Many applications require detection of both very small and very large signals. High-gain detector amplifiers provide low noise, but are easily swamped by large signals. Logarithmic amplifiers provide a wide range, but contribute to distortion. Auto-ranging circuits lose data when switching between low and high gain. Detecting over a wide range is especially challenging for modern, low-supply-voltage integrated circuits.
Transimpedance amplifiers can be used to convert an input current signal into an output voltage signal. This is useful for processing current signals generated by photodetectors from the detection of light. To detect a low-level light signal, a relatively large gain is needed for the transimpedance amplifier; for the detection of a high-level light signal, this relatively large gain can lead to a saturation of the output voltage signal from the transimpedance amplifier. Thus, there is a need to extend the dynamic range of transimpedance amplifiers so that both low-level light signals and high-level light signals can be detected since, in many cases, the exact signal level of the light is unknown prior to detection.
A hybrid detector amplifier was developed that combines a high gain and logarithmic amplifier in one. The hybrid amplifier compensates distortion inside the circuit, so the outputs are suitable for further amplification or datalogging. With two outputs to choose from, a computer, oscilloscope, or datalogger may record signals from the low-noise output as signal rises. Then it can instantly switch to recording the wide-range output, if necessary. This extends the recording device's useful resolution and range, while preserving signal fidelity.
The transimpedance amplifier system simultaneously generates a low-gain output signal and a high-gain output signal from an input current signal using a single transimpedance amplifier having two different feedback loops with different amplification factors to generate two different output voltage signals. One of the feedback loops includes a resistor, and the other includes another resistor in series with one or more diodes.
The amplifier system includes a signal linearizer to linearize one or both of the low- and high-gain output signals by scaling and adding the two output voltage signals from the transimpedance amplifier. The signal linearizer can be formed either as an analog device using one or two summing amplifiers, or as a digital device using two analog-to-digital converters and a digital signal processor (e.g. a microprocessor or a computer).
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