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
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.
this. At low frequency, the gain of amplifier A is unity. The amplifier
1
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
Automatic Gain Control
frequency dependent power limit. Also, potentiometer R can be set to
match the power rating and impedance of the speaker.
4
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.
The threshold is set by the sum of V of Q and the forward voltage
BE
1
drops of D and the LED, approximately 2.8V peak or 2.0 VRMS. Once
1
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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PERKINELMER [ PERKINELMER OPTOELECTRONICS ]
