Dynamic biasing for class A amplifier

Abstract: A Class A amplifier has a balanced input circuit (13) including first and second input transistors (24, 25) to receive inverting - and non-inverting + inputs and to provide an output signal on the collector of the first input transistor (24). First and second load transistors (28, 29) are associated with the input transistors (24, 25), the bases of the load transistors (28, 29) being connected to the collector of the second input transistor (25). An output circuit (14) has source (30) and sink (31) output NPN transistors connected to receive the output signal from the first input transistor (24), and a circuit (15) is provided for dynamically biasing at least one sink output transistor (31) to have a minimum biasing current in a quiescent state, and increased biasing current in a current sink state. The circuit (15) for dynamically biasing the output transistors (30, 31) has a pair of bias NPN transistors (34, 33) with their collector-emitter paths connected in series, and with the base of one (34) connected to receive the output of the first input transistor (24). The base of the other bias transistors (32)(33) is connected to the emitter of the one bias transistor (34) and to the base of the sink output transistor (31). The first and second bias transistors (32)(34) are sized to balance the load seen by the first and second input transistors (24,25).

Description: This invention relates to improvements in amplifier circuits, and more particularly to improvements in amplifier biasing methods and circuits, and still more particularly to improvements in dynamic biasing methods and circuits for class A amplifiers. Operational amplifiers often find use in circuits in which they are required to provide an output which has the capability of sourcing and sinking current to and from an associated load. In typical integrated circuit processes, sourcing current can be accomplished simply, since an NPN transistor can be used for a source transistor. However, sinking current from the load requires special design considerations. One circuit which is widely used to fulfill both sourcing and sinking requirements has a symmetrical output amplifier comprising both NPN and PNP transistors with the collector-emitter path of the NPN transistor being connected between the supply voltage source and the output, and with the collector-emitter path of the PNP transistor connected between the output and a reference potential, or ground. Such circuit structure, however, loses the advantages accompanying NPN transistor circuits, and, moreover, often require special offset compensation design. Furthermore, the design of the required PNP transistor devices is significantly more complicated in device fabrication than would be a corresponding NPN transistor device for the same or similar circuit considerations. In light of the above, it is an object of the invention to provide an amplifier which can sink relatively large amounts of output current, with relatively low quiescent currents in the output circuit. It is another object of the invention to provide a circuit of the type described which can achieve a relatively high current sinking capability without the use of PNP transistors, and without the use of a symmetrical PNP-NPN transistor output combinations. IBM Technical Disclosure Bulletin, Vol. 19, No. 5, Oct. 1976, New York, US, page 1719; P. Debord: 'RF Amplifier' discloses an amplifer having inputs for receiving first and second input signals and having an output stage comprising two NPN transistors. US Patent No. 4,482,868 discloses an amplifier having an input for receiving a single input signal and having an output stage comprising an NPN transistor and an n-channel MOS transistor. It is still a further object of the invention in which the offset currents in the circuit can be controlled by the scaling of the transistors in the output circuit, or in the biasing circuit for the output circuit. It is still another object of the invention to provide a circuit of the type described with a low inherent offset. The present invention provides an operational amplifier having an output circuit in which the active devices are high performance NPN transistors, which can both drive or source output currents and sink, or pull in, large amounts of current from the load. According to the invention there is provided an amplifier having first (IN+) and second (IN-) inputs and an output (OUT) for sourcing and sinking current from a load, comprising an input circuit comprising first and second input transistors to receive on respective ones of their bases the first (IN+) and second (IN-) inputs, said input circuit providing an output signal on the collector of said first input transistor and first and second load transistors associated with said respective input