Abstract:
A differential amplifier may include a first stage including a first transistor and a second transistor having the same polarity and assembled to constitute a differential amplifier. The first stage may be supplied by first and second mirror current sources. The differential amplifier may further include a common mode control circuit, which may include two inputs receiving a reference voltage VCM and a common mode voltage controlling the first and second mirror current sources, respectively. The differential amplifier may further include a Miller gain stage having inputs and for a setting gain-band product. The differential amplifier may further include an unlocking circuit, inserted between the common mode voltage and the Miller gain stage inputs, to cause the Miller gain stage to conduct on circuit start-up.

Description:
FIELD OF THE INVENTION 
     The invention relates to semiconductor amplifier circuits and, more particularly, to a differential amplifier circuit with an unlocking device. 
     BACKGROUND OF THE INVENTION 
     Differential amplifiers are widely used in the telecommunications field. They allow for processing of weak signals conveying voice signals, and, more generally, data. A differential structure is particularly preferred in data transfer networks of wired networks (Wide Area Network) found in Asynchronous Transfer Mode-type (ATM) networks or Asynchronous Digital Subscriber Line-type (ADSL) networks as well as their principal derivatives HDSL (commonly designated by the generic term XDSL). 
     Generally, a differential structure has the effective advantage of eliminating harmonics and second-order non-linearities in distortion noise. Furthermore, a differential structure ensures greater immunity to common mode interference, such as the interference that power supply circuits of electronic circuits experience. Differential amplifier circuits, particularly those using complementary-type MOS type transistors, which are used in bi-CMOS technology, frequently use power supply sources. Such power supply sources often experience difficult start-up transients and blocking phenomena preventing the amplifier from operating. 
     Circuits for overcoming this problem and avoiding power supply source blocking in a semiconductor circuit are known. French patent application No. 2,767,976 entitled “Dispositif d&#39;aide au démarrage pour une pluralité de sources de courant” discloses such a start-assisting device. The start-assisting device which, though not specifically adapted to an amplifier structure, is useful to supply devices for microprocessors and electronic apparatus. This device comprises a start-assisting device, which provides power supply sources with a start current for a transient period, until a steady state can be established. A complementary inhibiting device is thereafter required for steady-state operation. 
     Although differential amplifiers are especially important in integrated circuits used in telecommunications, it is still desirable to design start-assisting devices that are particularly adapted to their structure and can be easily and cost-effectively manufactured without requiring the addition of new circuits. However, a differential amplifier structure that directly and easily incorporates a suitable unlocking circuit is not yet available. 
     SUMMARY OF THE INVENTION 
     The invention is aimed at providing a differential amplifier structure having an efficient, simple unlocking device, which only requires the addition of a very limited number of other components. 
     Another object of the invention is to provide an unlocking device which is adapted to the architecture of a differential amplifier to be integrated into an integrated circuit, and, in particular, that uses CMOS-type components of bi-CMOS technology. 
     Another object of the invention provides a differential amplifier structure, which is adapted to telecommunication network requirements and, particularly, ADSL or HDSL-type links. 
     The invention achieves these objects through a differential amplifier structure having a first stage including first and second transistors of identical polarity, such as NMOS-type transistors, which are assembled to provide a differential amplifier. The first and second transistors are fed by first and second mirror current sources respectively, which are controlled by a control circuit supporting common mode. The common mode control circuit has two inputs receiving a reference voltage V CM  and a voltage representative of the common mode voltage of the amplifier. 
     A second Miller stage comprises third and fourth transistors of an opposite-type from the former transistor, for example PMOS. The inputs of the third and fourth transistors receive output signals from the first stage, which are used to increase the open loop gain of the amplifier circuit and to set the gain-bandwidth product of the amplifier. An additional unlocking circuit is inserted between the common mode voltage and the Miller stage inputs. The additional unlocking circuit causes controlled conduction of the third and fourth PMOS-type transistors until the common mode voltage reaches a value which is significantly close to the reference value. 
     Thus, it is ensured that the Miller stage is set for conducting and, hence, the first and second current sources are set for conducting, which ensures the amplifier operation. Once the common mode voltage rises back up to a normal operational value, the unlocking circuit locks and re-establishes impedance between the Miller stage output and the common mode voltage, thereby enabling the amplifier to operate in a linear mode. 
