Differential amplifier circuit

Abstract

The transistors of a differential amplifier are coupled at their common emitter electrode connection to a low voltage power supply terminal and at their collector electrodes through respective current sinks to a point of reference potential. The collector electrodes are maintained at a voltage level above the reference potential to permit input signal swings at the base electrodes of the transistors a comparable amount below the reference voltage level. Current sources are provided for satisfying that portion of the current demand of these sinks not furnished by the transistors. When the amplifier is connected to operate as a voltage follower, additional transistors may be provided to prevent an undesirable change in the polarity of the output signal, when the input signal swings substantially below the reference voltage level.

What is claimed is: 
     
       1. An amplifier comprising: a pair of input terminals receptive of input signals;   first and second like conductivity transistors each having collector, emitter and base electrodes, said emitter electrodes being connected to one another, and each base electrode being connected to a different input terminal;   first and second voltage rails;   first current supply means connected between the emitter electrodes of said first and second transistors and said first voltage rail, for providing a current flow of given value between said emitter electrodes and said first rail;   second and third current supply means connected between the collector electrodes of said first and second transistors and said second voltage rail, respectively, which together provide a flow of current substantially greater than that of said first current supply means;   fourth and fifth current supply means, the fourth connected between said first rail and said second supply means, the fifth connected between said first rail and said third supply means, said fourth current supply means and said first transistor supplying a total current equal to that of said second supply means, and said fifth current supply means and second transistor supplying a total current equal to that of said third supply means; and   means for maintaining said collector electrodes at .[.a reference voltage level of a value between that at said first and second rails.]. .Iadd.quiescent potentials of like value departing from the voltage on said second voltage rail in the same direction as the voltage on said first voltage rail and by an amount sufficiently small that the base electrode of either of said first and second transistors can accept input voltages with excursions past the voltage on said second voltage rail in the direction away from the voltage on said first voltage rail without appreciable forward conduction of current between the base electrode of that transistor and its collector electrode. .Iaddend.   
     
     
       2. The amplifier or claim 1, further including means for concurrently adjusting the current flow of said first, second, and third current supply means. 
     
     
       3. The amplifier of claim 1, wherein said maintaining means includes: said fouth and fifth current supply means each including third and fourth transistors, each opposite in conductivity to said first transistor, each having a collector electrode individually coupled to said first rail, an emitter electrode individually connected to the collector electrode of said first and second transistors, respectively, and a base electrode; and   means for supplying a predetermined operating voltage in common to the base electrodes of said third and fourth transistors.   
     
     
       4. The amplifier of claim 2, wherein said first current supply means includes: a first current mirror having a common terminal connected to said first voltage rail, an output terminal connected to the emitter electrodes of said first and second transistors, and an input terminal; and   said concurrent adjusting means including a second current mirror having a common terminal connected to said second rail, an input terminal receptive of a current bias signal, and an output terminal connected to said input terminal of said first current mirror.   
     
     
       5. The amplifier of claim 2, wherein said second and third current supply means include: said concurrent adjusting means including a first current mirror having an input terminal receptive of a current bias signal, and a common terminal connected to said second voltage rail; and   third and fouth transistors each having opposite conductivity as said first transistor, a collector electrode individually connected to the collector electrodes of said first and second transistors, respectively, and a base electrode connected in common to said input terminal of said first current mirror, the combination providing second and third current mirrors, respectively.   
     
     
       6. The amplifier of claim 3, wherein said means for supplying a predetermined operating voltge to said third and fourth transistors includes: a pair of diodes coupled in series between said first and second rails, one of said diodes being a Schottky diode, one of said diodes having an electrode connected in common to the base electrodes of said third and fourth transistors.   
     
     
       7. The amplifier of claim 3, which further includes: an output terminal, whereby an ouptut signal is developed across said output terminal and said second rail;   a feedback path from said output terminal to one of said pair of input terminals, for placing said amplifier in a voltage follower mode of operation, whereby whenever an input signal connected to the other input terminal has a like polarity as, and an amplitude substantially exceeding, a voltage at said second voltage rail, said amplifier tends to undesirably change the polarity of said output signal; and   means responsive to said input signal for preventing said undesirable change of polarity of said output signal.   
     
     
       8. The amplifier of claim 7, wherein said means for preventing said undesirable change of polarity of said output signal includes: fifth and sixth transistors, each being of the same conductivity as said first transistor, each having a base electrode individually connected to the pair of input terminals, respectively, each having an unconnected emitter electrode, and a collector electrode; and   seventh and eight transistors each being opposite in conductivity to said first transistor, each having a base electrode connected in common to said operating voltage supply means, said seventh transistor having an emitter electrode connected to the collector electrode of said sixth transistor, and a collector electrode connected to the collector electrode of said fourth transistor, said eighth transistor having an emitter electrode connected to the collector electrode of said fifth transistor, and a collector electrode connected to the collector electrode of said third transistor.   
     