transistors, the bases of said first and second load transistors being connected to the collector of said second input transistor, an output circuit comprising first and second output transistors of the same conductivity type connected to receive the output signal from the collector of said first input transistor a circuit for dynamically biasing at least one of said output transistors to have substantially minimum biasing current in a quiescent state, and sufficient biasing current to be enabled to sink current from said load in a non-quiescent state and a bias circuit to source current at least to said input and output circuits. More particularly, a Class A amplifier has a balanced input circuit including first and second input transistors to receive inverting and non-inverting inputs and to provide an output signal on the collector of the first input transistor. First and second load transistors are associated with the input transistors, the bases of the load transistors being connected to the collector of the second input transistor. An output circuit has source and sink output NPN transistors connected to receive the output signal from the first input transistor, and a circuit is provided for dynamically biasing at least the sink output transistor to have a minimum biasing current in a quiescent state, and an increased biasing current in a current sink state. The circuit for dynamically biasing the output transistors has a pair of bias NPN transistors with their collector-emitter paths connected in series, and with the base of one connected to receive the output of the first input transistor. The base of the other bias transistors is connected to the emitter of the one bias transistor and to the base of the sink output transistor. The first and second bias transistors are sized to balance the load seen by the first and second input transistors. The invention is illustrated in the accompanying drawing, in which: Figure 1 is an electrical schematic diagram of an amplifier in accordance with the principles of a preferred embodiment of the invention. A preferred embodiment of the invention is particularly well suited for use in Class A amplifiers, and more particularly for use in conjunction with operational amplifiers, for example of the type illustrated in Figure 1. The operation amplifier 10 has a non-inverting input, IN+, and an inverting input, IN-, to which input signals are applied, as well known in the art. An output is developed at an output terminal, OUT, to which a load (not shown) may be connected. An additional terminal receives a bias current, I SIAS , to establish a dc bias to the current sources of the general bias circuit, as below described, also as well known. In accordance with a preferred embodiment of the invention, the amplifier 10 has four main circuits, a general bias, or current source, circuit 12, a signal input circuit 13, a signal output circuit 14, and an output biasing circuit 15. The general bias circuit 12 includes FETs 18, 19, and 20 having their source to drain paths connected to the supply voltage, V SUPPLY to serve as current sources for the signal input and output circuits 13 and 14, and a pair of NPN transistors 21 and 22 establish the bias voltages on the gates of the FETs 18, 19, and 20, as above mentioned. The signal input circuit 13 has a pair of PNP transistors 24 and 25 connected to receive on their respective bases the non-inverting and inverting inputs, IN+ and IN-. It should be noted that the PNP transistors 24 and 25, although shown as bipolar junction transistors, can also be PMOS transistors, for example, which can easily be fabricated and used in CMOS or BICMOS circuitry. The PNP transistors each has its emitter-collector path connected between the source-drain path of the FET 19 and a respective one of a pair of NPN active load transistors 28 and 29. The active load transistors each has its collector-emitter path connected through a resistor (if necessary) to a reference potential, or ground. It will be noted that the output from the signal input circuit 13 is derived from the collector of the PNP transistor 24, to be conducted to the signal output circuit 14. On the other hand, in accordance with the invention, the bases of the active load NPN transistors 28 and 29 are connected to the collector of the input transistor 25. It will, therefore, be appreciated by those skilled in the art that this balanced arrangement of connections to the circuit of the input circuit 13, together with the sizing of the output bias transistors, below described, will result in little or no offset between the currents in the non-inverting and inverting signal paths. The signal output circuit 14 is designed in such a way that it can be fabricated entirely of NPN transistors, 30 - 35. Thus, it will be appreciated that the amplifier 10 will have the advantages