Abstract:
An amplifier output stage is described containing a preliminary stage, a final stage and a control device. The quiescent current that flows through transistors of the final stage is adjusted by the preliminary stage. For this, a current that is proportional to the quiescent current is generated in the control device from which control voltages are derived and controlled. The preliminary stage contains adjustable current sources for adjusting the quiescent current in a final step which are controlled by the control voltages.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of copending International Application No. PCT/DE99/02129, filed Jul. 9, 1999, which designated the United States. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to an amplifier output stage containing a final stage and a preliminary stage connected to the final stage and having a multiplicity of adjustable current stages for setting a quiescent current in the final stage. A control device is connected to the preliminary stage and has a control current from which control voltages are generated and regulated in accordance with the multiplicity of adjustable current sources of the preliminary stage. Each of the control voltages controls precisely one of the adjustable current sources in the preliminary stage. 
     Published European Patent Application EP-0115949 A1 describes a buffer circuit having a high impedance. The buffer circuit has a main buffer circuit section formed from a pair of complementary input transistors whose base terminals are connected to one another, and also an additional circuit section having the same circuit structure as the main buffer circuit section. A feedback signal generated by the additional circuit section is fed to the additional circuit section and to the main buffer circuit section in order that an output signal of the additional circuit section assumes a predetermined value. 
     In amplifiers which have to supply high currents for driving large loads, a complementary emitter/source follower in push-pull class AB operation is usually used as the final stage (see the reference by U. Tietze, Ch. Schenck, titled “Halbleiterschaltungstechnik” [Semiconductor Circuitry], 9th edition, pp. 516 to 518 and pp. 521 to 523). In this operating mode, a parallel-path current, referred to as a quiescent current, continuously flows through the complementary final-stage transistors in a non-driven state of the amplifier. Power is consumed as a result of this quiescent current, which results in that the final-stage transistors are heated. It proves to be particularly problematic to set this temperature-dependent quiescent current to a low and constant value over a large temperature range, in order to minimize the current consumption in the final stage and to avoid distortions. 
     IEEE Journal of Solid-State Circuits, Vol. 27, No. 4, pages 539-544, April 1992, titled “A CMOS Line Driver with 80-dB Linearity for ISDN Applications”, by H. Knorramabadi, discloses a line driver amplifier for an integrated services digital network (ISDN) U interface, in which a so-called error amplifier which drives the final stage is used. A disadvantage of such a configuration is that the error amplifier lies directly in the signal path of the amplifier and influences the bandwidth of the amplifier. Moreover, the quiescent current through the final-stage transistors depends on an offset voltage of the error amplifiers. 
     IEEE Journal of Solid-State Circuits, Vol. 32, No. 32, pages 169-176, February 1997, titled “A Rail-to-Rail, Constant Gain, Buffered Op-Amp for Real Time Video Applications”, by L. Moldovan and H. H. Li, and IEEE Journal of Solid-State Circuits, Vol. 29, No. 1, pages 63-65, January 1994, titled “Digital-Compatible High-Performance Operational Amplifier with Rail-to-Rail Input and Output Ranges”, by W. -CH. S. Wu, W. J. Helms, J. A. Kuhn, and B. E. Byrkett describe amplifiers in which the quiescent current in the final stage is set by an input stage and is thus directly dependent on the input signals and current or voltage fluctuations of the input stage. 
     In the reference titled “Integrierte Schaltungen” [Integrated Circuits], by A. Schlachetzki, and W. V. Munch, Teubner 1978, a complementary emitter follower with quiescent current setting is illustrated in FIG. 4.8 on page 144, in the case of which the quiescent current to be set is mirrored into the final-stage transistors T 1  and T 2  via transistors T 3  and T 4  connected as diodes. However, the quiescent current is not freely adjustable in this configuration. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide an amplifier output stage which overcomes the above-mentioned disadvantages of the prior art devices of this general type, which has better adjustability and regulation of the quiescent current in the final stage and in which the quiescent current regulation and setting lie outside the signal path of the amplifier. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, an amplifier output stage. The amplifier output stage contains a final stage, a preliminary stage connected to the final stage and has a multiplicity of adjustable current sources for setting a quiescent current in the final stage, and a control device connected to the preliminary stage and having a control current from which control voltages are generated and regulated in accordance with the multiplicity of adjustable current sources of the preliminary stage. Each of the control voltages control precisely one of the multiplicity of adjustable current sources in the preliminary stage, and the control current in the control device is proportional to the quiescent current in the final stage. 
