Differential output, power, CMOS, operational amplifier

Abstract: A power CMOS operational amplifier with a differential output (Vout-,Vout+), having an intrinsically stable current absorption under rest conditions, comprises two symmetric branches, each comprising a first folded cascode input inverting stage (M1-M10), a level shifting circuit (M11,M12), a second current mirror type noninverting amplifying stage (M13-M18) and a third output inverting stage (M19,M20), constituted by a complementary pair of transistors, connected in a common source configuration between the supply rails and driven by the output (A",B") of the second noninverting stage and by the output (A',B') of the level shifting circuit. Frequency compensation is accomplished by means of two capacitors (Cc1,Cc2) connected between each of the two output terminals (Vout-,Vout+) of the amplifier and the output (A,B) of the first inverting stage (M1-M10) and a node of the output branch of the noninverting current mirror stage (M13-M18). A single common mode feedback network (McM1-McM3) stabilizes both symmetric branches of the amplifier.

Description: FIELD OF THE INVENTION The present invention relates to a power operational amplifier, realized in CMOS technology and having a differential output. BACKGROUND OF THE INVENTION By utilizing a CMOS manufacturing process complex systems comprising analog portions and digital portions sharing the same supply rail (usually at 5V), may be combined in a single integrated circuit. The so-called power operational amplifier is often present among analog portions. This circuit is required for driving external loads of the integrated circuit (e.g. a loudspeaker or a telephone earpiece). These external loads may be characterized by a resistance as low as 50 Ohm and by a capacity as high as several hundreds picoFarad (pF). The operational amplifier must drive such a heavy load with a low distortion up to a frequency in the order of ten kiloHertz (kHz). To do so it must be able to deliver high current peaks in response to a signal and to have a low output impedance. Nevertheless it is desirable that, in absence of a signal, the circuit have a relatively low and well controlled current absorpion. Moreover the maximum signal voltage that can be supplied to the load should have a value as close as possible to the supply voltage (i.e. an output signal swing (dynamic) as large as possible). A further fundamental requirement is a good noise rejection on the supply rails. Finally it is important that the circuit occupy a relatively small integration area. In most cases, the operational amplifier is of the so-called single-ended output type, and in this most common configuration many types of amplifiers are known which generally employ an amplifying transconductance stage as the first or input stage, while a source follower stage or a push pull stage is used as output stage, as it is well known to a skilled technician. Many of these circuits are reviewed in the volume "Analog MOS Integrated Circuits For Signal Processing", edited by Gregorian, Temes. In an operational amplifier having a single-ended output any disturbance on the supply rails turns itself into a disturbance on the driven external load (this effect increases with an increase of the frequency of the disturbance itself). In a fully differential circuit, due to the physical symmetry of the structure, the disturbance is equally coupled with the two outputs and as a common mode event does not affect the load, even at very high frequencies. In other words a fully differential amplifier intrinsically possesses a very high rejection characteristic for disturbances on the supply rails and this assumes a particular importance in complex integrated systems, wherein quite considerable disturbances on the supply rails due to the switchings of the logic circuits integrated on the same chip may occur. The rare examples of known power CMOS amplifiers with a differential output are generally quite complex and, in spite of the intrinsic advantages of this type of operational amplifier, they haven't had widespread utilization. An example of known power CMOS amplifiers with a differential output is disclosed in the document DE-A-3725323 of the same applicant. The circuit comprises a dedicated circuit for controlling the rest or quiescent current and such a requirement of a specific control greatly complicates the circuit of the amplifier, in particular of the intermediate gain stage as shown in Fig. 5 of D1. Thus a demand and an utility exist for a CMOS power operational amplifier with a differential output, having a high allround performance and a relatively simple circuit easily integrable in a relatively small area. OBJECTIVE AND SUMMARY OF THE INVENTION This objective is attained by the present invention as it is defined in the appended claims whose object is a particularly simple power CMOS operational amplifier with a differential output, wherein a single and simple common mode feedback loop exerts a common mode control on all the cascaded stages of the operational amplifier. Moreover, the amplifier of the present invention has an intrinsically stable DC current level under rest conditions and thus does not need a dedicated control circuit for controlling this mechanism of instability, which usually is an effect of unavoidable asymmetries among elements forming pairs or groups of transistors which ideally should have been exactly similar among each other (i.e. offsets due to local differences inevitably introduced by the manufacturing process, process spread ). BRIEF DESCRIPTION OF THE DRAWINGS The different peculiar aspects and advantages of the circuit of the invention will become