Never Short a Penelope by AD9DP

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NEVER SHORT A PENELOPE by David Poole, AD9DP

The OPA2674 is essentially a dual wideband op amp that can swing between +/-5V into a 25 ohm or higher load on each channel or 4.5V into a 10 ohm load. As used in Penelope, the output transformer raises an ideal 50 ohm load to 75 ohms across the two drivers, neglecting reactance. Half that plus R4 or R13 yields a 41.5 ohm load on each channel. Maxed out with a single 12v supply, each channel swings from 1v to 11v into that load. If reasonably sinusoidal, the OPA2674 thus delivers 300 mW from each channel, or 600 mW in push pull. 29 mW are lost in each of R4 and R13 leaving 542 mW into the transformer primary. The transformer's .4db insertion loss over the HF bands leaves 494mW . That's the most we have to work with for an IPA electrically.

The OPA2674 is characterized as a DSL driver, so computing the safe operating area for sinusoidal inputs requires reworking the thermal model provided for the output H driver with a spreadsheet model of a sinusoidal half-cycle. In addition to 225mW of quiescent power dissipated in the OPA2674 regardless of its output state, instantaneous power is dissipated in the output driver(s) equal to (Vdd-Vout) times Iout. Averaging that over the half cycle determines that 156 mW is dissipated in one output transistor or the other over alternate CW half cycles. Both channels thus driven plus the 225mW quiescent dissipation total 536 mW. The OPA2674 has a 1 Watt total dissipated power rating in a 25 degree ambient, so the Penelope design is thermaly safe with a 50 ohm load. It's self limiting for that or higher impedance loads since the output can't swing greater than +/-5V into it.

Repeating the calculation for lower load resistance finds that the 1watt thermal limit will be exceeded for CW conditions on loads below 17.5 ohms per channel when both channels are driven as they are in Penelope. There's no grace for duty cycle because the SOIC-8 package has very little thermal mass - maybe enough for dit, but possibly not a dah. R4 and R13 increase that to 21.5 ohms in Penelope, which is 43 ohms across the primary or 28.6 ohms at the output port. The allowable loads thus have VSWRs below 1.75:1. An output short will produce a CW dissipation of around four watts that is likely to destroy the OPA2764 instantly. An open circuit at the Penelope board itself and some higher VSWRs are safe, but it depends on the angle of the reflection coefficient, e.g. an open quarter wave coax line can transform an open or high VSWR into a fatal short.

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