Line driver provides full rail excursions

Posted on Mar 25, 2013

The logic input is applied to opto-isolators Ul and U2 with, respectively, npn and pnp emitter follower outputs. Dc balance is adjusted by potentiometer R2. The emitter followers drive the gates of Ql and Q2, the complementary TMOS pairs. With a ±12 V supply, the swing at the common source output point is about 12 V peak-to-peak. By adding a ± 18-V boost circuit, as shown, the output swing can approach the rail swing.

Line driver provides full rail excursions
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This circuit applies the output to transformer Tl, which is rectified by diode bridge D3, regulated by U3 and U4, and then applied to the collectors of Ul and U2. Diodes Dl and D2 are forward-biased when 12-V supplies are used, but they are back-biased when the 18-V boost is used.

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