same time, reduce cost is to use thin- or thick-film networks for R1, R2, and R3. However, for the circuit to work, R3 and RS must match closely. So you will have to add a precision resistor, RADD, in series with R3 to bring their total resistance exactly up to RS's. EDN BBS /DI_SIG #1351

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