Electronic Design

Synthetic Inductor For Filters And Oscillators Is IR Controlled

Almost all semiconductor products are sensitive to light, but this feature has been especially optimized for photodetectors (photodiodes and phototransistors). These are employed to sense or measure radiant energy in all industrial, consumer, and scientific-instrumentation applications. With advances in fabrication technology, we now have photodiodes for wavelengths from far infrared through the visible spectrum and up to ultraviolet.

The design in Figure 1 uses a BPW41 photodiode in its photoconductive mode to synthesize an active inductor whose magnitude can be linearly controlled by the incident light level. This inductor can be conveniently and most effectively substituted in ladder-type passive filters to obtain light-dependent, low-sensitivity analog filters for optical instrumentation. Also, it may be used to control the frequency of optical oscillators using IR light. Furthermore, implementing an Elantec EL2082 current-mode multiplier lets light-dependent inductor-based designs be voltage-controlled by appropriate analog/digital control circuitry.

Op amp U1 (1/4 LM324) acts as a current scalar for the diode current IS. A resistive T network emulates a large feedback resistor via nominal resistor values. C1 is a small capacitance added to improve the transient response. Because the output current from R4 goes into the low-impedance, virtual-ground node, IX of the EL4083, a bootstrapping arrangement can be avoided. This also virtually shorts out the already low parasitic capacitance of the photodiode that would otherwise cause bandwidth and false-oscillation problems.

The EL2082 is configured to work as a voltage-controlled transinductive block. This IC's output current, and the scaled photodiode current, is fed into an EL4083 four-quadrant, current mode-multiplier. Its output current is connected to the input, forcing it to equal the total input current. Hence, the inductance seen from pin 3 of U2 will have an impedance value of:

LEQ = IREFR4R3 / IS(R1R3 + R2R3 + R1R2),where VC1 is set to 1 V, and IS is the light-sensitive diode current. The light-controlled feature is thus evident. De-pending on the source luminescence, the gain factor may be appropriately set through potentiometers R3 and R4. Note that the BPW41 has a built-in optical filter that gives appreciable current for the IR spectrum. Implementing a different photodiode will make the circuit suitable for another spectrum.

To avoid using any inductors (and the EMI associated with them), the inductance can be replaced by a transistor-RC network at pin 2 of U2. This technique is adapted from an earlier Idea for Design, "Reactance Circuit Stabilizes Photosensor, Eliminates Ambient Light Interference," (electronic design, Lorenz Ziegler, Sept. 20, 1999, p. 106), which is reproduced in Figure 2.

Alternatively, employing an additional EL2082 (U4) also eliminates the need for any inductors (Fig. 3). The impedance is now given as:

IS(R1R3 + R2R3 + R1R2)

VC1 and VC2 are tied to 1 V, and the bias voltage is ±5 V. Both circuits were tested using a standard 100-W light bulb.

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