clear through the following description of an embodiment thereof and by reference to the attached drawings, wherein: Figure 1 is a complete circuit diagram of an operational amplifier made in accordance with to the present invention; and Figure 2 is a functional diagram of an external capacitor network, employed for controlling the amplifier's common mode. DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to Fig. 1 , the operational amplifier is formed by two essentially symmetrical branches. Each branch comprises a first input inverting stage of the so-called folded cascode type which is formed by transistors M0, M1, M2, M3, M4, M5, M6, M7, M8, M9 and M10. The terminals VB1, VB2, VB3, VB4 and VB5 are circuit biasing terminals to which a respective constant voltage may be applied. The operation of this type of differential transconductance input stage is well known to a technician skilled in the art. When in response to a signal applied between the two differential input terminals: V in+ and V in- , the input voltage V in+ increases and the voltage V in- decreases, an amplified voltage rise on the output node B and a symmetric voltage decrease on the output node A occur. Each of the two symmetric branches of the differential amplifier comprises also a level shifting circuit, which is formed by transistors M11A and M12A for one branch, and by M11B and M12B for the other branch. This level shifting circuit is essentially a source follower whose sole function is to shift the voltage level and therefore any signal present on a respective input node: A or B , is reproduced on the respective output node: A' or B'. In absence of this level shifting circuit the noninverting amplifying stage that follows would impose a DC voltage level on the output node of the first input inverting stage too low for a correct operation of the latter. The voltages of the output nodes of the level shifting circuits: A' and B' , are applied to the input nodes of second noninverting amplifying stages, respectively, which are formed by transistors M13A, M14A, M15A, M16A, M17A and M18A for one branch and by transistors M13B, M14B, M15B, M16B, M17B and M18B , for the other symmetric branch of the amplifier, respectively. This second stage is essentially a noninverting amplifying stage. In absence of an input signal the currents through the MOS transistors are all equal to each other as the current mirror formed by transistors M15 ( A or B ) and M16 ( A or B ) keeps identical the currents in the two branches of the circuit of these second stages. If the input voltage increases, the current through transistor M13 ( A or B ) and therefore also through transistor M15 ( A or B ) increases. This current is mirrored in transistor M16 ( A or B ), which thus conducts a current larger than the one supplied by the constant current generator M14 ( A or B ). Therefore also the voltage on the respective output node ( A" or B" ) of the second noninverting amplifying stage increases in function of the voltage on its input ( A' or B' ). The third amplifying stage, i.e. the output stage, is an inverting stage formed by a p-channel transistor ( M19A and M19B , respectively) and by an n-channel transistor ( M20A and M20B , respectively) connected in a common source configuration between the two supply rails of the circuit. The gate of the p-channel transistor is driven by the output signal of the second noninverting amplifying stage, while the gate of the n-channel transistor is driven by the output signal of the level shifting circuit, which is a replica of the output signal of the first inverting input stage. When the voltage at the input (nodes A' and B' ) of the second noninverting amplifying stage increases, the current through the n-channel transistor of the output stage, which is directly coupled with this input node ( M20 ), increases, while the current through the p-channel transistor M19 of the output stage, whose gate is connected to the output of the second noninverting amplifying stage, decreases. In fact, as said above, also the voltage of this output node increases as the output voltage of the first input stage thus causing a decrease of the voltage between the gate and the source of transistor M19 . Because of this unbalance between the currents through the p-channel and the n-channel transistors of the output stage, caused by the input signal, a current flow through the output node and the external load occurs. Because of the presence of three amplifying stages, the circuit has a very high overall gain. By virtue of the preceding amplifying stages, the gates of the transistors of the two symmetric output stages have very large voltage swings, close to the supply voltage. For this reason, the two output stages, if their physical structure is properly sized, may deliver a very large current to the load, in function of the input signal. The circuit of the differential amplifier may usefully comprise a common mode feedback circuit, of a type usually defined as "dynamic" type, which is extremely easy to realize. This common mode control circuit utilizes, as shown in Fig. 2 , two capacitors C1 and C2 connected in-series to each other and between the two amplifier's outputs terminals: V out+ and V out- . Connected in parallel to each of these two capacitors, there is a second capacitor, C s1 and C s2 , respectively, and each of these capacitors is periodically disconnected, by means of the integrated ( CMOS ) switches SW1, SW2 and SW3 , which are driven by a logic control signal, from the respective capacitor it is combined with and is charged to a predetermined common mode voltage at the VCM output of the amplifier minus a certain fixed bias voltage VB1 (this circuit