Feature: Electronic design
the amplifier. For certain major loop gain, the better the intrinsic linearity of the amplifier’s forward path, the better the linearity of the closed loop amplifier. However, maximising forward-path linearity can only be
Reappraising the differential complementary folded cascode as the sole voltage gain stage in linear audio frequency power amplifiers
By Michael Kiwanuka,
B.Sc. (Hons) Electronic Engineering
W
hen designing a linear audio frequency power amplifier, the engineer should consider the following: 1. Maximise the forward-path gain of the amplifier;
2. Maximise the forward-path linearity of the amplifier;
3. Minimise the number of forward-path poles. Te higher the amplifier’s forward-path gain and the fewer its
forward-path poles – ideally giving no more than a single pole roll-off at unity major loop gain frequency – the greater the major loop negative feedback that can in principle be applied around
42 December 2021/January 2022
www.electronicsworld.co.uk
achieved at the expense of the amplifier’s forward-path gain. Tis is because improving forward-path linearity mandates the use of minor (local) negative feedback in each of the gain stages in the amplifier’s forward path, which necessarily reduces its forward- path gain. Minor loop negative feedback not only linearises the gain stage to which it is applied, it also serves to compensate the major negative feedback loop, by simultaneously reducing the gain of each stage and modifying its frequency response. Clearly, a balance must be struck between linearising the amplifier’s forward path by using minor loop degenerative feedback and retaining as much forward-path gain as possible, without having to increase the number of gain stages in the forward path to compensate for the loss of forward-path gain. It’s been shown that, ideally, an amplifier should have no
more than two stages of gain in its forward path to facilitate the compensation of the major feedback loop. Although nested Miller (minor loop negative feedback) compensated operational amplifiers with three or more stages of gain in the forward path exist, for example such an amplifier gives only half the bandwidth of a two-gain-stage forward path with a single Miller minor negative feedback compensation loop, thereby curtailing the available major loop gain – each additional stage of gain halves the forward-path bandwidth. Feed-forward paths can be introduced to bypass one or more nested loops in the forward path of a three- or more-stage amplifier, but this can only be justified in low-voltage, low-power applications.
A two-stage voltage amplifier Figure 1 shows a two-stage voltage amplifier, used to drive the unity voltage gain output stage of a linear audio frequency power amplifier. Tis will be our benchmark for other commonly used single-stage complementary folded cascode voltage amplifiers. An ideal voltage-controlled voltage source (VCVS) buffers the major feedback network from the output of the voltage amplifier; its closed loop linearity can be assessed in LTspice. Te amplifier in Figure 1 has a major loop gain of 31dB at
20kHz, a unity major loop gain frequency of 697kHz and total harmonic distortion of 0.0003%, or three parts per million at 20kHz for 20.6V peak output. Its input is a differential transadmittance stage (TAS), which, in fact, is a voltage-controlled current source (VCCS) with output current proportional to the differential input voltage. Series-derived, series-applied minor loop negative feedback is implemented in the TAS with emitter degeneration resistors R3 and R4. Tis setup trades some of the input stage’s transadmittance (or transconductance at DC) for improved linearity, and allows the amplifier to be compensated by a proportionately-smaller minor negative feedback loop capacitor C2 across the second stage. Te current mirror facilitates differential-to-single-ended conversion, and forces equality of collector currents in the
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