Limiters
May, 2022
Arthur Harrison, editor

Note: Neither author nor copyright declaration is indicated in the primary source document.

The limiter is a specialized form of audio compressor used in applications such as  commercial radio broadcasting, sound recording, and, in some forms, electric guitar "distortion" and "fuzz" effects that are used to emulate the sonic characteristics of vacuum-tube amplifiers. This article considers the design principles for diode-based limiters and their wide range of characteristics.

Limiter designs can vary in complexity from a pair of diodes to multi-stage voltage controlled amplifiers (VCAs) with split frequency bands. Diode-based limiters are simpler in design, instantaneous in response, and have a more natural-sounding loudness response than VCA topologies. However, diode-based limiters also introduce a greater degree of distortion in the limiting region than VCA types. On the other hand, VCA-type limiters may exhibit "breathing" or "pumping" effects.

A diode has extremely high resistance until the voltage across it is large enough to forward bias it, at which point, current flows. A diode's voltage drop remains fairly constant while forward-biased and that property is exploited in diode-based limiter circuits. Since audio signals are AC, two diodes together can symmetrically limit both the positive and negative halves of a waveform.

Two general categories of limiters may be described as those with "soft" and "hard" responses, although there is a continuous spectrum of response between those two extremes. The output voltage of a hard limiter, shown below in Figure 1, has maximum peak and valley values that are equal to the forward bias voltages of the diodes. Since (for silicon diodes) that value is about 0.7 volts, the design assumes that the voltage values at the slider of the left potentiometer will exceed +/-0.7 for limiting to occur. In practical designs, the voltage at the potentiometer slider would be in the region of +/-1 volt with respect to ground, with the voltage difference between the slider and the diodes dropping across resistor R1.


Figure 1

This type of limiter in Figure 1 can achieve an output with a more dynamic characteristic with the addition of a second resistor to form a voltage divider with R1, as illustrated below in Figure 2. When configured with two resistors, this limiter becomes a simple type of compressor with a compression factor determined by the ratio of R1 and R2. Figure 2's configuration may be characterized as a "soft" limiter. Increasing R2 will give a more dynamic sound at the expense of level control. The circuit will still limit a waveform above the diode forward bias voltages, but also lets through an unmodified portion of the signal by taking the output at the top of R2. The graph in Figure 3 compares the response of both types of limiters.


Figure 2

Figure 3

The harsh characteristic of audio signals processed by hard limiters is partially due to the odd-order distortion harmonics that are generated during limiting. Germanium diodes (such as the ubiquitous type 1N34A) have a smoother, vacuum tube-like limiting quality because they distort with more even-order harmonics, which are pleasant to the human ear. Germanium diodes have a forward bias voltage of about 0.3 volts and are sometimes used in series-groups for both the positive and negative-conducting paths. Parallel pairs of light emitting diodes (LEDs) may provide stronger even-order harmonics than paralleled-pairs of silicon or germanium diodes. A paralleled-pair of one red and one green LED may produce yet more pleasant harmonics, because their forward-voltage differences will increase even-order distortion.

The limiter in Figure 4 is an "active" design with the LED limiters in parallel with the feedback resistor of an operational amplifier. LED1 provides limiting for negative input excursions, and LED2 for positive input excursions. The negative and positive limiting thresholds are values between 1.4 and 2.6 volts, which is the range of forward bias voltages for a majority of red, green, and yellow LEDs. For this configuration, the response will transition from hard to soft as the value of resistor R2 goes from infinity to a finite value. Since the operational amplifier is configured in an inverting mode, the output will be a voltage inversion of the input. If R1 and R2 are made equal in value, then the response of the circuit will have unity gain, and be linear, for input voltages below the diodes' forward bias thresholds. When that voltage value is exceeded, the gain will diminish in accordance with the resistance presented by the forward-biased diode. An important distinction between this active configuration and the previous configurations is that the output, when limited, will have an amplitude that correlates to the input amplitude, as opposed to being limited exclusively by the diodes' threshold values.

Figure 4

Figure 5, below, shows two limiter configurations based on zener diodes. Circuit A limits when the input exceeds the breakdown voltage plus the forward voltage of a zener. Since both zeners are in inverse-series, the design works for both polarities of the signal. Circuit B requires only one zener diode and limits when the input is greater than the breakdown voltage of the zener combined with the forward voltages of two of the bridge-configured diodes. Zeners have the advantage of being easier to match for symmetric limiting than other types of diodes, and they are available over a wide range of voltages.


Figure 5

The circuits shown so far have fixed limiting levels. One way to vary levels is to apply a bias voltage to the normally-grounded side of the diodes. The circuit in Figure 6A permits manual adjustment of the positive and negative portions of the waveform separately. Circuit 6B puts the audio signal through an inverting amplifier, the output of which connects to the bottom of the zener array. Limiting occurs when the audio signal exceeds (Vf + Vz ) / [1 + (R3 x RP1 / R2 x RP1x)], where Vf is the forward bias voltage of a zener and Vz is the breakdown voltage. P1 adjusts the feedback of the inverting amplifier and thus the limiting level.

Figure 6

 Broadband limiters work across the entire frequency range. There are times when limiting should be frequency dependent. For example, if the audio signal is noisy, then limiting the high frequencies can result in smoother, more intelligible sound. The circuit in Figure 7 is one example of a bandwidth restricted limiter. The low frequency limiting level of the diode array is modulated by the output of the low pass filter, so that only the high frequencies are affected.


Figure 7

Analog tape recorders may be used in a signal processing chain to archive compression. They provide certain desirable sonic characteristics including a subjective "warmth."  Part of these characteristics results from the analog tape media's limiting of high-frequency signal content. The circuit in Figure 8 emulates this process. A high-frequency (above 2.4 kHz) pre-emphasis is applied in the first op amp section, facilitated by resistor R1 and capacitor C1 at its negative input. After the diode limiting stage, the signal is de-emphasized in another network with the same corner frequency, facilitated by the R5 and C2. Although only two diodes are shown, practical implementations may include more diodes, both silicon and germanium, to achieve the desired limiting characteristic. The emphasis and de-emphasis scheme reduces the high frequency distortion components that occur due to limiting.


Figure 8


References

Primary Source Document, https://www.261.gr/limiters.html
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Orban, Robert, "Apparatus and Method for Peak-Limiting Audio Frequency Signals," U.S. Patent No. 4,208,548, June 17, 1980.
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