     One embodiment of the amplifier output stage contains the preliminary stage, the final stage and the control device. In this case, the quiescent current through the transistors of the final stage is set by the preliminary stage, which is connected to the control device. The preliminary stage has controllable current sources, by which the voltage at the control terminals of the final-stage transistors is set. This voltage defines the quiescent current through the final-stage transistors. In the control device, a control current is set, which is proportional to the quiescent current flowing through the final-stage transistors. From this, control voltages are generated for setting the current sources in the preliminary stage, which in turn set the quiescent current in the final stage. It is advantageous that the control device lies outside the signal path of the amplifier output stage and, consequently, does not limit the bandwidth of the amplifier output stage. In the control device, moreover, the control current and thus the quiescent current can be set to an arbitrary and exact value. 
     In a preferred embodiment of the invention, the preliminary stage of the amplifier output stage has two emitter or source follower circuits that the final stage decouples from an amplifier input stage which drives the amplifier output stage. This advantageously reduces the load on the amplifier input stage since the preliminary-stage transistors driven by the amplifier input stage represent a lower capacitive load than the final-stage transistors for the input stages. Each emitter or source follower circuit has a controllable current source that sets the quiescent current through the transistors of the final stage. In this embodiment, it is advantageous that only one transistor lies in the signal path of the amplifier output stage, the limiting frequency of which transistor is so high that the bandwidth of the amplifier is not restricted. 
     A preferred embodiment of the control device has a first and second emitter or source follower circuit, which each simulate the emitter or source follower circuits of the preliminary stage in the non-driven state. Each emitter or source follower circuit has a controllable current source. The inputs of the two emitter or source follower circuits are connected to a voltage corresponding to the output voltage of the amplifier output stage in the quiescent state. It is advantageous that similar operating conditions apply to the emitter or source follower circuits of the preliminary stage and of the control device. 
     In a preferred embodiment, the final stage has a first and second transistor which are operated in push-pull class AB operation. Advantages include the lower harmonic distortion factor compared with a final stage in class A operation and the lower current consumption compared with a final stage in class B operation. 
     In a preferred embodiment, the control device furthermore has a first and second transistor and an adjustable current source. In this case, the two transistors simulate the two transistors of the final stage and the current source feeds a control current into both transistors. It is advantageous that the control current fed in is adjustable and proportional to the quiescent current in the final stage. 
     In a preferred embodiment, the control device has a first and a second mirror transistor, the first mirror transistor mirroring the current through the first transistor and the second mirror transistor mirroring the current through the second transistor. The voltage at the control terminals of the mirror transistors is advantageously used for generating the control voltages. 
     The control device particularly preferably has a first and a second control transistor, the control transistors transferring the input voltage at the inputs of the emitter follower or source follower circuits to the first and second transistor. Consequently, the simulated final stage has a quiescent output potential which advantageously corresponds to the input voltage at the inputs of the emitter follower or source follower circuits and the quiescent output potential can be set by way of the input voltage of the emitter follower or source follower circuits. 
     In a particularly preferred embodiment of the control device, a first and second operational amplifier regulates the current sources contained in the emitter or source follower circuits in such a way that the voltage at the output of the operational amplifier can be used for setting the current sources in the preliminary stage. It is advantageous here that the voltage is dependent only as a result of the offset voltage of a few mV of the operational amplifiers and on the ratio of a few transistors of the current mirror and the final stage. This regulation of the control voltage has the major advantage that it is very accurate and disturbing influences (OP offset voltage, transistor ratios) have a negligible influence on the control voltage. 
