Page 117 - Op Amps Design, Applications, and Troubleshooting

100 AMPLIFIERS

For our example, we compute overall voltage gain as

A v(overall) = -2.4 x 0.97 = -2.33

Notice that the method described for computing overall voltage gain does not
include the effects of the variations in open-loop gain at different frequencies.
Although this additional consideration could be included as with the direct-
coupled amplifier, it is not normally necessary because our calculations are
accomplished at the lowest input frequency. If you want to compute the gain at
some relatively high frequency, then you should include the effects of reduced op
amp internal gain.
Another point that you may wish to consider involves phase shift. In addition
to the 180-degree phase shift provided by the op amp itself, the signal also receives
a phase shift from the two RC networks. The preceding calculations compute only
the amplitude of the signal. If the phase is also an important consideration, then the
same basic equations still apply but you can express the values as complex num-
bers. The final answer, then, not only will include the magnitude of the gain as
computed, but will also reveal the amount of phase shift given to the signal.
The voltage gain calculation for the noninverting circuit shown in Figure
2.27(b) is similar, but will be considered as three separate gains that are multiplied
together to find the overall gain. The three individual gains are

1. RI/CI network gain (actually a loss)
2. RL/CQ network gain (actually a loss)
3. The gain of the op amp circuit as determined by R F and R }

The gains of the input and output RC circuits are computed in the same way
that we computed the gain of the output RC circuit in Figure 2.27(a). Let us first
calculate the input RC circuit gain. Our initial step is to compute the reactance of
C| at the lowest frequency (assumed to be 800 hertz).

Next we find the net impedance of RI and Q.

Finally we compute the voltage gain (loss) of the RC network with Equation (2.38).