Page 243 - Op Amps Design, Applications, and Troubleshooting

High-Pass Filter 225

We will select a standard value of 0.033 microfarad for both capacitors.

Compute the Value of R 3. Resistor R 3 is included to reduce the effects of the
op amp bias current that flows through JR^ R 3 is set equal to RI, or simply

In our case, we will use a 20-kilohm resistor for R 3.

Select the Op Amp. The following op amp parameters are the most essential
when designing a high-pass filter circuit:

1. Bandwidth
2, Slew rate

Additionally, if it is necessary to use high-resistance values, then every effort
should be made to use an op amp with low-bias currents.

Bandwidth. When we construct a high-pass filter with an op amp, we inher-
ently build a bandpass filter. That is, our filter circuit, by design, will attenuate all
frequencies below the cutoff frequency. Ideally, all frequencies above the cutoff
frequency should be passed with minimal attenuation. In practice, however, the
gain of our op amp falls off at high frequencies. Thus, the very high frequencies
are attenuated by the reduced gain of the op amp.
When selecting an op amp for a particular application, we must ensure that
the amplifier gain is still adequate at the highest expected frequency of operation.
Since the op amp is configured for unity gain, we simply need to be sure that it has
a unity gain bandwidth (f UG) that is higher than the highest input frequency. In the
present case, the highest input frequency is cited as 5000 hertz. Therefore, our
choice of op amps must have a unity gain frequency greater than that. This should
be an easy task.

Slew Rate* The slew rate limitation of the op amp restricts the highest fre-
quency mat we can properly amplify at a given amplitude. Since the maximum
input amplitude was not specified in the design goals, we will assume that the
output may produce a full swing between ±VSAT- Equation (5.7) can be used to
estimate the required op amp slew rate for our present application:

Both bandwidth and slew rate requirements are within the capabilities of a stan-
dard 741 op amp. Let us plan to use this device in our design.