Current limiting circuit
First Claim
1. A circuit for monitoring the base-emitter voltage of a transistor for limiting the current therethrough, comprising:
- a first transistor having base, emitter and collector terminals;
a first source of current coupled to said first transistor for supplying base drive to said first transistor;
a second transistor having a collector for conducting a first current, a base coupled to the base of said first transistor, and an emitter having a first area;
a third transistor having a collector for conducting a second current, a base coupled to the base of said first transistor, and an emitter having a second area, said first area being substantially larger than said second area;
impedance means coupled between the emitters of said second and third transistors and the emitter of said first transistor, said impedance means and said first and second areas causing said first and second currents to be substantially equal at a predetermined value of the base-emitter voltage of said first transistor; and
means for reducing the base drive to said first transistor when said first current is substantially equal to said second current.
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Accused Products
Abstract
A current limiting circuit is provided wherein the monitored voltage is applied to the base of first and second transistors, the first transistor having a substantially larger emitter area than the second transistor. Impedance means are connected to the emitters of the first and second transistors such that they function in conjunction with the emitter area ratio causing the current flowing through the first transistor to be higher than that flowing through the second transistor at low base voltages, while at higher base voltages the current flowing through the second transistor is higher than flowing through the first transistor. At some intermediate voltage the current flowing through both the first and second transistors is equal. This property may be used, for example, in a voltage regulator so as to divert base drive current from the output transistor at the point where the first and second currents are equal.
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Citations
7 Claims
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1. A circuit for monitoring the base-emitter voltage of a transistor for limiting the current therethrough, comprising:
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a first transistor having base, emitter and collector terminals; a first source of current coupled to said first transistor for supplying base drive to said first transistor; a second transistor having a collector for conducting a first current, a base coupled to the base of said first transistor, and an emitter having a first area; a third transistor having a collector for conducting a second current, a base coupled to the base of said first transistor, and an emitter having a second area, said first area being substantially larger than said second area; impedance means coupled between the emitters of said second and third transistors and the emitter of said first transistor, said impedance means and said first and second areas causing said first and second currents to be substantially equal at a predetermined value of the base-emitter voltage of said first transistor; and means for reducing the base drive to said first transistor when said first current is substantially equal to said second current. - View Dependent Claims (2)
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3. A current-limited voltage regulator, comprising:
- voltage regulator means adapted to receive a reference voltage and for producing a regulated output voltage;
current source means coupled to said voltage regulator means and responsive to an input voltage for supplying a drive current to said voltage regulator means; a first transistor for conducting a first current therethrough having a base coupled to said voltage regulator means and receiving therefrom a first voltage proportional to said regulated output voltage, a collector, and an emitter having a first area; a second transistor for conducting a second current therethrough having a base coupled to said first voltage, a collector and an emitter having a second area, said first area being substantially larger than said second area; current diverting means coupled to the collector of said first transistor, said current source means and said voltage regulator means; impedance means coupled to the emitters of said first and second transistors, said impedance means and said first and second areas reducing said first current with respect to said second current as said first voltage increases; and current mirror means coupled to said first transistor, said second transistor and said current diverting means for diverting drive current from said voltage regulator means when said first current falls below said second current. - View Dependent Claims (4, 5, 6, 7)
- voltage regulator means adapted to receive a reference voltage and for producing a regulated output voltage;
Specification