Switching Amplifiers

Stair-Step Modulation

Class SS modulation synthesizes the desired output by sequentially switching full bridge outputs on and off at the signal frequency.

Since the output transistors turn on only once each cycle, this modulation is used when the maximum output frequency is desired. Each bridge contributes one step to each polarity of the output waveform. The outputs of each bridge, which are in series, are held in a low impedance state until called upon to contribute to the sum. The distortion decreases inversely with the number of Output PWBs used. The distortion for a 4 bridge Class SS amplifier is about 10% at full output.

Amplitude, frequency, and phase can be closely and rapidly controlled, but amplitude modulation results in increased distortion, since lowering the amplitude means using fewer steps. Power supply modulation, although slow, may be used to control output amplitude without increasing distortion.

Class SS modulation is well suited for high power applications where distortion is not a problem. As the power rating increases, more Output PWBs are used which reduces the relative size of each step and improves the waveform.

Pulse Width Modulation

PWM requires that the output transistors switch at 4 to 10 times the maximum signal frequency. Amplifiers using this technique and voltage feedback have 60 dB of dynamic range and distortion of less than 0.2%. Current feedback can be used to synthesize a resistive output impedance. This impedance is also known as damping factor and may be made as great as twice the matching load impedance at low frequencies.

A PWM amplifier may be used to reproduce arbitrary waveforms. It comes close to the performance of a linear amplifier with higher efficiency. Generally, the total losses in the output stage are less than 10% of the output VA and the overall efficiency including power supply and transformers is at least 75%.

High power amplifiers, which use multiple Output PWBs, offer improved performance by using a polyphase carrier modulation technique to cancel ripple components. The output includes the amplified input signal plus sidebands around harmonics of the switching frequency. A good general purpose filter will allow full power into a resistive load up to 1/4 of the switching frequency. Capacitive loads will lower the filter frequency and absorb much of the switching harmonic current. Inductive loads will raise the filter frequency and reduce the high frequency output. In some applications, no output filtering is necessary.

Switchmode Noise and Distortion

Instruments, Inc. PWM Amplifiers are full bridge output circuits that achieve a high degree of cancellation of the switching frequency harmonic sidebands. The harmonic sidebands (ripple) consist of odd harmonics of the input signal (f_i) centered around even harmonics of the switching frequency (f_s).

At full output (100% sinewave modulation), single bridge unfiltered ripple includes:

When two bridges are modulated with phase shifted switching frequencies, the 2f_s harmonics cancel. Four bridges, such as our model S11-8, will also cancel the 4f_s harmonics and so on.

A reasonable output filter is a 2 pole, (Q = 1 with resistive load), low pass cornering at f_s/4. This reduces the largest ripple harmonic to -50 dB from a single bridge, -68 dB from a 2 bridge and -86 dB from a 4 bridge amplifier.

As the input frequency is increased above f_s/10, the harmonic sidebands spread out and intrude into the signal bandwidth (aliasing). Around f_s/4, the output filter can resonate individual sideband harmonics and appear as distortion. Operation between f_s/4 and f_s/2 (the "Nyquist" limit) is possible if significant distortion is tolerable (10%).

At low modulation, the ripple sidebands reduce both in width and amplitude. The ripple to signal ratio is fairly constant. Voltage feedback reduces the signal band noise floor to about 90 dB below full output. It originates in the PWM comparators. Larger amplifiers average the output of more comparators which results in a greater dynamic range.

Distortion is mainly due to the dead time necessary to prevent simultaneous conduction of the upper and lower switches in each half bridge. At about 30 dB below full output, the modulation is equal to the deadtime, and "crossover" distortion is at a maximum. Voltage feedback may reduce this to about 0.2% It can appear larger if the distortion harmonics resonate with the output filter. Capacitive loads will lower the output filter resonance.

Theoretical Spectrum of Unfiltered Output, Sinewave Input.

100% Modulation f_i = 20 kHz, f_s = 200 kHz

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