Page 85 - Op Amps Design, Applications, and Troubleshooting

68 AMPLIFIERS

3. Load resistance
4. Maximum input voltage

As an example of the design procedure, let us design a noninverting amplifier
with the following characteristics:

1. Voltage gain 8
2. Frequency range DC to 5 kilohertz
3. Load resistance 27kilohms
4. Maximum input voltage 800 millivolts RMS

Determine an Initial Value for R h There are endless combinations of R F and
RI that will produce the desired circuit voltage gain. The smaller the values of R F and
JR/, the higher the value of feedback current. The feedback current subtracts from the
maximum available output current. Thus, we want to avoid extremely small values.
The larger we make R F and R f, the more the circuit operation is affected by
certain nonideal characteristics. In general, neither resistor should be less than 1.0
kilohms nor more than 680 kilohms unless there is a compelling reason for them to
be so. With this rule of thumb in mind, we select R/ as 4.7 kilohms.

Determine the Value of R F. R F can be computed from the voltage gain equa-
tion, Equation (2.28):

For the present design example, we compute R F as follows:

We select the nearest standard value of 33 kilohms to use as R F.

Determine the Required Unity Gain Frequency. You will recall from our
discussions on bandwidth that the error between the calculated or ideal gain and
the actual gain increases as frequency increases. We can, however, estimate the
required unity gain frequency by applying Equation (2.22).