Function Generator 2

Posted on Feb 5, 2014

How to take the triangle wave waveform and bend it into a sine wave. To help adjust its transfer function, resistors R32 and R33 are added. Increasing or decreasing these resistors causes the response to be more or less linear. How do we take our differential output current from the the Q1, Q2 pair above and convert it back to voltage Here`s a handy circuit that`s easier to analyze than it looks.

Function Generator 2
Click here to download the full size of the above Circuit.

The transistor outputs are modeled as current sources. Although the transistors do have an output impedance across current sources, the value is typically so high where it won`t significantly affect circuit operation. Let`s see how this circuit works. Notice R30 and R31 attenuate the signal by ~10x so that the differential pair is not over driven with too large of an input. Choosing R34=5k sets up about 0. 5mA flowing in each transistor. The resistors for the current to voltage converter were initially chosen as R41, 42, 43, 43=2k. You can increase R42 and R44 to increase the output. CIRCUIT INSIGHT Run a transient simulation and plot the triangle wave output V(3). In another plot window, add the transistor collector currents IC(Q30) and IC(Q31). Finally go back to the original plot and add the sine wave output at V(36) to see the difference of currents converted back to a voltage. In many classic function generator circuits, this is how the triangle wave becomes a sine wave! HANDS-ON DESIGN Now, let`s monkey with the controls. Reduce linearizing resistors R32, 33 from 120 to just 12 ohms. Rerun the simulation and check out V(36). What happened to the beautiful sine wave. Looks like there`s too much clipping! Try increasing R32, 33 to 240 ohms. What happened to the output V(36). Looks like the gain is too linear where the output starts resembling the triangle wave. Finally, you can scale the output by adjusting R42, 44. Try...

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