VT33CT datasheet(67/76 Pages) PERKINELMER

67 / 76 page

Application Notes—Analog Optical Isolators

Limiters

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.

Signal Limiters

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:

eout / ein = RPHOTOCELL / R1

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. R2

limits the maximum gain and is needed to prevent the amplifier, A1

from going open loop when there is no input signal, in which case the

cell “off” resistance is much higher than 10 M

Ω.

Amplifier A2 operates as a high input impedance rectifier that drives

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 A1 is regulated at a voltage determined by the forward drop of

the LED and the closed loop gain of amplifier A2. A2 amplifies the

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 R4

changes the gain of the rectifier. Omitting R4 will double the output

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.

Figure 4a. Peak Sensing Compressor

Figure 4b. Output Characteristics

VT33CT Datasheet(PDF) 67 Page - PerkinElmer Optoelectronics

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