Amplifiers are fundamental circuit-design elements. They drive everything from earbuds to antennas. Placed ahead of analog-to-digital converters (ADCs), they reshape signals from sources as diverse as strain-gauges to ultrasound probes. Through proper selection of feedback passives, they can be configured into high-pass, low-pass, band-pass, and band-elimination filters. Feed them with multiple signals, and they produce harmonic series of all the components of those inputs—good for some applications, a headache in others.
You’d think this venerable component had little left in the innovation department, but it continues to evolve nonetheless. What follows is a review of amplifier basics with a twist. It assumes some engineering knowledge of how transistors are configured into amplifiers, so it will focus more on what’s new and what may not be so widely understood in a universe dominated by digital thinking.
AMPLIFIER ABCS These ABCs don’t get into how to bias a transistor, but rather how the transistor is operated. It once was a matter of where the device was operated on its transfer curve (Fig. 1). Now, the amplifier class alphabet extends beyond the old classifications.
In class A, the active device is biased so that it operates in its linear conduction region during the entire input cycle. In class B amplifiers, there are two devices. The input waveform is split— one active device conducts during half of an input cycle, the other during the other half. Class AB amplifiers resemble class Bs, except they bias their active devices so that both conduct during some portion of the input cycle.
In class C, a single device is biased whereby it conducts during only a small portion of an input cycle. Energy is driven into a high-Q LC tank circuit that continues to ring at its resonant frequency during the portions of the cycle when the active device isn’t conducting. An analogy would be continuously tapping a big bell with a small hammer at a rate equal to the resonant frequency of the bell.
In the beginning, classification had something to say about linearity versus efficiency. Class A amplifiers can be made very linear, but their efficiency is limited. In theory, a class A amp can achieve 50% efficiency with inductive output coupling or 25% with capacitive coupling. Class B amplifiers are subject to “crossover” distortion, but their efficiency is theoretically as high as 78.5%.
Class AB sacrifices some of that efficiency for lower distortion. Class C amps can achieve up to 90% efficiency. The resonance effect of the LC-tuned circuit minimizes distortion. Class B amplifiers are sometimes called “push-pull” because the outputs of the active devices have a 180° phase relationship. (For a different kind of push-pull configuration, see fig. 4, as well as “Bridge- Tied Load Amplifiers,” at www.electronicdesign.com, Drill Deeper 18998.)
Beyond class C, the simple relationship between alphabetical designations and the input signal’s application to the active device’s transfer characteristic starts to break down. For example, in class D amps there are two devices, as in a class B amp. But they’re driven between saturation and cutoff by a square wave with a frequency significantly higher than the highestfrequency component of the input waveform.
The pulse width or pulse density of the square wave is variable, and one or the other is controlled by the input signal. At the amplifier output, the switching frequency and its harmonics are attenuated by a low-pass filter, leaving only the amplified input waveform.
With field-effect transistors operating most of the time in either cutoff or saturation, losses are primarily from the transistors’ forward-voltage drops. Class D amps can achieve efficiencies as high as 90%, with distortion levels approaching class AB. Suppressing radiated and conducted interference from the switching circuitry is challenging.
Classes E and F are related to class C, since they’re RF amplifier topologies that use LC tank circuits. Where class C amplifiers are widely used below 100 MHz, class E amps are used at VHF and microwave frequencies.
Class E amps differ from class C amps in that the active device is used as a switch, rather than being operated in the linear portion of its transfer characteristic. Figure 2 is an equivalent circuit for a class E amplifier. The switch S represents the active device. L and C are the series-tuned circuit, and R is the load. CS represents all of the parasitic capacitances across the active device, and the choke isolates the power source.
Class F amplifiers resemble class E amplifiers, but use a more complex load network. In part, this network improves the impedance match between the load and the switch. Also in part, it’s designed to eliminate the even harmonics of the input signal so that the switching signal is more nearly a square wave. This improves efficiency by making the switch run at saturation or cutoff longer.
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