Page 194 - Op Amps Design, Applications, and Troubleshooting

Wren-Bridge Oscillator 177

Compute the Frequency Determining Values. We will base our design on
the accepted practice that RI - R 2 - R and Q = C 2 - C. We will choose a value for C
and compute the associated value for R. The initial selection of C is somewhat
arbitrary, but will generally produce good results with values at least 100 times
greater than the input capacitance of the op amp. For our present example, let us
choose an initial capacitance value of 1000 picofarads. We can now compute the
required value of R with Equation (4.1).

We will choose a standard value of 15 kilohms. If the computed value of R is
below 1 kilohm or greater than 470 kilohms, you might want to select a different
value for C and recompute JR.

Compute R F and Up R F and R/ are selected to produce both of the following
conditions:

1, When the FET is biased off, the gain of the op amp will be less than 3.
2, When the FET is in its "resistive" range, the gain of the op amp can exceed 3.

We will compute R/ as follows:

In our present case, we compute Rj as

Let us select a standard value of 390 ohms for Rj. We will set R F to be equal to Rj.
That is,

More specifically,

This ensures that conditions for oscillation will be met.