4. GENERAL GUIDANCE ON PULSE-SHAPING
4.1
HOW TO REMOVE OVERSHOOTS AND IMPROVE SETTLING-TIMES
If a pulse has an overshoot as shown in Fig. 4.1(a), below, it is possible to remove the overshoot
without seriously affecting the rise time by using an RC net instead of a simple capacitor (which
would seriously degrade rise time if it were large enough to completely remove the overshoot).
Fig. 4.1(a)
Fig. 4.1(b)
To see how this works, consider the inverting amplifier shown in Fig. 4.1(b). Let us assume the
overshoot is 10% of the true pulse height without the RC net in the circuit. At early times (high
frequencies) we need to reduce the gain by 10%. At these frequencies, ZC is small. Therefore we
need to choose R1 so that:
R1 || R
F
= 0.9R
F
to reduce the gain by 10%. To choose C1, observe the time constant, TC, for the overshoot.
Simply set R1C1 = TC, and the net will remove the overshoot.
For the non-inverting amplifier shown in, the formula is a little more complex.
Fig 4.1(c)
R is an internal resistor, whose value (in A2 and A3) is 95 ohms. Assuming a large open-loop gain,
the initial gain, G
0
, is given by:
G
0
= 1 + R
F
/R
-12-
 |
 |
 |
 |
 |