Abstract:
A miniature low-noise photodiode amplifier system that minimizes offset and drift has the output of a photodiode connected to the non-inverting input of an operational amplifier. The operational amplifier has a split voltage supply and the output of the operational amplifier is transmitted to a rectifying diode. A second amplifier to increase the output of the operational amplifier is not required when this miniature low-noise photodiode amplifier system is employed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is based on, and claims the benefit of and priority to, U.S. provisional patent application Ser. No. 60/090,753 filed on Jun. 26, 1998. 
    
    
     TECHNICAL FIELD 
     The invention relates to light amplification and, more particularly, to amplification of an output of a photodiode. 
     BACKGROUND INFORMATION 
     Spectral analysis of living tissue can be used to detect various forms of cancer and other types of diseases. In spectral analysis, light illuminates tissue region under examination and a light detector detects optical properties of the illuminated tissue region by measuring light energy modified by its interaction with the tissue region in a pre-determined frequency and amplitude domain. Optical properties include absorption, luminescence, fluorescence, frequency and time-domain responses to various materials injected to the tissue region and other electromagnetic responses. Diseased tissue may be identified by comparing a spectrum obtained to spectra of normal tissue obtained under the same controlled conditions. 
     Current devices available for tissue characterization using spectral analysis include night vision sensing systems with filtering adapted to be used with endoscopes and multichannel fiber optic delivery systems. The latter systems typically include a light source, a first optical conduit, a light applicator and receiver, a second optical conduit, a spectrometer and a display unit. The receiver used to receive the reflective light can be a photodiode. A photodiode amplifier can be used to amplify the output from the photodiode. 
     Photodiode amplifiers are typically used in conjunction with an operational amplifier circuit. An output of a photodiode can be amplified in a photoconductive mode illustrated in FIG.  1 . In the photoconductive mode, an amplifier circuit  100  includes a photodiode  110 , an operational amplifier (Op-Amp)  115 , and a feedback resistor  130 . A cathode  110   a  of the photodiode  110  is coupled to an inverting input  115   a  of the operational amplifier  115  via a resistor  120 . An anode  110   b  of the photodiode  110  is coupled to a voltage source  140 . A non-inverting input  115   b  of the operational amplifier  115  is coupled to ground  125 . An output  115   c  of the operational amplifier  115  is fed back to the inverting input  115   a  through the feedback resistor  130 . A capacitor  135  is connected in parallel with the feedback resistor  130  to filter out unwanted noise. 
     Alternatively, an output of the photodiode  110  can be amplified in a photovoltaic mode as illustrated in FIG.  2 . The only difference between the photoconductive mode and the photovoltaic mode is the manner in which the photodiode is connected to the operational amplifier. In the photovoltaic mode, the cathode  110   a ′ of the photodiode  110 ′ is coupled to the inverting input  115   a ′ of the operational amplifier  115 ′ and the anode  110   b ′ of the photodiode  110 ′ is connected to the non-inverting input  115   b ′ of the operational amplifier  115 ′. 
     In both the photoconductive and the photovoltaic modes, a current output from the photodiode  110 ,  110 ′ is applied to the operational amplifier  115 ,  115 ′. The operational amplifier  115 ,  115 ′ provides a voltage output equal to the value of the resistance of the feedback resistor  130 ,  130 ′ multiplied by the current from the photodiode  110 ,  110 ′ which passes through the feedback resistor  130 ,  130 ′. In an “ideal” situation, the gain of this system is directly related to the feedback resistor  130 ,  130 ′ based on the following equation: 
     
       
         V OUT =I L R f   
       
     
     Such an ideal situation is fictional and is rarely realized in practice as other components must be added to compensate for offset currents and voltage drift. These components tend to decrease the gain of the system. Since the gain is decreased and the output is negative, a second amplifier needs to be coupled to the first amplifier in order to provide a positive output voltage with moderate gain. Compensation still takes place with the second amplifier, and this system will still tend to drift slightly. The outcome is a larger system that is not very sensitive and which usually requires re-calibration steps, alignment or other compensatory actions, and is more costly because of the number of components, and the complexity of manufacture. 
