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Dennis Feucht

High-Efficiency Analog Amplifiers, Part 2: Series and Parallel Segmented Amplifiers

Dennis Feucht
steve.taranovich
steve.taranovich
1/8/2018 4:37:57 PM
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Re: Reverse Vbe problem
I have fixed the error under the text---thanks Dennis

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D Feucht
D Feucht
1/8/2018 3:39:43 PM
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Re: Reverse Vbe problem
Brad,
First, there is an error in the text. The line under the first circuit diagram,

"...the diode conducts and QL+ is cut off." should read QU+ instead of QL+.

Second, as to your point, yes, a series base or emitter diode would be required (or some similar fix) to keep from breaking down the b-e junctions.


Actually, I prefer the parallel-segmented scheme, perhaps because I've used it, even though it requires a few more parts. For it, only one BJT conducts per segment and it is thus somewhat more efficient. For an IC implementation, the  extra parts are trivial. The comparator input characteristics can be sloppy; a commodity-grade LM339 would be quite adequate.

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bpeeters
bpeeters
1/8/2018 2:49:07 PM
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Reverse Vbe problem
Doesn't look like there are enough diodes in the Series Segmented Amplifier circuit.  With zero signal, for example, QU+ is going to experience most of +Vg1 across its base-emitter junction reverse biased.  Unless +Vg1 is very low voltage, e.g., less than 8 volts or so, this is likely to exceed the reverse-breakdown spec of the NPN QU+ transistor.  This could probably be fixed with another diode in series with the QU+ emitter and some careful biasing resistors to ensure the emitter can follow the base down below the Vg1+ supply voltage.  Alternatively, might be able to protect the base-emitter junction with a diode in series with the base.

It's a neat concept to reduce wasted power in an analog amplifier, but I suspect the schematic shown is "simplified" for illustrative purposes.  A real-world implementation would have a lot of additional details needed to make it actually work and not blow out the transistors, right?

Best, Brad

 

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