Miscellaneous active electrical nonlinear devices – circuits – and – Specific input to output function – Logarithmic
Patent
1997-05-06
1998-09-22
Tran, Toan
Miscellaneous active electrical nonlinear devices, circuits, and
Specific input to output function
Logarithmic
327561, H03G 1108
Patent
active
058120084
DESCRIPTION:
BRIEF SUMMARY
This invention relates to improvements to a logarithmic converter which may be used within a closed loop power control circuit.
Transmitters for GSM (Global System for Mobile Communications) and DCS (Digital Cellular System) communication systems require amplification of input power over a closed loop power control circuit to produce output power over a dynamic range of at least 30 dB. To ensure that amplification is possible over as wide a range of input frequencies as possible, a power circuit with a linear gain characteristic is required.
Typically amplification of input power is achieved by the use of a variable attenuation element in the power control circuit. Such devices have a gain characteristic that is approximately dB linear, i.e. an input voltage into the device undergoes exponential gain to produce an output voltage. Therefore use of a compensating element is required to ensure that a linear gain characteristic is achieved in the power control circuit.
It is well known that a non-linear product of an operational amplifier can be linearised by placing a similar non-linearity into the feedback path of the operational amplifier. Linearisation of the power gain produced by voltage controlled attenuators has been achieved similarly by using an element with a similar non-linear gain characteristic placed into a feedback path to the voltage controlled attenuator. A suitable non-linear element is a logarithmic converter which typically uses the exponential current versus voltage characteristic of a p-n junction, such as may be provided by a diode.
The general aim of the invention is to provide a further logarithmic converter for use in a closed loop power control circuit.
According to the invention a logarithmic converter for use in a closed loop power control circuit, comprises amplifying means for amplifying an input signal, the amplifying means including at least non-inverting and inverting inputs and an output, and two feedback means, separably connected between the output and an input of the amplifying means, wherein the first feedback means produces a feedback signal which results in the amplifying means exhibiting a non-linear gain characteristic and the second feedback means contains at least one reactive element operable above a given input level, so that in use the second feedback means acts to limit the feedback signal from the first feedback means above certain signal levels.
This allows the gain characteristic produced by the first feedback means to be available over a larger range of input signal values, particularly when an alternating input signal is supplied.
The logarithmic converter may produce either a non-inverting gain or an inverting gain. Connection of the first feedback means determines the gain characteristic, with connection of the first feedback means between the output of the amplifying means to the non-inverting input resulting in a non-inverting gain. It will be understood that an inverting gain characteristic will be achieved by connection of the first feedback means between the output of the amplifying means and the inverting input.
The first feedback means preferably comprises reactive components including further amplification means. The further amplification means may be provided by an emitter coupled pair of transistors. The transistors may be n-p-n or p-n-p doping combinations.
In a further embodiment of the invention, the further amplification means may be provided by two Darlington transistor pairs, wherein each Darlington pair is coupled to the other by the unconnected emitters of the Darlington pairs, so as to produce a logarithmic converter where movement of a temperature independent point of the gain characteristic of the logarithmic converter is permitted.
The reactive element of the second feedback means is preferably a zener diode. This acts to contain large increases in conversion gain at higher output voltages.
The second feedback means may further be provided with at least one resistive element and at least one capacitive element to allow suppression o
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patent: 4709215 (1987-11-01), McClanahan et al.
patent: 5004906 (1991-04-01), Takasaki et al.
patent: 5012140 (1991-04-01), Bateman
patent: 5014013 (1991-05-01), Kotzian
patent: 5528191 (1996-06-01), Yasuda
Patent Abstracts of Japan, vol. 16, No. 420(E-1259), JP4144305, Fujitsu Ltd .
Nokia Telecommunications Oy
Tran Toan
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