1. Field of the Invention
This invention relates to impedance reducing circuits and more particularly to such circuits where the impedance is reduced by injecting a carrier current which is very large compared to the input current signal and then is precisely removed to facilitate obtaining a voltage output which is proportional to the logarithm of the signal current.
The invention finds particular application to video systems having relatively high frequency responses, such as in the order of 4 megahertz. The signal current is developed in response to a photodetector sensing light from a light emitting diode scanner. The signal current from such a photodetector is approximately 0.5 ua. The capacitance of the photodetector is approximately 50 picafarads. The input impedance of the circuit must be less than 1 k ohms. The input resistance into a semiconductor junction is (26 k/I) ohms, where I is the input current in microamperes. The required input resistance is obtained by superimposing a D.C. current 26 to 50 microamperes on the signal current.
In order to eliminate the effect of variations in light intensity of the light emitting diodes making up the scanner, a second photodetector monitors the light emitting diode. The signal from the second detector is D.C. coupled to a second impedance reducing circuit in the same manner as the signal from the first photodetector is coupled to the first impedance reducing circuit. The signal currents which are amplified by the impedance reducing circuits are converted to voltage outputs which are proportional to the logarithms of the signal currents. The two voltage output signals are then divided by subtracting the logarithms of the two signals. This is done by means of a differential amplifier.
In order to accurately form the logarithm function, the amount of D.C. current, not due to the signal from the photodetector, which is allowed to flow through the log producing junction must be very small in comparison with the signal. The D.C. carrier current is removed during a reset period when the light-emitting diodes are turned off by generating a take-out current proportional to the D.C. carrier current. The circuit for removing the D.C. carrier current will not respond to the signal from the photodetector. However, removal of the D.C. carrier current, i.e., the take-out current, increases very slightly during signal time and the amplifier of take-out circuit then swings sufficiently positive to reduce the take-out current below that required to satisfy the condition of minimum current amplification within the impedance reducing circuit. Although the carrier current is substantially removed, a small amount remains so that the transistor having the log producing junction will not cut off.