1. Technical Field of the Invention
The present invention generally relates to the art of electrical current sources/sinks and, more particularly, to a current source with improved output impedance using bipolar transistors.
2. Description of Related Art
Many circuits require current sources/sinks that are relatively constant and highly insensitive to changes in supply. In addition, a most desirable characteristic of current source/sink circuits is a high output resistance or impedance. This improves the accuracy for the output signal and results in a high voltage gain for the circuit if the circuit is used as an active load in an amplifier for example. When using PNP/NPN bipolar transistors cascode current sources/sinks, two problems in particular arise, base currents and base current modulation, contribute to undesirable variations to the output. Base currents, and hence base current errors, result from finite transistor Beta. In contrast, finite Early voltage and/or changes in the output voltage lead to base current modulation errors. The above-described errors within the cascode transistor limit the output impedance of the final circuit. It would be desirable to provide a current source/sink circuit that removes these limitations.
The present invention provides technical advantages as a circuit and method that provides a cascode current source/sink with high output impedance. In one embodiment, the dependency on the external load is reduced by directing a compensation current corresponding to change in base current in the cascode in an approach such that the compensation current cancels out the error of the cascode. In a further embodiment, the biasing is arranged such that change in base current of the cascode is summed at the emitter of the cascode such that the collector current of the cascode remains unchanged.
For a more complete understanding of the present invention, reference is made to the following detailed description taken in conjunction with the accompanying drawings wherein:
The numerous innovative teachings of the present application will be described with particular reference to the presently preferred exemplary embodiments. However, it should be understood that this class of embodiments provides only a few examples of the many advantageous uses and innovative teachings herein. In general, statements made in the specification of the present application do not necessarily delimit any of the various claimed inventions. Moreover, some statements may apply to some inventive features, but not to others.
Throughout the drawings, it is noted that the same reference numerals or letters will be used to designate like or equivalent elements having the same function. Detailed descriptions of known functions and constructions unnecessarily obscuring the subject matter of the present invention have been omitted for clarity.
By definition, a current source will supply a constant current irrespective of the magnitude and frequency of the applied voltage. A bipolar junction transistor is itself a voltage controlled current source. However, for practical BJT current sources, the output current may vary as the applied voltage changes. A cascode current device is a commonly used cure all to improve a current source/sink's immunity to change in voltage. Referring to
The above-described cascode configuration has the effect of reducing the dependency of the circuit 100 on the output voltage Vout such that the reaction of the output transistor Qout to changes in output voltage Vout is reduced. However, when cascoding a current source/sink with a PNP/NPN bipolar transistor the output impedance of the final circuit is limited by the Beta and Early voltage of the cascode transistor Q1 and impact ionization within the cascode transistor Q1. As Vout is varied the bias current Ibc of the cascode transistor Q1 changes causing error in Iout.
Referring now to
Although a preferred embodiment of the method and system of the present invention has been illustrated in the accompanied drawings and described in the foregoing detailed description, it is understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications, and substitutions without departing from the spirit of the invention as set forth and defined by the following claims.
Number | Name | Date | Kind |
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4345217 | Pace | Aug 1982 | A |
4855618 | Brokaw | Aug 1989 | A |
5512815 | Schrader | Apr 1996 | A |
6664842 | Pobanz | Dec 2003 | B1 |
Number | Date | Country | |
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20040239410 A1 | Dec 2004 | US |