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-1, QN-2, ..., Q1 all conduct the current of QN, in saturation.
- 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.