Application Notes—Analog Optical Isolators
Figure 5. Peak Sensing Compressor with Constant Bandwidth
Variable bandwidth can be avoided if the AOI is used in a voltage
divider circuit at the input of a fixed gain amplifier. For the same range
of input signals, the amplifier gain must be 500 and the voltage divider
must have a range of 1000:1. This configuration is shown in Figure 5.
The AOI has been changed to a lower resistance unit to be able to
the voltage limit is determined by the allowable diaphragm excursion.
For constant voltage on the speaker, the displacement doubles when
the frequency is reduced by half. The maximum displacement is
determined by the mechanical design of the speaker and exceeding
the limit will produce extreme distortion and may even cause
mechanical damage.
work over the wider range. Also, A is now a high input impedance,
1
non-inverting stage to avoid a high insertion loss. This circuit is useful
when the input voltage is high, which allows the use of a lower gain
amplifier.
Speaker Power Limiting
Speakers that are driven from high power amplifiers must be protected
from excess drive. While ordinary program levels may be well within
the rating of the speaker, peaks do occur that can be destructive. The
simplest solution is to use a compressor or limiter. Unfortunately, the
maximum power that may be applied is not constant over the
frequency range. Therefore, the limit must be set to provide protection
at the lowest frequency that is expected.
Figure 7. Maximum sine wave Voltage and Power for a Typical Woofer
To understand the requirements for effective speaker protection, a brief
review of speaker power limitations follows. Figure 7 is a typical
maximum sine wave voltage limit for a low frequency speaker
commonly called a “woofer”. Above 200 Hz, the maximum allowed
voltage or power is constant. The operating temperature at which wire
insulation and coil bonding fail establishes this value. Below 200 Hz,
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