Therefore, in the case of a 10% overshoot, we need an early-time (high-frequency) gain G1 equal
to 0.9 G
0
, where
R1 || R
F
G1 = 1 +
R
Accordingly, R1 and C1 are chosen so that
1 + R
F
/R
= 1.1 ; and (R1 + R)C1 ~ TC.
R1 || R
F
1 +
R
Of course, this whole method only works if the effect is linear.
PULSE SHAPING WITH LOSSY CABLES
A lossy cable will distort the output pulse, giving rise to a pulse shape illustrated in Fig. 4.2(a).
Fig. 4.2(a)
Fig. 4.2(b)
This is not of the form:
{
-t/TC1}
{
-t/TC2}
V X
{ 1 – e
} X
{ 1 – e
}
and therefore an exact compensation cannot be achieved with one RC net. The basic idea of
compensation is similar to that employed to remove overshoots, but now we need to boost the
gain at early times.
The circuit needed (for a non-inverting amp) is shown in Fig. 4.2(b).
Choose R and C to remove as much of the undershoot as possible; concentrate on the long time-
constants first. If a single net is not sufficient, add a second net to pick up the early gain. Two
nets are generally sufficient to solve the problem, but you could add more if the cable were really
lossy.
-13-
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