Page 144 - Op Amps Design, Applications, and Troubleshooting

High-Current Amplifier 127

Note that the negative value simply indicates an inversion.

Determine the Value of Rj. The value of the input resistor is chosen to
establish the required input resistance of the circuit. In our case, anything
greater than 10 kilohms should suffice. Let us choose a standard value of 18
kilohms.

Calculate the Value of R F. R F is calculated with the inverting amplifier gain
equation, Equation (2.6).

We will select a standard value of 56 kilohms.

Determine the Required Unity Gain Frequency. The required unity gain
frequency is computed in a manner similar to that in previous discussions, using
Equation (2.22).

Select a Value for Rp. The value of the potentiometer K P is essentially arbi-
trary. As it is made smaller, its power rating requirement becomes higher, and the
current draw from the supply becomes greater. If R P is made excessively large,
then the effects of bias currents that flow through R P are more pronounced. As a
guideline, select R P to be approximately equal to R f/10, but consider 1.0 kilohm to
be the minimum practical value. In our case, we have