VT33CT datasheet(68/76 Pages) PERKINELMER

68 / 76 page

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

work over the wider range. Also, A1 is now a high input impedance,

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.

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,

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.

Figure 7. Maximum sine wave Voltage and Power for a Typical Woofer

VT33CT Datasheet(PDF) 68 Page - PerkinElmer Optoelectronics