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

Segmented amplifiers are analog amplifiers that switch on different transistors over the input-waveform voltage range, thereby limiting the voltages across them and hence their power dissipation. This is a preferred way to increase analog amplifier efficiency.

The first circuit concept in the development of segmented amplifiers , called a Class G amplifier, is shown below.

For the positive half of the circuit, whenever vI Vg1 , the diode conducts and QU+ is cut off. Then the maximum voltage that is applied to QL+ is Vg1 . For vI > Vg1 , QU+ conducts, cuts off the diode and output current flows from the +Vg supply through both QU+ and QL+ . (The base of QU+ must be biased higher than QL+ for both to conduct, and QL+ saturates.) The same kind of behavior occurs in the circuit of opposite polarity.

The above circuit is a two-segment, series-segmented amplifier. The more general form of series segmentation is shown below for the positive side. Characteristics of series segmentation are:

  • QN must sustain Vg
  • QN-1 , QN-2 , …, Q1 all conduct the current of QN , in saturation.
  • VBE(N) = VBC (N – 1)
  • Simpler circuit than parallel-segmented scheme

The parallel-segmented (||-segmented) scheme is shown below.

Instead of conducting in series, the transistors are in parallel and only one conducts for each voltage segment. For the highest voltages of the input waveform, only QN conducts. The comparator output is high (open collector) and vI is applied directly through a diode to the base of QN . The emitter of QN-1 is pulled up by the output – by the emitter of QN – and is cut off. The diode in its collector prevents current from the input from flowing through the QN-1 b-c junction in reverse to the Vg(N – 1) supply. When vI < Vg(N – 1) , then the comparator output becomes low, cutting off QN , and QN-1 conducts. The behavior is similar for the other N – 2 parallel stages. Each has a comparator that controls whether it is off or not.

Characteristics of the ||-segmented circuit are:

  • Only one Qn conducts per voltage segment
  • QN must sustain only Vg Vg(N – 1)
  • Input hysteretic comparators add circuit components
  • Qn have same V , I ratings – same part type, same max power dissipation

Although the ||-segmented scheme has more components, these can be reduced significantly for N > 2 by noting that the control behavior is the same as LED bar-graph encoders in dot mode, ICs which can be used to drive, via translator transistors, the segment Qs of the parallel scheme. The LM3914 is such a driver. The circuit concept plan for such a scheme is shown below.

An alternative driver implementation is shown below, where the comparator on the left is an output of the bar-graph encoder and on the left is the op-amp that drives the bases of the translator BJTs. The loop is closed through the active output driver Qn .

Then by the noninverting op-amp formula,

For a four-segment scheme, a commodity quad comparator, the LM339, can be used instead of the LM3914, as shown below.

What has yet to be derived is a formula for the segment voltages, for resistive and constant-current loads. This is addressed in Part 3.

3 comments on “High-Efficiency Analog Amplifiers, Part 2: Series and Parallel Segmented Amplifiers

  1. Brad.Peeters
    January 8, 2018

    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


  2. D Feucht
    January 8, 2018

    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.

  3. Steve Taranovich
    January 8, 2018

    I have fixed the error under the text—thanks Dennis

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