is well known in the literature and is described in the text: "Design Of MOS VLSI Circuits For Telecommunication", by Tsividis, Antonietti, page 207). These pairs of capacitors are such that, if the average value of the output voltages V out+ and V out- changes, also the voltage of the node INCM , to which they are connected, varies of the same amount. This node, INCM , an acronym for Common Mode INput node of the amplifier, is constituted, as shown in Fig. 1 , by the gate of transistor MCM1 . This transistor, together with transistors MCM2 and MCM3 , forms a simple voltage inverter, whose output is connected to the gates of transistors M3 and M4 of the input stage (node VA ). When a common mode voltage increase occurs on the operational amplifier outputs, also the voltage of the node INCM increases, according to what has been said above. The voltage present on this node is inverted by the circuit formed by transistors MCM1 , MCM2 and MCM3 , and is applied to the gate of transistors M3 and M4 . By assuming an increase of the voltage of the node INCM , a decrease of the voltage applied to the gates of transistors M3 and M4 is obtained and, therefore, a decrease of the current flowing through these transistors. Since in absence of an input signal these currents balance those flowing through M9 and M10 , in case of a common mode unbalance, also an unbalance of these currents occurs. In the case considered, the decrease of the currents flowing through M3 and M4 makes the output nodes ( A and B ) of the first input stages raise in voltage and this voltage increase causes a similar voltage increase at the output nodes of the second noninverting amplifying stages, A" and B" , and a voltage decrease on the operational amplifier output terminals: V out+ and V out- . The assumed increase of the output voltages is thus compensated and nullified by the common mode feedback circuit which forces, as a consequence, a voltage decrease. A remarkable advantage of this solution is that the feedback circuit for controlling the amplifier common mode does not introduce any factor of nonideality in the amplifier response because is formed solely by capacitors, which are linear and nondissipating elements, and by a very simple voltage inverting circuit which may be easily designed so as to avoid the introduction of any disturbance. The latter circuit introduces merely a very high frequency pole whose effects are negligible. The frequency compensation networks of the differential operational amplifier employ the capacitors Cc1A and Cc2A for the first of the two symmetric branches and Cc1B and Cc2B for the second branch of the differential amplifier, respectively, as shown in Fig. 1 . For each of the two symmetric branches, the two capacitors are connected between the respective output node of the amplifier and the output node ( A or B ) of the input stage and the source of transistor M18 ( B or A ), respectively. The transistor M18 ( B or A ) of the output branch of the second noninverting amplifying stage, has its gate kept at a fixed voltage. This connection nection is original and differs from the known referred to as: "Miller's compensating nested connection", wherein the second capacitor of each pair ( Cc2A and Cc2B ) is connected, by contrast, to the output node of the respective second amplifying stage (see J.H. Huijsing: "Low Voltage Operation Amplifier With Rail To Rail Input And Output Range": JSSC, Dec. 85). The advantage of the frequency compensating network of the differential operational amplifier of the invention consists in that the pole at the output of the operational amplifier is advantageously shifted toward the high freqency range without any problem thus enhancing the amplifier stability. This arrangement, previously employed solely in operational amplifiers having only two amplifying stages (see D.B. Ribner, M.A. Copland: "Design Technics For Cascoded Op. Amps. With Improved PSRR And Common Mode Input Range"; JSSC, December '84), proves itself more effective if implemented in an amplifier having three amplifying stages, because the number of poles increases and thus becomes more important to increase as much as possible the frequency of the nondominating poles so that they do not affect the response. A further remarkable advantage of the amplifier of the invention is that it does not need any additional circuit for controlling the current level under rest conditions. Its peculiar circuit architecture ensures, in fact, an intrinsic control of the bias current level. As pointed out hereinabove, a bias current increase may be due, in certain integrated structures, to accidental asymmetries among elements of pairs or groups of transistors which ideally should be identical to each other (offsets due to the fabrication process spread). In the circuit of the amplifier of the instant invention, any kind of presumable offset will not produce a significant increase of the total power dissipation under rest conditions. In fact, a skilled technician will easily recognize, by analyzing the effects of different presumable offsets, that they will always cause opposite current variations in n-channel transistors and in p-channel transistors of the two symmetric output stages. Thus the excess current in any one of the two transistors must always flow through the external load thus causing a consequent variation in the output voltage and the intervention of the negative external feedback network which is always present between input and output nodes of any operational amplifier. In presence of an output unbalance this negative feedback causes an input unbalance which counteracts the output unbalance, thus restoring the proper current levels.

