Dynamic current supply

Abstract

A dynamic current supply is connected between the emitter electrode of an output transistor and a point of reference potential, to progressively increase the magnitude of the flow of current therebetween, as the output signal at the emitter electrode approaches the point of reference potential. The dynamic current supply includes a current mirror, the output of which is in the path of and controls the flow of current. An MOS transistor, the conductivity of which is a function of the difference in potential between the operating voltage supplied to the output transistor and the output signal, supplies the input or control current to the current mirror.

What is claimed is: 
     
       1. In combination: a power terminal for receiving an operating voltage;   a reference terminal for receiving a reference voltage;   an input terminal;   an output terminal;   an output transistor having a collector electrode connected to said power terminal, a base electrode coupled to said input terminal, and an emitter electrode connected to said output terminal;   a controlled-current-supply transistor having a principal conduction path between a first electrode thereof and a second electrode thereof, the conductivity of said path being directly related to the potential between said first electrode and a third electrode of said controlled-current-supply transistor, the first and second electrodes of said controlled-current-supply transistor being connected respectively to said reference terminal and to said output terminal; and .Iadd.in addition to the foregoing elements, .Iaddend.   means.Iadd., connected to the emitter electrode of said output transistor, .Iaddend.for sensing the potential .[.appearing.]. at said output terminal .Iadd.with respect to one of said operating and reference voltages .Iaddend.to generate a control signal applied between the first and third electrodes of said controlled-current-supply transistor to increase its conduction as the potential between said reference terminal and said output terminal decreases in value.   
     
     
       2. The combination of claim 1, wherein said controlled-current-supply transistor has a direct current conductive connection between its first and third electrodes which includes a semiconductor diode means. 
     
     
       3. The combination of claim 2 wherein said means for sensing the potential .[.appearing.]. at said output terminal includes: a first resistor;   an MOS transistor having a gate electrode connected to said emitter electrode, a drain electrode connected to the third electrode of said controlled-current-supply transistor, and a source electrode connected to one end of said first resistor, the other end of which is coupled to said power terminal.   
     
     
       4. The combination of claim 3 further including: switching means connected between the other end of said first resistor and said power terminal, responsive to a switching signal, for selectively connecting and disconnecting said operating voltage from said first resistor.   
     
     
       5. The combination of claim 2, wherein said means for sensing the potential .[.appearing.]. at said output terminal includes: a first resistor;   an MOS transistor having a gate electrode connected to said emitter electrode, a drain electrode connected to the third electrode of said controlled-current-supply transistor, and a source electrode connected to one end of said first resistor;   bias potential supply means coupled between said power terminal and said reference terminal, for providing a bias voltage to the other end of said first resistor, the amplitude and polarity of said bias voltage being such that for one polarity of said output signal substantially no control current will flow, whereas for an opposite polarity of said output signal a control current will flow having a magnitude determined by the value of said first resistor and the difference between said bias voltage and the potential at said output terminal.   
     
     
       6. The combination of claim 2, wherein said semiconductor diode means comprises a self-biased transistor;   
     
     
       7. The combination of claim 2, wherein said direct-current-conductive connection between the first and third electrodes of said controlled-current-supply transistor includes a resistor in series connection with said semiconductor diode means.   
     
     
       8. The combination of claim 5, wherein said bias potential supply means includes: second and third resistors;   an emitter-follower transistor having a collector electrode coupled to said power terminal, an emitter electrode connected to the .[.and.]. .Iadd.end .Iaddend.of said first resistor remote from the source electrode of said MOS transistor, and a base electrode connected to a first end of said second resistor, the second end of which is coupled to said power terminal; and   a diode poled for reverse conduction in a connection between the base electrode of said emitter-follower transistor and one end of said third resistor, the other end of which is connected to said reference terminal, the amplitude of said bias potential being determined by the breakdown voltage of said diode poled for reverse conduction and the resistances of said second and said third resistors.   
     
     
       9. The combination of claim 8 including: switching means connected between the second end of said second resistor and said power terminal, responsive to a switching signal, for selectively applying and removing said operating voltage from said bias potential means.   
     
