Application Notes—Analog Optical Isolators
from going open loop when there is no input signal, in which case the
cell “off” resistance is much higher than 10 M
Limiters
Ω.
If the magnitude of an AC signal varies over a wide range, it may be
necessary to amplify or compress the signal before any audio
processing can be performed. In other cases, the audio power has to
be limited to prevent damage to an output device. Circuits that perform
this function on a continual basis need a non-linear element to produce
variable gain. However, most non-linear elements introduce distortion.
This is unacceptable in a high fidelity audio circuit and other critical
applications. Using an AOI, simple circuits can be made to perform this
function without introducing distortion or generating any noise.
Amplifier A operates as a high input impedance rectifier that drives
2
the LED. The forward drop of the LED is 1.6 – 2.0V, and when the peak
output of the rectifier exceeds this value, current will flow into the LED.
As the signal increases, more current flows into the LED, driving the
photocell resistance lower thus decreasing the amplifier gain. The
output of A is regulated at a voltage determined by the forward drop of
1
the LED and the closed loop gain of amplifier A . A amplifies the
2
2
signal by a factor of two, and a 1.8V peak (1.27 VRMS) is required to
activate this AOI. This results in the output voltage being held to 0.64
VRMS over a input range of 1 – 600 mV. Changing the value of R
4
Signal Limiters
changes the gain of the rectifier. Omitting R will double the output
4
voltage because the rectifier gain drops to one. Putting a resistor in
series with the LED will cause the regulated voltage to rise as the input
is increased (see Figure 4b). As the amplifier gain changes, the
amplifier bandwidth also changes. When the signal is low, the amplifier
will have the highest gain and lowest bandwidth.
Any circuit that performs as a limiter or compressor must have low gain
when the signal magnitude is high and high gain when the signal is
low. The gain is adjusted so that a wide dynamic range is compressed
into a small one. In other signal processing applications, the signal
may need to be virtually constant.
The circuit such as shown in Figure 4a will keep the output level
constant when the input voltage varies over a range of 50 – 60 db.
Amplifier A1 operates as an inverting amplifier with a gain:
e
/ e = R
/ R
out in
PHOTOCELL 1
The feedback resistor is a photocell and has an “off” resistance of 10
megohms, minimum, and an “on” resistance of 5000 ohms with 5.0 mA
in the LED. Using the components shown, the gain of this stage varies
between 500 with no signal and 0.5 with maximum signal applied. R
2
limits the maximum gain and is needed to prevent the amplifier, A
1
Figure 4a. Peak Sensing Compressor
Figure 4b. Output Characteristics
62
PERKINELMER [ PERKINELMER OPTOELECTRONICS ]
