Page 316 - Op Amps Design, Applications, and Troubleshooting

294 SIGNAL PROCESSING CIRCUITS

signals at some time, the highest frequency that can be amplified without slew
rate distortion is computed using Equation (2.11),

where v 0(max) is the maximum swing on the output of the op amp and the 0.5-
voltS'per-microsecond slew rate is given by the manufacturer for the 741 op amp.
So, if we expect to rectify maximum amplitude signals, we wiM be limited to 6.12
kilohertz or less. On the other hand, if the application uses only lower amplitude
signals, then a wider bandwidth can be expected.

7.2.3 Practical Design Techniques

Let us now design a dual, half-wave rectifier circuit that satisfies the following
design goals:

1. Peak input voltage ±250 millivolts to ±5.0 volts
2. Peak output voltages ±375 millivolts to ±7.5 volts
3. Minimum input impedance 3000 ohms
4. Highest input frequency 25 kilohertz
Compute the Required Voltage Gain. The design goals give no indication
that we are to design for unequal gains on the two different half cycles. The required
amplifier gain, then, is computed by applying the basic gain equation, Equation (2.1).

Of course, we could also have used the higher values of 5 volts and 7.5 volte for
input and output voltage, respectively. This calculation gives us the absolute
value of required voltage gain. By virtue of the circuit configuration, we know the
gain will be inverting (i.e., A v = -1.5).

Select &i. You will recall from our discussions of amplifiers in Chapter 2 that the
input impedance of an inverting amplifier is determined by the value of input resis-
tor. In our present case, resistor RI must be large enough to satisfy the minimum
input impedance requirement. Additionally, there is little reason to go beyond a few
hundred kilohms. For the present design example, let us select RI as 20 kilohms.

Compute RI and R 3, In our current design, resistors R 2 and R 3 will be equal
because the gains for the two half cycles must be equal. We compute the value of
R 2 (or R 3) by applying a transposed version of the inverting amplifier gain equa-
tion, Equation (2.6).