Page 124 - Op Amps Design, Applications, and Troubleshooting

AC-Coupled Amplifier 107

Computing the ratio of the two cutoff frequencies (using the higher frequency
as the numerator) gives us the index, from Equation (2.42), needed for the lookup
operation in Table 2.5:

Using this value as the index into Table 2.5 gives us an approximate value of
1.33 for k. The lower cutoff frequency for the entire circuit in Figure 2.27(b) can
now be estimated with Equation (2.43):

At this point, the bandwidth of the circuit can be estimated with Equation (2.5):

bw=f u-f L
= 100^2-642 Hz
= 99.36 kHz

Slew-Rate Limitations. The slew rate of the op amp will limit the upper cutoff
frequency for high-amplitude output signals. The slew-rate limiting frequency is
calculated in the same manner as described for previous amplifier configurations.

2.7.3 Practical Design Techniques

The design of either the inverting or the noninverting AC-coupled amplifier is a
relatively easy process. Following are the sequential steps:

1. Design the basic amplifier circuit according to the guidelines presented for
the direct-coupled inverting or noninverting amplifier circuits.
2. Compute the values for the input and output RC coupling components.

As an example, let us design a noninverting AC-coupled amplifier that has the fol-
lowing characteristics:

1. Midpoint voltage gain of 12
2. Lower cutoff frequency of 500 hertz
3. Upper cutoff frequency of at least 15 kilohertz
4. Input impedance of at least 3000 ohms

Determine the Value of Jfy We will select R { as 6.8 kilohms. Although this
selection is somewhat arbitrary, we are keeping within the guidelines of choosing
resistance between 1000 ohms and 680 kilohms. Additionally, the selection of Rj