Page 333 - Op Amps Design, Applications, and Troubleshooting

Ideal Clamper 311

by finding the parallel resistance of tine (-) input, the diode's reverse resistance,
and the input resistance of the following stage. All of these values are computed
using DC parameters. For our present example, let us assume that the output of
the clamper is driving a standard 741 configured as a voltage follower. First, we
compute the reverse resistance of the diode. The manufacturer's data sheet indi-
cates that the diode will have about 25 nanoamperes of reverse current with 20
volts applied. From this we can estimate the reverse resistance as

The effective EC resistance of the (-) input can be estimated from the bias current
data provided by the manufacturer. The data sheet indicates that the bias current
will be about 300 nanoamperes (at 0°C). We can assume a worst-case voltage equal
to the maximum peak-to-peak input (10 volts in our case). Thus, Ohm's Law will
allow us to estimate the effective resistance of the (-) input as

Finally, the approximate DC resistance of the (+) input of a standard 741 con-
nected as a voltage follower (not shown in Figure 7.14) can be computed. The data
sheet indicates a maximum bias current of 800 nanoamperes. Again, we will
assume a voltage equal to the highest peak-to-peak input. Our estimate for the
input resistance of the follower stage is, then,

The value of R x is simply the parallel combination of these three estimated resis-
tances. That is,

Finally, the lower frequency limit of the clamper can be estimated with Equation
(7.11).