     
       10. The combination of claim 5, which further includes: a pull-down transistor having a collector electrode connected to the emitter electrode of said output transistor, an emitter electrode connected to said reference terminal, and a base electrode; and   a bias voltage connected across the base-emitter electrodes of said pull-down transistor, of such value to ensure a predetermined minimum magnitude of current flow between said reference terminal and the emitter electrode of said output transistor.   
     
     
       11. An amplifier comprising, in combination: first and second terminals for an operating voltage source;   a transistor having base, emitter and collector electrodes, connected at its collector electrode to the first of said terminals; .Iadd.and in addition to the foregoing elements, .Iaddend.   controllable impedance means coupling said emitter electrode to said second terminal, further including means responsive to the voltage at said emitter electrode .Iadd.with respect to the voltage at one of said first and second terminals.Iaddend., when it approaches the voltage at said second terminal .Iadd.more closely than a predetermined amount, .Iaddend.for reducing the impedance of said controllable impedance means between said emitter electrode and said second terminal to a value such that the voltage between said two terminals reduces to lower than 1 IV be , 1 V be  being the voltage between the base and emitter electrodes of a transistor which is necessary to support conduction through the transistor.   
     
     
       12. An amplifier as set forth in claim 11, wherein said means responsive to the voltage at said emitter electrode comprises an input current path of a current mirror amplifier, and said controllable impedance means comprises an output current path of said current mirror amplifier, and comprises further, means for supplying an amount of input current to said input current path as a function of the voltage at said emitter electrode. 
     
     
       13. An amplifier as set forth in claim 12, wherein said output current path comprises the emitter-to-collector path of a transistor whereby, in the limit, the voltage between said emitter electrode and said second terminal can reduce to V sat ., the voltage between the emitter and collector electrodes of this transistor, at saturation. 
     
     
       14. An amplifier as set forth in claim 12, wherein said means responsive to voltage further includes means for reducing to substantially zero the flow of current through said output current path when the voltage at said emitter electrode is at a particular value between the voltages at said first and second terminals. 
     
     
       15. An amplifier comprising, in combination: first and second terminals between which an operating voltage may be applied;   a signal input terminal;   a signal output terminal;   an output transistor having a collector electrode connected to said first terminal, a base electrode coupled to said input terminal, and an emitter electrode connected to said output terminal;   solid-state means having a current path connected between said emitter electrode and said second terminal, and a control electrode receptive of a control signal for controlling the impedance of said current path; and .Iadd.in addition to the foregoing elements, .Iaddend.   means coupled to said output terminal, to said second terminal, and to said control electrode, responsive to an output signal at said output terminal .Iadd.with respect to the voltage of one of said first and second terminals.Iaddend., for applying a control signal to said control electrode, for reducing the impedance of said current path, as the output signal amplitude approaches the voltage level at said second terminal .Iadd.more closely than a predetermined amount.Iaddend..   
     
     
       16. The amplifier of claim 15, further including: a current mirror amplifier having input, output and common terminals, said common terminal being connected to said second terminal, an input current path between said input and common terminals and an output current path between said output and common terminals; and voltage sensing means coupled to said input current path for controlling the current passing through said input current path, said means coupled to said output terminal comprising said voltage sensing means and said input current path of said current mirror amplifier, and said solid state means comprising said output current path of said current mirror amplifier. 
     
     
       17. The amplifier of claim 16, wherein said voltage sensing means includes a MOS transistor having a source electrode coupled to said first terminal, a gate electrode connected to said output terminal, and a drain electrode connected to said input terminal of said current mirror amplifier. 
     
     
       18. The amplifier of claim 15, wherein said solid-state means comprises a transistor having emitter, base and collector electrodes, said current path comprising the collector-to-emitter path of said transistor, and said control electrode comprising said base electrode. 
     
     
       19. The amplifier of claim 16, wherein said current mirror amplifier includes: a first transistor having a collector electrode connected to the output terminal of said current mirror amplifier, an emitter electrode connected to said common terminal of said current mirror amplifier, and a base electrode; and   a second transistor having a base electrode and a collector electrode connected in common to the base electrode of said first transistor and to the input terminal of said current mirror amplifier, and an emitter electrode coupled to said common terminal of said current mirror amplifier.   
     
     
       20. The amplifier of claim 15, further including: a second transistor, said second transistor including a collector electrode connected to the emitter electrode of said output transistor, an emitter electrode connected to said second terminal and a base electrode to which a bias voltage may be applied.