     
       9. A differential amplifier comprising, in combination: first and second terminals across which an operating voltage may be applied;   first and second transistors, each having base, emitter and collector electrodes, connected to one another at their emitter electrodes;   two signal input terminals, one coupled to each base electrode;   a current source, and first and second current sinks, said source coupling said emitter electrodes to said first terminal, said first current sink coupling one of said collector electrodes to said second terminal, and said second current sink coupling the other of said collector electrodes to said second terminal, said two current sinks demanding together a total current which is substantially more than is available from said current source;   means coupled to said collector electrodes for maintaining said collector electrodes at .[.a voltage level.]. .Iadd.respective quiescent voltage levels .Iaddend.between that at said first terminal and that at said second terminal, .Iadd.departing from the voltage level at said second terminal by an amount sufficiently small that the base electrode of either of said first and second transistors can accept input voltages with excursions past the level at said second terminal in the direction away from the voltage level at said first terminal without appreciable forward conduction of current between the base electrode of that transistor and its collector electrode; .Iaddend.and   means coupled to said collector electrodes for supplying the additional current demanded by said two current sinks.   
     
     
       10. A differential amplifier as set forth in claim 9, wherein said means for maintaining said collector electrodes at a voltage level comprises a reference voltage source and two semiconductor junctions, one connected in the forward direction between said reference voltage source and one collector electrode, and the other connected in the forward direction between said reference voltage source and the other collector electrode. 
     
     
       11. A differential amplifier as set forth in claim 10, wherein said means for supplying the additional current comprises second and third current sources, each including a transistor having base, emitter and collector electrodes, and both of different conductivity type than said first and second transistors, one connected at its emitter electrode to the collector electrode of said first transistor and the other connected at its emitter electrode to the collector electrode of the second transistor, both connected at their base electrodes to said reference voltage source, and both coupled at their collector electrodes to said first terminal, said semiconductor junctions comprising the base-emitter junctions of said transistors of said second and third current source. 
     
     
       12. A differential amplifier as set forth in claim 11, wherein said reference voltage source comprises: a direct current impedance means connected at one terminal to said first terminal; and   a pair of diodes connected in series in the forward direction between the other terminal of said impedance means and said second terminal, one of said diodes comprising a Schottky diode.   
     
     
       13. A differential amplifier as set forth in claim 11, further including: an output terminal individually coupled to the collector electrodes of the transistors of said second and third current source, for providing an output signal reference to the voltage at said second terminal;   a feedback path between said output terminal and one of said input terminals for supplying to said one input terminal a signal of the same amplitude as the input signal applied to the other input terminal; and   means responsive to said input signal for preventing an undesirable tendency of said amplifier to reverse the polarity of said output signal, whenever the amplitude of the input signal substantially exceeds the amplitude of a voltage at said second terminal.   
     
     
       14. A differential amplifier as set forth in claim 13, wherein said means for preventing said undesirable change of polarity of said output signal includes: third and fourth transistors, each being of the same conductivity as said first transistor, each having a base electrode connected to an individual one of said two signal input terminals, respectively, each having an unconnected emitter electrode, and a collector electrode; and   fifth and sixth transistors each being opposite in conductivity to said first transistor, each having a base electrode connected in common to said reference voltage source, said fifth transistor having an emitter electrode connected to the collector electrode of said fourth transistor, said sixth transistor having an emitter electrode connected to the collector electrode of said third transistor;   the transistors of said second and third current sources each having a collector electrode individually connected to the collector electrodes of said sixth and fifth transistors, respectively. .Iadd.   
     
     
       15.  An amplifier comprising: a pair of input terminals receptive of input signals;   first and second like-conductivity transistors each having collector, emitter and base electrodes, said emitter electrodes being connected to one another, and each base electrode being connected to a different input terminal;   first and second voltage rails;   first current supply means connected between the emitter electrodes of said first and second transistors and said first voltage rail, for providing a current flow of given value between said emitter electrodes and said first rail;   first current mirror amplifier means being connected within said first current supply means, having an input connection, having at least a first output connection to the emitter electrodes of said first and second transistors, and having a common connection to said first voltage rail;   second and third current supply means connected between the collector electrodes of said first and second transistors and said second voltage rail, respectively, which together provide a flow of current substantially greater than that of said first current supply means;   second current mirror amplifier means being connected within said second and third current supply means, having an input connection, having at least first and second output connections, which first and second ouptut connections are respectively to the collector electrode of said first transistor and to the collector electrode of said second transistor, and having a common connection to said second voltage rail;   fourth and fifth current supply means, the fourth connected between said first rail and said second supply means, the fifth connected between said first rail and said third supply means, said fourth current supply means and said first transistor supplying a total current equal to that of said second supply means, and said fifth current supply means and second trasistor supplying a total current equal to that of said third supply means; and   means for maintaining said collector electrodes at a reference voltage level of a value between that at the first and second rails; and   means for concurrently adjusting the current flow of said first, second, and third current supply means including   means for applying a control current to the input connection of one of said first and second current mirror amplifier means and   means for connecting a further output connection of said one of said first and second current mirror amplifier means to the input connection of the other of said first and second current mirror amplifer means. .Iaddend..Iadd.   
     
     
       16. The amplifier of claim 15 wherein said maintaining means includes: said fourth and fifth current supply means each including third and fourth transistors, each opposite in conductivity to said first transistor, each having a collector electrode individually coupled to said first rail, an emitter electrode individually connected to the collector electrode of said first and second transistors, respectively, and a base electrode; and   means for supplying a predetermined operating voltage in common to the base electrodes of said third and fourth transistors. .Iaddend. .Iadd.17. The amplifier of claim 16 with means for converting the collector currents of said third and fourth transistors to single ended form, which means includes:   an output terminal for signals for which the collector electrode of said third transistor is direct coupled, and   third current mirror amplifier means having an input connection to which the collector electrode of said fourth transistor is direct coupled, and having an output connection to said output terminal. .Iaddend.