     In a particularly preferred embodiment of the invention, the amplifier output stage is embodied using CMOS technology. Advantages of this embodiment include the capability of integration with further circuits using CMOS technology, and also the widespread use of CMOS and the relatively small area requirement compared with other technologies. 
     In a preferred embodiment, the current mirrors of the control device of the amplifier output stage have transistor ratios which conduct the current impressed into the two current mirrors almost completely through in each case one of the two transistors of the current mirrors. As a result, a potential which, applied to the control terminals of the transistors of the final stage, produces a defined quiescent current through the transistors of the final stage is advantageously established at the control terminal common to both current mirror transistors. 
     Through a preferred dimensioning of the ratios of in each case a transistor of the current mirror and a transistor of the final stage, the quiescent current through the transistor of the final stage can be set very precisely. A ratio of 20 is particularly preferred here, which ratio can be established very simply through the dimensioning of the transistors. 
     In accordance with an added feature of the invention, the control current in the control device is a fraction of the quiescent current flowing through the first transistor and the second transistor of the final stage. The fraction is determined by a channel width/length ratio of the second transistor of the control device, by a channel width/length ratio of the second transistor of the final stage, by a channel width/length ratio of the first transistor of the control device, and by a channel width/length ratio of the first transistor of the final stage. 
     The integration of the amplifier output stage on an integrated circuit is a preferred embodiment. A great advantage here is the good adjustability of the ratios of the transistor geometries. 
     In a particularly preferred embodiment, the emitter or source follower circuits of the preliminary stage and of the control device are placed locally next to one another on an integrated circuit. This advantageously reduces the influence of process fluctuations on the quiescent current through the transistors of the final stage. Moreover, temperature differences between the individual circuit sections of the amplifier output stage have a negligible influence on the quiescent current. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in an amplifier output stage, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of an exemplary embodiment of an amplifier according to the invention; and 
     FIG. 2 is a circuit diagram of a structure of a control device for setting a quiescent current in a final stage of the amplifier according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The exemplary embodiment described below is concerned with an embodiment of the invention using CMOS technology. In all the figures of the drawing, sub-features and integral parts that correspond to one another bear the same reference symbol in each case. Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown an amplifier output stage containing a preliminary stage  2 , a final stage  4  and a control device  3 . The preliminary stage  2  is connected to the final stage  4  through two lines  61  and  62  and the control device  3  is connected to the preliminary stage  2  through two lines  71  and  72 . A signal is fed in through an input  1  of the amplifier output stage, the signal coming, for example, from an input stage having a high gain. An amplified signal is available at an output  5 . 
     The preliminary stage  2  contains two source follower circuits. 
     A first source follower circuit contains a first n-channel MOSFET TN 1 A, whose drain terminal is connected to a first voltage supply VDD of the amplifier and whose gate terminal is connected to the input  1  of the amplifier output stage. The first transistor TN 1 A is furthermore connected via a resistor RNA, which is connected to a source terminal, to a drain terminal of a second n-channel MOSFET transistor TN 2 A, whose source terminal is connected to a second voltage supply VSS. 
     A second source follower is constructed in a mirror-inverted manner with respect to the first source follower using p-channel technology and contains a first p-channel MOSFET TP 1 A, whose drain terminal is connected to the first supply voltage VDD of the amplifier and whose source terminal is connected via a resistor RPA to a drain terminal of a second p-channel MOSFET TP 2 A, whose source terminal is in turn connected to the second supply voltage VSS. A gate terminal of the second transistor TP 2 A is connected to the input  1  of the amplifier output stage. 
