Amplifiers – Having light-controlled or activated device
Patent
1996-11-06
1998-07-28
Mullins, James B.
Amplifiers
Having light-controlled or activated device
250214AG, 330282, 330308, 359194, H03G 330, H03F 1700
Patent
active
057867305
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to an amplifier circuit, and more specifically to a circuit of variable gain for amplifying a signal of variable magnitude so as to provide an output signal of substantially uniform magnitude.
BACKGROND OF THE INVENTION
A photodiode is a convenient means for measuring the intensity of electromagnetic radiation in the visible or infra-red regions of the electromagnetic spectrum. Photodiodes have low internal noise which allows measurement of very low light levels, and intrinsically fast response time which allows measurement of very short light pulses. However, the output signal produced by a photodiode exposed to low light levels may be very small and, for this reason it is customary to amplify the photodiode output signals before they are processed further. The signals output from a photodiode have a current proportional over a wide range to the intensity of light falling thereon but most signal processing is applied to signals of variable voltage. It is desirable to be able to convert the current signals output from the photodiode into corresponding voltage signals and therefore a current-to-voltage converter or so-called transimpedance amplifier is commonly used for this purpose. These amplifiers have many desirable properties, including low noise and wide bandwidth, and generally comprise a high gain inverting amplifier, commonly an integrated circuit operational amplifier connected in a virtual earth configuration, with a feedback resistance connected from output to input.
The designing of an optimum current-to-voltage converter, when the maximum output current produced by a photodiode is known, is a matter of routine. Photodiodes are able to operate over such a large range of light levels that no single design of amplifier can be optimum over the entire range. In order to achieve the lowest amplifier circuit noise the feedback resistance should be as large as possible. However, high feedback resistance increases the circuit gain, and at high light levels the amplifier circuit will be unable to handle the maximum signal produced by the photodiode. If the gain is low enough to accommodate the maximum signal that can be produced by the photodiode, the noise from the amplifier circuit will exceed the signal from the photodiode at low light levels.
It is therefore convenient and indeed desirable in many cases to provide an amplifier circuit of variable gain. It is also often desirable that the gain be electronically controllable, for example by making the feedback resistance of the amplifier circuit an electronically controllable element. Various methods of implementing such electronic control are well known.
As is well known to those possessed of the appropriate skills the relationship between a signal input to a linear circuit and a signal output from the circuit can be described by the so-called transfer function of the circuit which may be expressed as the ratio of two complex polynomials in complex frequency S. The dynamic behaviour of the circuit can be determined by examining the closed loop transfer function for mathematical poles and zeros, that is to say values of complex frequency S at which the denominator of the transfer function is zero (i.e. a pole) and at which the numerator is zero (i.e. a zero). Poles in a transfer function are caused by reactive elements (eg. capacitors or inductors) in the circuit and the value of a pole indicates whether the circuit will become unstable and oscillate.
As the feedback resistance of the transimpedance amplifier circuit is varied, its bandwidth also changes. At high gains (large feedback resistance), stray capacitance across the feedback resistance together with capacitance in the photodiode causes a response pole which increasingly limits the maximum high frequency bandwidth of the photodiode and amplifier circuit in combination. At low gains (small feedback resistance) the combined effect of the phase shift of the inverting amplifier and the phase shift caused by the photodiode capacitance and feedback r
REFERENCES:
patent: 4314152 (1982-02-01), Fenk
patent: 4459475 (1984-07-01), Flint et al.
patent: 4498001 (1985-02-01), Smoot
patent: 4540952 (1985-09-01), Williams
patent: 4868902 (1989-09-01), Sato
patent: 4870369 (1989-09-01), Bartenstein et al.
patent: 4939475 (1990-07-01), Prasse
patent: 5012202 (1991-04-01), Taylor
An Articed entitled "A 50-Mbit/s CMOS Monolithic Optical Receiver", by David M. Pietruszynski et al., IEEE Journal of Solid-State Circuits 23 (1988), No. 6 New York, NY, pp. 1426-1433.
An Article Entitled "Circuit Options Boost Photodiode Bandwidth", by Jerald Graeme, Electrical Design News, 37 (1992), May 21, No. 11, Newton, MA, US, pp. 155-162.
Japanese Patent Abstract 61-41213 (Sumitomo Electric Ind. Lts), dated Feb. 27, 1986.
Ardis Robert B.
Mullins James B.
Ostroff Irwin
Stewart Hughes Limited
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