VT800 datasheet(69/76 Pages) PERKINELMER

69 / 76 page

Application Notes—Analog Optical Isolators

This reduced low frequency power rating can be accommodated by

using a limited circuit which reduces the limit threshold when the

frequency is below 200 Hz. Figure 8a shows a very simple circuit to do

this. At low frequency, the gain of amplifier A1 is unity. The amplifier

has a 6 db/octave gain roll-off starting at 25 Hz and levels off at 100

Hz. Therefore it will take a signal that is four times as large at 100 Hz

as at 25 Hz before limiting action starts. Breakpoints in the Frequency

vs. Gain curve shown in Figure 8b can be set to match the speaker

frequency dependent power limit. Also, potentiometer R4 can be set to

match the power rating and impedance of the speaker.

The threshold is set by the sum of VBE of Q1 and the forward voltage

drops of D1 and the LED, approximately 2.8V peak or 2.0 VRMS. Once

the threshold has been exceeded, current is injected into the LED of

the AOI which attenuates the signal voltage. This voltage divider can

be placed anywhere in the signal path. Once the limiter comes into

play, the system frequency response will no longer be flat, but no

distortion is introduced.

Automatic Gain Control

Automatic gain control (AGC) circuits have electrically programmable

references or set points, but in other respects are the same as limiters

or compressor circuits. Each has a forward gain amplifier and a loop

which controls the gain of that amplifier.

Figure 8a. Speaker Power Limiter with Frequency Compensation

Figure 8b. Amplitude vs. Frequency for the Amplifier

Figure 8c. System Voltage Limits

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