     The preliminary stage  2  is connected to the final stage  4  through the two lines  61  and  62 . The gate terminals of the second transistor TN 2 A of the first source follower and of the first transistor TP 1 A of the second source follower are connected to the control device  3  through the lines  71  and  72 . The control device  3  controls the voltages which are present at the gate terminals of the transistors TN 2 A and TP 1 A via the lines  71  and  72 , so that the transistors TN 2 A and TP 1 A act like controllable current sources. Variation of the voltages adjusts currents I 10  and I 11  that flow through the first and second source followers. As a result, it is possible to set the voltages that are present via the lines  61  and  62  at the gate terminals of transistors TPAUS and TNAUS of the final stage  4 . The setting of these voltages results in the setting of the quiescent current IQ that flows through the two transistors TPAUS and TNAUS of the final stage. In addition, the quiescent potential is set at the output terminal  5  of the final stage  4  of the amplifier. 
     The final stage  4  is constructed as a complementary source follower or common-source circuit and contains the p-channel MOSFET TPAUS, whose drain terminal is connected to the first supply voltage VDD of the amplifier, and the n-channel MOSFET TNAUS, whose source terminal is connected to the second supply voltage VSS. A source terminal of the transistor TPAUS is connected to a drain terminal of the transistor TNAUS and forms the output  5  of the amplifier. If the final stage  4  is not driven, a parallel-path current IQ corresponding to the quiescent current of the final stage flows through the two transistors TPAUS and TNAUS. The final-stage transistors are dimensioned for a high current yield and, to that end, have a very large channel width. 
     The control device  3  serves for setting and regulating the quiescent current in the final stage  4  and has two source follower circuits TN 1 B, RNB, TN 2 B and TP 1 B, RPB, TP 2 B analogous to the preliminary stage (see FIG.  2 ). The control device further has a final-stage circuit TP 2 S, TN 2 S, two operational amplifiers for quiescent current regulation OPP, OPN, two mirror transistors TPS 1 , TNS 1 , two control transistors TNS 3 , TPS 3  and an adjustable current source I 2 . 
     The first source follower is constructed like the first source follower TN 1 A, RNA, TN 2 A of the preliminary stage and contains the n-channel MOSFETs TN 1 B and TN 2 B, whose source and drain terminals are connected via a resistor RNB. The drain terminal of the transistor TNlB is connected to the first supply voltage VDD of the amplifier and the source terminal of the transistor TN 2 B is connected to the second supply voltage VSS. 
     The second source follower is constructed like the second source follower TP 1 A, RPA, TP 2 A of the preliminary stage and contains the p-channel MOSFETs TP 1 B and TP 2 B, whose source and drain terminals are connected to one another via a resistor RPB. The drain terminal of the transistor TP 1 B is connected to the first supply voltage VDD of the amplifier and the source terminal of the transistor TP 2 B is connected to the second supply voltage VSS. 
     The gate terminals of the transistors TN 1 B and TP 2 B are connected to a voltage source  10 . These two source follower circuits  13  simulate the two source follower circuits of the preliminary stage  2  in the non-driven, i.e. quiescent, state of the amplifier. To that end, a voltage VCM of the voltage source  10  is fixed at a value within the range defined by the first VDD and second VSS supply voltage and corresponds to the quiescent output potential of the amplifier output stage. The voltage VCM is preferably set to the potential (VDD+VSS)/2, which is intended to be established at the output  5  of the amplifier output stage when the amplifier is not amplifying any signals. 
     The control transistor TPS 3  and TNS 3  serves for establishing the voltage VCM of the voltage source  10  at a node  15  and  16 . Consequently, TPS 2 , I 2  and TNS 2  simulate the final stage  4  in the quiescent state with the output potential VCM and a quiescent current I 2 . In order to obtain the potential VCM at the nodes  15  and  16 , the control transistors TPS 3  and TNS 3  are dimensioned in such a way that the threshold voltage losses through the transistors TP 2 B and TN 1 B cancel one another out. 
     The current I 2  impressed by the adjustable current source is then set proportionally to the quiescent current IQ through the transistors TPAUS and TNAUS of the final stage  4 : 
     
       
           I 2= IQ/N.    