     SUMMARY OF THE INVENTION 
     In one aspect, the invention features an amplifier circuit. The amplifier circuit includes an operational amplifier, a light-sensitive device, a feedback module, and a rectifier. The operational amplifier includes an inverting input, a non-inverting input, and an output. The light-sensitive device is in electrical communication with the non-inverting input of the operational amplifier. The feedback module is in electrical communication with the output of the operational amplifier and the inverting input of the operational amplifier. The rectifier is in electrical communication with the output of the operational amplifier. 
     In one embodiment according to this aspect of the invention, the rectifier is a rectifying diode. In another embodiment, the light-sensitive device is a photodiode. In another embodiment, the feedback module includes a resistor. In one detailed embodiment, the feedback module further includes a capacitor. In another embodiment, the amplifier circuit further includes a resistance in electrical communication with the inverting input of the operational amplifier. In still another embodiment, the amplifier circuit further includes a resistance in electrical communication with the rectifier. 
     In another aspect, the invention involves a spectrometer comprising an amplification circuit, a light source for illuminating tissue, and a light-sensitive device. The amplifier circuit includes an operational amplifier, a feedback module, and a rectifier. The operational amplifier includes an inverting input, a non-inverting input, and an output. The feedback module is in electrical communication with the output of the operational amplifier and the inverting input of the operational amplifier. The rectifier is in electrical communication with the output of the operational amplifier. The light-sensitive device is in electrical communication with the non-inverting input of the operational amplifier. The light-sensitive device detects optical properties of the illuminated tissue. 
     In one embodiment of this aspect of the invention, the light source is internal to the spectrometer. In another embodiment, the light source is external to the spectrometer. In one embodiment, the amplifier circuit comprises a plurality of operational amplifiers. In another embodiment, the spectrometer further comprises a light filter disposed between the illuminated tissue and the light-sensitive device. In one detailed embodiment, the spectrometer further comprises a second light-sensitive device and a second light filter disposed between the illuminated tissue and the light-sensitive device. In another detailed embodiment, the first light filter passes through a first range of wavelengths of light and the second light filter passes through a second wavelengths of light. 
     In still another aspect, the invention relates to a method for amplifying an output of a photodiode. According to the method, an amplifier circuit comprising a light-sensitive device, a rectifier, and an operational amplifier with an inverting input, a non-inverting input, and a output is provided. A first voltage is applied to the inverting input of the operational amplifier. An optical signal is detected through the light-sensitive device which converts the optical signal to a second voltage. The second voltage is applied to a non-inverting input of the operational amplifier, wherein the first voltage and second voltage have opposite polarity. The output generated by the operational amplifier is transmitted to the rectifier, and an output with the same polarity as the second voltage is transmitted through the rectifier. 
     In one embodiment according to this aspect of the invention, the method further includes the step of stabilizing the output transmitted through the rectifier. In another embodiment, the first voltage from the output of the operational amplifier is applied to the inverting input of the operational amplifier. In still another embodiment, the method further comprises filtering noise in connection with the optical signal. 
     The foregoing and other objects, aspects, features, and advantages of the invention will become more apparent from the following description and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. 
     FIG. 1 shows an amplifier circuit. 
     FIG. 2 shows another amplifier circuit. 
     FIG. 3 shows an amplifier circuit in accordance with the invention. 
     FIG. 4 shows a curve of photodiode amplifier output vs. input 
     FIG. 5 is a block diagram of a spectrometer according to the invention. 
     FIG. 6 is a schematic diagram of the spectrometer. 
     FIG. 7 shows a spectroscopy probe according to the invention. 
     FIG. 8 is a block diagram of a system for spectral analysis including a spectrometer module in communication with external components. 
