Electronic Design

Tunable Diode Laser Photocurrent Amplifier Has 80-dB Logarithmic Gain

Tunable diode lasers (TDLs) are of increasing importance in many optoelectronic applications, including wavelength division multiplex communications and chemical analytical spectroscopy. TDLs are unexcelled as coherent, high-intensity, wavelength-agile, tunable monochromatic light sources. They provide high throughput and sensitivity in a wide variety of precision optical instrumentation.

For example, two dual-channel TDL spectrometers, designed for water vapor and carbon dioxide abundance and isotopic ratio measurements, were part of the science payload of the (ill-fated) Mars Polar Lander (see "Flash-ROM-Based Multichannel Arbitrary Waveform Generator," W. Stephen Woodward and Randy D. May, Electronic Design, April 19, 1999, p. 86).

Optical measurements incorporating high-performance sources like TDLs need similarly high-performance optoelectronic detectors and signal-processing circuitry. This photocurrent amplifier incorporates a number of design characteristics to accommodate the peculiarities of analytical spectrometer alignment (see the figure). At the same time, these features aim at preserving the signal quality (SNR) in the inherently noisy environment of aerospace applications.

The performance of any chemical analytical spectrometer directly de-pends upon two factors. Such devices rely on both the availability and the use of an intense source that matches the absorption spectrum of the analyte molecule of interest (e.g., CO2 or H2O). Fabricating TDLs with output wavelengths that accurately fit the predetermined spectra of the chosen analyte molecules is a specialized and still-evolving art. Consequently, some laser performance parameters (e.g., power output) must be relaxed in the interest of achieving reasonable device yields.

A useful TDL spectrometer detector-amplifier needs a stable, wide-range gain adjustment to accommodate up to several orders of magnitude of variation in detector-incident light levels. To achieve this functionality, the amplifier shown exploits the exponential transconductance versus VBE behavior of the CA3046 bipolar transistor array:

QA − QE: AQ = 2 × EXP(11,000 × VBB/TKELVIN).

The 1.24-V voltage reference U2 cooperates with QD to sink an approximately −500-µA temperature-compensated bias current from the wiper of R1. An additional +1.5 µA/K must be factored in to accommodate the 0.3%/K TKELVIN dependence of the transistor's exponential gain behavior. Therefore, as R1's wiper is adjusted from CCW to CW, QB's VB (VBB) varies linearly from 0 to −250 mV (at 25°C + 0.3%/K). This causes the overall amplifier current gain (I2/I1) to shoot from 2 to 20,000 (6 to 86 dB).

Positive feedback components QC and R3 improve the amplifier linearity by compensating for the emitter-resistance-related log-nonconformity of QA and QE. This nonlinearity-canceling "tweak" comes from adding a bias term to VBB equal to I2 × 0.8 Ω = 800 µV/mA. That's enough to cancel QA's and QB's paralleled emitter resistances of 1.6 Ω each, which is typical of the transistors in a CA3046 array.

Regrettably, ground-loop related noise is common in avionics applications where digital and analog components are often forced to operate in close proximity. Immunity to ground-loop-induced noise is provided by the current-mode amplifier output signal. The voltage compliance of the I2 output is ±5 volts.

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