attendant in such construction, such as ease of layout design and fabrication, the ability to sink relatively large currents from any load connected to the output terminal, OUT, and so forth. In contrast to the widely seen arrangement in the prior art of complementary NPN and PNP transistors, the output circuit 10 has two NPN transistors 30 and 31 having current flow paths between the supply voltage, V SUPPLY , and ground, with the output signal being derived at the emitter-collector connection between them. The signal path for the output transistors 30 and 31 is provided by the transistors 32 - 35 of the output biasing circuit 15. Thus, the signal output from the input circuit 13 is applied to the bases of NPN transistors 34 and 35, which act as emitter follower amplifiers to conduct the signal to the respective collectors of NPN transistors 33 and 32. Additionally, the biasing for the output transistors 30 and 31 and more particularly, the current sinking transistor 31, is provided by the NPN transistors 32 - 35. In operation, for example, in the situation in which the circuit is used to source a current, or drive a current to the load at the output terminal, OUT, the transistor 30 is suitable, because it can provide substantial current, being based upon the current provided by the FET 20 current source, multiplied by the current gain of the transistor 30. On the other hand, in the situation in which the circuit is used to sink or pull in a current, the transistor 31 must be relied upon. If the transistor 31 were to be set up merely as a current source, without control, as might be expected using NPN transistor devices, the transistor 31 would be unable to sink substantial current from the load at the output terminal, except that to which it is fixed or established to provide as a current source. That is, in order to provide a large current sinking capability, a large quiescent bias current must ordinarily be established for the transistor 31. According to the invention, however, dynamic biasing is provided to transistor 31, principally by transistors 34 and 32. The transistor 34 controls the current flowing both through itself and through transistor 32. Consequently, when a need arises to sink large currents in transistor 31, the current in both transistors 34 and 32 increases, which, in turn increases the current which is available through transistor 31. Transistor 33 is also a part of the signal path, and receives current from the current source provided by FET 20. It will be appreciated that the operation of the amplifier 10 is intended to be linear (or Class A), and, therefore, the current from FET 20 would also flow through the transistor 33, except as required for the base current of transistor 30. As the base of transistor 34 is pulled up, the currents in both transistors 32 and 33 increase. However, the increase in current through transistor 33 causes the base of transistor 30 to be pulled down, or fall in voltage. This would be a situation which would be encountered, for example, when positive current were being supplied, and a change is made to an operating mode in which current is to be sinked from the load. Under this condition, the current in transistor 33 would increase, tending to rapidly turn off the output transistor 30. However, as the current in transistor 33 reaches the limit of current which can be supplied by the current source FET 20, the transistor 33 would begin to saturate. To prevent transistor 33 from saturating due the rapid fall in voltage on its base, transistor 35 establishes an alternate current path, as transistor 33 tries to conduct more current than is available from FET 20. Transistor 33, therefore, serves as an "anti-saturation transistor" or clamp. Frequency compensation across the anticipated operating frequency range of the amplifier is provided by the capacitor 41 and resistor 44, in a known manner. In sum, the arrangement of transistors 34, 32, 33, and 31 provides a much more controlled current than heretofore available. The current in transistor 32 can be made to be equal to the sum of the currents in active load transistors 28 and 29, for example, by device sizing, so the base current in transistor 34 is approximately equal to the current taken up by the connection of the collector of transistor 29 to the bases of transistors 28 and 29. So dynamic biasing is achieved, together with low inherent offset, and with currents which are independent of the entire process. Although the invention has been described and illustrated with a certain degree of particularity, it is understood that the present disclosure has been made only by way of example, and that numerous changes in the combination and arrangement of parts can be resorted to by those skilled in the art without departing from the scope of the invention as hereinafter claimed.