    
    
     DESCRIPTION 
     Referring to FIG. 3, an amplifier circuit  10  includes a photodiode  15 , an operational amplifier  3 , a feedback module  5 , a first resistor  11 , a rectifier  21 , and a second resistor  23 . The photodiode  15  has its anode  15   a  coupled to a non-inverting input  16  of the operational amplifier  3  and its cathode  15   b  connected to ground  13 . The inverting input  9  of the operational amplifier  3  is connected to ground  13  through the resistor  11 . The output  7  of the operational amplifier  3  is coupled to the feedback module  5  and the rectifier  21 . The feedback module  5  is coupled to the inverting input  9  of the operational amplifier  3 . In one embodiment, the feedback module  5  includes a resistor. In another embodiment, the feedback module  5  includes a resistor and a capacitor. When light is not applied to the photodiode  15 , the offset currents will force the output  7  of the operational amplifier  3  to reach the maximum negative supply voltage. The operational amplifier  3  will remain at the maximum negative supply voltage until light is applied to the photodiode  15 . When light is applied to the photodiode  15 , a positive voltage is applied to the non-inverting input  16  of the operational amplifier  3 . By splitting the voltage to the operational amplifier  3  by applying the operational amplifier output  7  to the inverting input  9  of the operational amplifier  3  and by applying voltage from the photodiode  15  to the non-inverting input  16  of the operational amplifier, a split voltage is applied to the operational amplifier  3 . 
     The rectifier  21  allows current to flow in only one direction. In one embodiment, the rectifier  21  is a rectifying diode. When no light is applied to the photodiode  15 , the rectifier  21  becomes reverse biased, and no current can flow to the output  25 . A voltage reading at this point would now be zero. When light is applied to the photodiode  15 , the operational amplifier output  7  swings positive, forward biases the rectifier diode  21 , and allows current to flow to the output  25 . The voltage present at the output  25  is dependent on the amount of light reaching the photodiode  15 . For added stability, a load resistor  23  can coupled to the rectifier  21  in one direction and ground  13  in the other direction. 
     FIG. 4 shows a graph of an output voltage of the amplifier circuit of FIG. 3 as a function of the input voltage applied to the operational amplifier of the circuit. FIG. 4 illustrates that the output voltage is always positive. The output voltage remains low in the threshold region (I) where the input voltage is between about 0.0001 volts and 0.0158 volts. Once the input voltages are in the range between 0.0158 and 0.177 volts the output is now in the operable region (II). Once the input voltage is above 0.177 volts (III), the output is in the saturation region (III). The sensitive voltage range is determined by the values of the first resistor  11  and the resistor in the feedback module  5 . At 0.177 input volts this circuit reaches its saturation level. 
     Referring now to FIG. 5, an amplifier circuit  50  includes a first photodiode  15 , a second photodiode  15 A, a first operational amplifier  27 , a second operational amplifier  27 A, a first light filter  30  and a second light filter  30   a.  The first light filter  30  is positioned in the optical pathway of light to the first photodiode  15 . The first light filter  30  transmits light having a wavelength within a first range of wavelengths. The second light filter  30   a  is positioned in the optical pathway of light to the second photodiode  15 A. The second light filter  30  transmits light having a wavelength within a second range of wavelengths. The first photodiode  15  is coupled to the first operational amplifier  27 . The second photodiode  15 A is coupled to the second operational amplifier  27 A. Both photodiodes  15  and  15 A are connected to ground  13 . The voltage outputs of the system are  25  and  25 A. 
     FIG. 6 shows a detailed circuit diagram of the amplifier circuitry of FIG.  5 . Photodiodes  15  and  15   a  are in electrical communication with operational amplifiers  27  and  27   a  respectively. The photodiodes  15  and  15   a  are filtered by respective filters  30  and  30   a.  For this particular system, the two photodiodes  30  and  30   a  can be filtered to pass 440 nm in filter  30  and 370 nm light respectively. The output of each amplifier is fed into respective feedback modules  35  and  35   a.  Feedback module  35  is comprised of two series resistors  40  and  41  in parallel with a capacitor  42 . Feedback module  35   a  is also comprised of two series resistors  40   a  and  41   a  in parallel with a capacitor  42   a.    
     Referring to FIG. 7, a tissue spectroscopy probe  301  includes a spectroscopic component module  306 , a power source  310 , and a control module  308  disposed inside a housing  303 . The spectroscopic component module  306  includes a light source  306   a  and light detectors  306   b.  The light source  306   a  illuminates the tissue  304 , and the detectors  306   b  detect spectroscopic properties of the illuminated tissue  304 . The light source  306   a  can be, for example, a laser or a diode capable of emitting light at a pre-determined wavelength. Light filters  306   c  can be disposed between the detectors  306   b  and the illuminated tissue  304  to allow light of a predetermined wavelength to pass through to the detectors  306   b.  The spectroscopic component module  306  may comprise one or more light sources  306   a  and one or more light detectors  306   b.  The light source  306   a  and the light detectors  306   b  are electrically coupled to the power source  310  through cables  312 . 
