Miscellaneous active electrical nonlinear devices – circuits – and – Specific input to output function – Logarithmic
Patent
1995-02-02
1996-10-29
Tran, Toan
Miscellaneous active electrical nonlinear devices, circuits, and
Specific input to output function
Logarithmic
327352, H03G 1108, G06F 7556
Patent
active
055700551
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the field of logarithmic amplifiers. More particularly, the present invention relates to a high performance, low power logarithmic amplifier cell for use in a multi-stage logarithmic amplifier providing accurate logarithmic intercept and slope response.
2. Discussion of the Related Art
Until recently, logarithmic amplifiers were practically unavailable which are useful for both low frequency and high frequency applications, and which are inherently free of sensitivity to temperature variations and production tolerances of the devices comprising the active elements so as to provide high accuracy log-law performance over a wide temperature range and in the presence of large production variations.
U.S. Pat. Nos. 4,929,909 and 4,990,803 (hereafter "the '909 patent" and "the '803 patent", respectively), assigned to the same assignee as the present invention and incorporated herein by reference, describe a logarithmic amplifier which does not suffer from the above-noted limitations.
FIG. 1 is a block diagram of the logarithmic amplifier disclosed in the '803 patent. The amplifier includes a plurality of amplifier/limiter/detector stages (hereinafter referred to as "gain" stages or "logarithmic amplifier gain stages") 100-1 through 100-n, a gain bias generator 102, a scale bias generator 104, and an offset generator 106, which are common to all gain stages. All of the gain stages working together produce a logarithmic output signal corresponding to the input signal. FIG. 2 is a block diagram for each of the gain stages 100-1 through 100-n. Each gain stage 100-i includes an emitter follower 112-i, having a differential input and balanced output, a full-wave detector 114-i, and a limiting amplifier 116-i. The emitter follower 112-i introduces a slight gain loss of approximately 0.07 dB. The limiting amplifier 116-i can be designed to provide a gain of 10.07 dB, so that the overall stage gain is 10 dB. P Although the logarithmic amplifier of the '909 and '803 patents performs well, that demodulating logarithmic amplifier requires relatively complicated gain stages having a relatively large number of active components, both positive and negative power supplies and requires approximately 300 milliwatts of power for a ten stage amplifier.
SUMMARY OF THE INVENTION
The present invention provides, in accordance with one aspect thereof, a logarithmic amplifier gain stage for supplying, in response to an instantaneous input signal, an output signal corresponding to a logarithmic value of the input signal. Alternatively, in response to an alternating input signal, the gain stage produces a demodulated output signal corresponding to a logarithmic value of the input signal. The logarithmic amplifier gain stage includes a transistor amplifier having an input that receives the input signal and an intermediate output that supplies an intermediate output signal; and a full-wave detector having an input coupled to the intermediate output of the transistor amplifier that receives the intermediate output signal, and an output that supplies the output signal. The full-wave detector includes a rectifier comprising transistors having different effective emitter areas. The different emitter areas can be formed by physical differences in emitter areas of the rectifier transistors. In another embodiment of the invention, the effect of different emitter areas can be synthesized by using a bias voltage between a base and an emitter of at least one transistor in the rectifier. Other embodiments of the invention include means for extending the usable frequency range by increasing a slew rate of the gain stage without consuming additional power. The logarithmic amplifier gain stage further includes means for reducing a variation in a phase of the output signal with a variation in a level of the input signal.
The logarithmic amplifier gain stage of the present invention is less complex and consumes less power than the prior art, and requires powe
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Patent Abstracts of Japan, vol. 007, No. 115, (E-176), May 19, 1993 & JP-A-58 036 008 (Sharp KK).
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Analog Devices Inc.
Tran Toan
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