Claims: An amplifier (10) having first (IN+) and second (IN-) inputs and an output (OUT) for sourcing and sinking current from a load, comprising: an input circuit (13) comprising: first (24) and second (25) input transistors to receive on respective ones of their bases the first (IN+) and second (IN-) inputs, said input circuit providing an output signal on the collector of said first input transistor (24); and first (28) and second (29) load transistors associated with said respective input transistors (24,25), the bases of said first (28) and second (29) load transistors being connected to the collector of said second input transistor (25) ; an output circuit (14) comprising first (30) and second (31) output transistors of the same conductivity type connected to receive the output signal from the collector of said first input transistor; a circuit (32,34) for dynamically biasing at least one of said output transistors (30,31) to have substantially minimum biasing current in a quiescent state, and sufficient biasing current to be enabled to sink current from said load in a non-quiescent state; and a bias circuit (12) to source current at least to said input and output circuits. The amplifier of claim 1 wherein said circuit (32,34) for dynamically biasing said output transistors (30,31) comprises a first pair of bias transistors (32,34) having collector-emitter paths connected in series between a voltage source and a reference potential, the base of one of said bias transistors (34) receiving said output of said first input transistor (24), and the base of said another (32) of said bias transistors being connected to the emitter of said one (31) of said bias transistors and to the base of said at least one of said output transistor of the output circuit which is enabled to sink current from said load in a non-quiescent state. The amplifier of claim 1 or claim 2 wherein the first (24) and second (25) input transistors control the current in first and second input signal paths, respectively, in accordance with said first (IN+) and second (IN-) input signals. The amplifier of claim 3 wherein said first (34) and second (32) bias transistors are sized to balance the loads presented to said first and second input signal paths. The amplifier of claim 4 wherein said first (34) and second (32) bias transistors reduce the current offset in the first and second input signal paths. The amplifier of claim 4 or claim 5 wherein the emitters of said first (34) and second (32) bias transistors are sized to balance the loads presented to said first and second input signal paths. The amplifier of any one of claims 2 to 6 wherein said first (30) and second (31) output transistors and said first (34) and second (32) bias transistors are all of the same conductivity type. The amplifier of claim 7 wherein said first (30) and second (31) output transistors and said first (34) and second (32) bias transistor are NPN transistors. The amplifier of any one claims 1 to 8 further comprising a circuit (20,33) for supplying bias current to said first output transistor (30). The amplifier of claim 9 wherein said circuit (20,33) for supplying bias current to said first output transistor (30) includes a current source (20) and a transistor (33) having an emitter-collector path in series between said current source (20) and a reference potential, and a base to receive said output signal from said first input signal path. The amplifier of claim 9 or claim 10 further comprising an anti-saturation circuit (35) connected to prevent said transistor (33) in said circuit for supplying bias current to said first output transistor (30) from saturating when said first output transistor (30) changes from conducting to non-conducting states. The amplifier of claim 11 wherein said anti-saturation circuit comprises a transistor (35) having a collector-emitter path connected between said voltage source (V SUPPLY ) and the current source (20) and having a base connected to receive the signal on the first input signal path. The circuit of any one of the preceding claims wherein said first and second load transistors (28,29) comprise a first NPN transistor (28), the collector of the first NPN transistor (28) being connected to said first input signal path and the emitter of the first NPN transistor being connected to a first resistor connected to a reference potential; and a second NPN transistor (29), the collector of the second NPN transistor (29) being connected to the second input signal path, the base of the second NPN transistor (29) being connected to the second input signal path and the base of the first NPN transistor (28), and the emitter of the second NPN transistor (29) being connected to a second resistor connected to a reference potential. The amplifier of claim 2 wherein said circuit for dynamically biasing said output transistors further comprises a second pair of bias transistors, one (33) having a collector-emitter path connected between said bias circuit and a reference potential, and having a base connected to the base of said bias transistor of said first pair of bias transistors, and another bias transistor (35) having a collector-emitter path connected between said voltage source and said bias circuit and having a base connected to receive said signal on the collector of said first input transistor (24). The amplifier of claim 1 wherein said circuit for dynamically biasing said output transistors comprises first (34) and second (32) bias transistors, said first bias transistor (34) being connected to control the current flowing both through itself and through said second bias transistor (32) in response to said output signal from said input circuit (13), whereby when said output transistors are required to sink large currents, the current in both said first (34) and second (32) bias transistors increases, to increase the current available to said output transistors (30,31). The amplifier of any preceding claim wherein said amplifier is a Class A amplifier. The amplifier of any preceding claim wherein said amplifier is an operational amplifier. The amplifier of any preceding claim wherein said inputs to receive said first and second input signals are inverted and noninverted signal connections. The amplifier of any preceding claim wherein said bias circuit (18,19) further sources current to said first (28) and second (29) load transistors.

Inventors: BIEN DAVID EDWARD, BIEN, DAVID EDWARD
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS, INC.
Application year: 1992
Main classification: H03F3
IPC codes: H03F3