       
     
     Through the channel width/length ratio W/L of the MOSFETs TNS 2  and TNAUS, and TPS 2  and TPAUS, the factor N results as follows: 
     
       
           W/L ( TNS 2)= W/L ( TNAUS )/ N    
       
     
     
       
           W/L ( TPS 2)= W/L ( TPAUS )/ N.    
       
     
     In order to obtain a ratio of the transistors TNS 2  and TNAUS, and TPS 2  and TPAUS, which is as exact as possible, these transistors locally lie as close together as possible in an integrated circuit. Inaccuracies on account of technological fluctuations are thus largely avoided. 
     The current I 2  is in each case mirrored into a very small current through the mirror transistors TPS 1  and TNS 1  and a voltage VGTPS 2  and VGTNS 2  is established at nodes  12  and  11 , which voltage corresponds to the control voltage for the transistors TPAUS and TNAUS of the final stage  4 , in order to obtain a quiescent current IQ through the transistors of the final stage. To that end, the following must hold true for the channel width/length ratio W/L of the MOSFETs TNS 1  and TNS 2 , and TPS 1  and TPS 2 : 
     
       
           W/L ( TNS 1)&lt;&lt; W/L ( TNS 2) and  W/L ( TPS 1)&lt;&lt; W/L ( TPS 2).  
       
     
     The first operational amplifier OPP compares the voltage VGTPS 2  with the voltage VDTP 1 B of the non-driven source follower circuit TP 2 B, RPB, TP 1 B and, via the operational output of the amplifier which is fed back to the control terminal of the transistor TP 1 B, sets the voltage VDTP 1 B like the voltage VGTPS 2 . The feedback output voltage is used, via a terminal  720 , for controlling the current source TP 1 A of the preliminary stage  2 . In this way, the voltage VGTPS 2  is transferred to the transistor TPAUS of the final stage  4 , where it sets the desired quiescent current IQ. 
     The second operational amplifier OPN compares the voltage V GTNS2  with the voltage V DTN2B  of the non-driven source follower circuit TN 2 B, RNB, TN 1 B and, via the output of the operational amplifier which is fed back to the control terminal of the transistor TN 2 B, sets the voltage V DTN2B  like the voltage V GTNS2 . The feedback output voltage is used, via a terminal  710 , for controlling the current source TN 2 A of the preliminary stage  2 . In this way, the voltage V GTNS2  is transferred to the transistor TNAUS of the final stage  4 , where it sets the desired quiescent current IQ. 
     In the case of low supply voltages VDD and VSS or high absolute values of the transistor threshold voltages, the resistors RNA, RNB, RPA and RPB can be omitted. 
     Consequently, the setting of the quiescent current IQ through the final transistors TPAUS and TNAUS depends exclusively on an offset voltage of the two operational amplifiers OPN and OPP of the control device  3  and the accuracy of a factor N resulting from the ratio of the transistors TNAUS and TNS 2 , and TPAUS and TPS 2 . 
     By virtue of the fact that the offset voltage of the two operational amplifiers is very low relative to the voltage at the gate terminals of the transistors TN 2 B and TN 2 A, and TP 1 B and TP 1 A (a few mV or 1-2 orders of magnitude smaller than the gate voltages), the influence of the offset voltages on the accuracy of the quiescent current regulation is negligible. 
     Temperature fluctuations likewise have only little effect on the quiescent current since, particularly in the case of integration of the amplifier output stage, the temperature differences between the individual transistors are negligible on account of the local proximity to one another. 
     In addition to the transistors of the input stage and of the final stage, only the transistors TN 1 A and TP 2 A of the preliminary stage  2  lie in the signal path of the amplifier. The resulting pole in the transfer function of the amplifier only slightly influences the bandwidth of the amplifier since the pole is at very high frequencies. 
     In order to obtain a low current consumption of the amplifier, the quiescent current IQ through the transistors of the final stage is set to be as small as possible. The quiescent current IQ in the final stage is set freely by setting the current I 2  of the adjustable current source, which is a fraction of the quiescent current IQ.