     The housing  303  includes a distal window  305 , and the spectroscopic component module  306  is disposed adjacent the window  305 . The probe  301  further includes a proximally mounted actuation switch  307 , and indicators  309  and  311 . In one embodiment, the indicator  309  is a red light, which is actuated to indicate cancerous tissue, and the indicator  311  is a green light, which is actuated to indicate normal tissue. The probe  301  is sized and shaped to fit inside a body cavity, which provides access to a tissue  304  to be examined, while the proximal end of the probe  301  remains outside the body for manipulation and control as well as for allowing the operator to observe the indicators  309 ,  311 . 
     The power source  310  is electrically coupled to the control module  308 . In one embodiment, the power source  310  includes a plurality of batteries, which provide DC power to the light source  306   a,  the light detectors  306   b,  the control module  308  and the indicators  309 ,  311 . The control module  308  performs a variety of functions including: regulating the power delivered to the light source  306   a;  converting the detected light from an analog to a digital signal; and providing the logical function and display driver to the indicators  309  and  311 . The light detectors  306   b,  the indicators  309 ,  311  and the control module  308 , as used as a display driver for the indicators  309 ,  311 , can be implemented by utilizing the circuit as described in FIG.  5  and FIG.  6 . 
     The probe  301  may be tapered, cylindrical or elongated in shape. The housing  303  may be constructed of a flexible material such as vinyl or polyethylene. The flexible housing  303  permits the probe  301  to be inserted inside the body cavity with greater comfort. Other materials suitable to form the housing  303  include plastics, metals or composites such as carbon fiber or glass fiber composites that exhibit low thermal conductivity. In one embodiment, the housing  303  is constructed of a material having a low thermal conductivity. Low thermal conductivity of the housing material prevents the person from feeling the coldness of the metal instruments disposed inside the housing  303  and prevents any heat that may be generated from the internal electronics from propagating out of the housing  303 . 
     The tissue spectroscopy probe  301  is an embodiment of the circuit illustrated in FIG.  8 . Referring to FIG. 8, the spectrometer module  401  includes a light source  403  and a multi-channel light detector  405  in close proximity to each other and to a region of interest  406 . The region of interest  406  may be living tissue located inside a body. The light source  403  and the light detector  405  are located in close proximity to the region of interest so that they may both emit and/or couple the light energy efficiently with minimum intervening space or material. The light source  403  is in communication with a power supply or source  407  through a DC power line  409 , and the light detector  405  is in communication with the power source  407  through a bias supply line  410 . The light source  403  may be internal or external to the module  401 . The power source  407  may provide direct current (DC) of either high or low voltage, alternating current (AC) of an appropriate frequency, or a pulse. AC power may be supplied to the light source  403  for the purpose of modulating the light source with a modulator  417 . Alternatively, current with complex waveforms may be supplied to the light source  403 . A diode may be placed in the circuit at the light source  403  to rectify some of the AC power so that it can be used to bias the detector  405 . In the disclosed embodiment, a metering device  419  is placed at the source of power and employs a current sampler  420  in line to monitor and display the power applied to the light source  403 . This configuration may be used to help calibrate the instrument during use. 
     One or more output lines  413  extend from the detector  405  to a microprocessor  425  and a display  411  through an amplifier  421  and an A-D converter  423 . The output lines  413  may be shielded to reduce noise pickup. The output of the detector  405  is amplified through an amplifier  421  and sent to an analog-to-digital (A-D, A/D, or A-to-D) converter  423 . The digitized signal can then be sent to a microprocessor  425  or other logical device for subsequent spectral analysis. 
     Variations, modifications, and other implementations of what is described herein will occur to those of ordinary skill in the art without departing from the spirit and the scope of the invention as claimed. Accordingly, the invention is to be defined not by the preceding illustrative description but instead by the spirit and scope of the following claims.