Current sources, such as sensors, generally convert physical quantities to electrical signals for further processing by electronic circuits. To assist with this signal processing, signals from the current sources are often amplified prior to processing. Current sources are coupled to an amplifier directly, through a passive component, such as a resistor or capacitor, or through an active component, such as a transistor.
When current sources are coupled to other circuit elements in an application circuit, undesirable and non-ideal properties of the current sources, such as capacitance and resistance, are introduced into the application circuit. This is especially true of solid-state current sources such as photodetectors. Additionally, elements of the application circuit, such as interconnections and bias circuits needed to operate the current sources, may introduce additional undesirable and non-ideal properties. Further, undesirable signals, often in the form of undesirable amplitudes or frequencies, may be introduced by the current sources, the application circuit, or the application environment.
These undesirable and non-ideal properties and signals introduced by the current sources may degrade the amplifier performance and overall application circuit performance and impose additional requirements on the design of both the amplifier and the application circuit. This is especially true for application circuits sensitive to fast dynamic changes in current source signals.
Another problem with current sources occurs when output signals from several current sources are connected together prior to amplification by one or more amplifiers. The signal current may flow between the current sources, which reduces the amount of current delivered to the amplifier. This signal loss may degrade system performance. Additionally, undesirable and non-ideal properties of the current sources, such as capacitance and resistance, as described earlier, may accumulate with each additional current source and further degrade system performance.
A further problem with current sources occurs when a grid of current sources, typically in a resistor-coupled configuration, is used in position-sensitive applications where the resistor-coupled grid can exhibit a signal leakage effect. In the grid configuration, signal current should flow primarily along one row and one column in the grid. Signal leakage occurs when signal current that flows through adjacent coupling resistors into adjacent rows and columns. Signal leakage reduces the amount of current delivered to the desired row and column amplifiers, which degrades spatial uniformity and position resolution.