Claims: Power CMOS operational amplifier with differential output comprising two symmetric branches, each having a first differential inverting input stage composed by a source-coupled pair of input transistors (M1, M2) capable of producing on a respecting output node (A, B) a voltage signal replica of a voltage applied to the respective input (Vin+, Vin-) terminal of the differential amplifier, at least an intermediate gain stage, a power output stage composed of a p-channel transistor (M19A, M19B) and by a n-channel transistor (M20A, M20B) functionally connected in a common source configuration between supply rails of circuit with their drains connected in common to a respective differential output terminal (Vout+, Vout-) of the amplifier, and a common mode control feedback network, characterized by the fact that said intermediate gain stage comprises a level shifting circuit (M11A, M12A, M11B, M12B), having an input coupled to the output node (A, B) of said first stage and an output node (A', B'), capable of outputting a replica of the voltage signal present at its input, shifted in level by a predetermined amount; a noninverting amplifying stage comprising a current mirror (M16A, M15A, M15B, M16B) having an input branch which comprises at least one transistor (M13A, M13B), whose gate constitutes an input and is connected to the output (A', B') of said level shifting circuit, and an output branch which comprises at least a transistor (M18A, M18B) having a drain connected to an output node (A", B") of the stage for producing on said output node an amplified voltage replica of the output voltage produced by said level shifting circuit; the gate of the n-channel transistor (M20A, M20B) of said power output stage being driven by the output signal produced bv said level shifting circuit and the gate of the p-channel transistor (M19A, M19B) of said power output stage being driven by the output node (A", B") of said noninverting amplifying stage; a frequency compensating network formed by a first capacitor (Cc1A, Cc1B) connected between said differential output terminal (Vout+, Vout-) and said output node (A, B) of said first input inverting stage and by a second capacitor (Cc2A, Cc2B) connected between said differential output terminal (Vout+, Vout-) and a source of said transistor (M18A, M18B) of the output branch of said noninverting amplifying stage; said common mode control feed back loop being single (MCM1, MCM2, MCM3) and exerting a control on all the cascaded stages of the amplifier. Power CMOS operational amplifier according to claim 1, characterized by the fact that said feedback network for common mode control is formed by: a first (C1) and a second capacitor (C2) connected in series to each other between said differential output terminals (Vout+, Vout-) of the amplifier; a third (Cs1) and a fourth (Cs2) capacitor intermittently connected in parallel to said first (C1) and to said second (C2) capacitor, respectively, by means of switches (SW1, SW2, SW3), driven by a logic control signal, and, alternately, to three terminals to which are functionally applied a preset common mode constant reference voltage (VCM) and a constant reference bias voltage (VB1); an intermediate node of said series connected capacitors (C1, C2) being connected to a common mode input terminal (INCM) of the amplifier, which is coupled to the gate of a first input transistor (MCM1) of a voltage inverting circuit (MCM1, MCM2, MCM3), whose output is coupled to the gates of two output transistors (M3, M4) of the first inverting input stages of the differential amplifier, for compensating, through an increase or through a decrease of the current flowing through said output transistors (M3, M4) of the first inverting input stage, a decrease or an increase of the voltage present on said common mode input terminal (INCM) of the amplifier. Power CMOS operational amplifier according to claim 1, wherein both said inverting input stages have a folded cascode configuration. Power CMOS operational amplifier according to claim 1, wherein said transistor of the output branch of the current mirror (M18A, M18B) has a gate held at a fixed voltage (VB1).

Inventors: NICOLLINI GERMANO, PERNICI SERGIO, NICOLLINI, GERMANO, PERNICI, SERGIO
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1991
Main classification: H03F3
IPC codes: H03F1, H03F3