In Part 1, we looked at Class A, B, AB, C, and D amplifiers. These designations are standardized, fully defined, and widely recognized. Now we’ll look at a few other topologies which are less well known but also used.

The Class G amplifier is similar to the Class AB amplifier except that is uses two or more supply voltages, one at lower voltage and the other at higher voltage, Figure 1. It is used exclusively for audio applications. When the power output demand is at a lower level (as set by the user or the application), the amplifier automatically uses only the lower-voltage rail. However, when the desired output signal power increases, the Class G amplifier automatically switches to the higher supply voltage. As a result, this amplifier is more efficient than Class AB amps because it uses the maximum supply voltage only when it is needed and beneficial, while Class AB amplifiers always use the maximum supply voltage.

Note that there is a potential issue with implementing a Class G amplifier, in that usually only a relatively low voltage is available in portable or battery-powered applications. To overcome this, the Class G amplifier often comes with a charge-pump voltage converter to generate a second, higher-value rail voltage. Of course, the charge pump is not 100% efficient so it is important to make sure that the potential efficiency gain from using that second, higher-voltage rail is not negated by the losses in the charge-pump voltage booster.

The Class H amplifier, also suitable only for audio applications, is similar to the Class G design. It adjusts its supply voltage to minimize the voltage drop across the output stage and thus minimize associated losses. Depending on the specific approach, the Class H amplifier may use multiple discrete voltages or even a continuously adjustable supply. This topology does not require multiple power supplies since it uses (and adjusts) the existing power rail via a feedback loop between output power level and supply-rail value.

The Doherty Amplifier does not have a letter designation, but instead uses the name of its developer. It was first implemented in the 1930s and is applicable only for RF amplification. As with other amplifier topologies, it is intended to minimize inefficiency and dissipation. It is especially useful when there is a large range between average signal power and peak signal power.

The Doherty amplifier uses two amplifiers, with one as a “carrier” amplifier biased as Class AB, and the other as a “peaking” amplifier biased for Class C operation to provide a high-efficiency power boost when needed for higher-power input excursions, Figure 2. Although this topology was in relatively little use for many decades, partially due to its complexity as well as its non-intuitive nature and RF-only aspects, it is now receiving a great deal of attention due to its efficiency when amplifying the complex waveforms of various smartphone modulation schemes.

In operation, the input signal is evenly split using a 3-dB quadrature coupler. The two outputs from the coupler are 90⁰ out of phase, they are brought back into phase and reactively combined using the quarter-wave transmission line of the peaking amplifier. The two signals in parallel create an impedance of Z₀/2 Ω which is stepped up to Z₀ Ω by the quarter-wave transformer. Clearly, this is a complex operation with RF lines, couplers, impedance transformers and more, but it yields high efficiency for signals with high peak/average ratios.

Other less-common and less standardized amplifier designations include the Class E design which uses LC tank circuits for filtering (like class C amplifiers), but where active device becomes a switch. There are also Classes F, S, and T amplifiers but these are highly specialized and primarily offer small variations on the better-known classes.