     Preferably, the unlocking circuit is provided by two MOS-type transistors causing short-circuiting between the common mode voltage and the Miller stage inputs. The drain terminal of each NMOS transistor (which can also be NPN transistors) or unlocking transistor is connected to a corresponding gate of one of the third and fourth transistors. Furthermore, both unlocking transistors have a source connected to the terminal representative of the common mode voltage. The gates of the unlocking transistors are connected to a reference voltage V CM  and cause the Miller stage transistors to conduct when the true value of the common mode is significantly smaller than the reference value V CM . 
     In a particular embodiment, the amplifier may comprise a cascade stage comprising, for instance, bipolar transistors. More particularly, the differential amplifier may comprise a first stage comprising a first and a second NMOS-type transistor (which can also be NPN-type transistors) assembled as a differential pair. The gates of the first and second NMOS-type transistors may receive input signals via feedback resistors from a common source connected to a third current source. 
     The differential amplifier may further comprise a second Miller gain stage comprising third and fourth PMOS-type transistors, for example. The third and a fourth PMOS-type transistors may be assembled as a common source, each associated with a current source and a capacitor. The second Miller gain stage output may be connected to output terminals and having one input. 
     The differential amplifier may further comprise fifth and sixth PMOS-type transistors, for instance. The fifth and sixth PMOS-type transistors provide the first and second current sources, respectively, feeding the first and the second NMOS-type transistors, which form the differential pair of transistors. The fifth and sixth transistors may be assembled as a common source and each having a drain. The drains of the fifth and sixth transistors are connected to corresponding drains of the first and second transistors in the differential pair, as well as a gate controlled by the common mode supporting circuit. 
     The differential amplifier may further comprise an unlocking circuit comprising seventh and eighth NMOS-type transistors, for example, having gates that receive a reference value V CM . The unlocking circuit may short-circuit the common mode voltage and the gates of the third and fourth transistors, which form the Miller stage when the common mode voltage goes below the reference value V CM . 
     The invention is especially adapted to designing wide-band amplifiers used in wired telecommunications networks, and, more particularly, to wired telecommunication networks found in the Asynchronous Digital Line Subscriber-type networks and their derivatives. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates an exemplary differential amplifier circuit comprising a Miller gain stage, which may be advantageously provided with an unlocking circuit, according to the present invention; 
     FIG. 2 illustrates a differential amplifier structure shown in FIG. 1 including the unlocking circuit, according to the present invention; 
     FIG. 3 illustrates a second embodiment of the differential amplifier shown in FIG. 2 including an additional cascade circuit, according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates an example of a differential amplifier comprising a Miller gain stage and which can be advantageously provided with the unlocking circuit described below. The differential amplifier comprises a pair of NMOS-type transistors  10  and  20 . Although the preferred embodiment will describe the use of NMOS-type transistors to form the differential pair, it is clear that those skilled in the art will readily adapt the structure to an architecture in which the differential pair will be based on PMOS-type transistors. The amplifier is fed by a power supply source supplying a voltage V dd . 
     The source electrodes of NMOS transistors  10  and  20  are connected to a current source  1  (I 3 ) which, in turn, is connected to ground or other reference voltage. Each transistor of the differential pair  10  and  20  is fed through its drain by a current source based respectively on a PMOS transistor  11  and a mirroring PMOS transistor  21 . The source and drain terminals of transistor  11  are connected to supply terminal V dd  and to the drain terminal of transistor  10 , respectively. The source and drain terminals of transistor  11  are connected to supply terminal V dd  and to the drain terminal of transistor  20  respectively. 
     Transistors  11  and  21  are mounted in a current mirror configuration cooperating with a common mode supporting stage, which comprises a second differential pair associated with a current source  2  (I 4 ) and a PMOS-type transistor  5 . More particularly, the second differential pair comprises two transistors  3  and  4 , sources of which are connected to a current source  2  (I 4 ) which, in turn, is connected to ground. The drain electrodes of transistors  3  and  4  are connected to the drain of transistor  5  and supply terminal V dd , respectively. 
     The gate of transistor  3  is connected to the midpoint of a resistive bridge, comprising two resistors  17  and  27  generally of equal values, the ends of which are connected to output terminals O 1  and O 2  of the differential amplifier. The bridge resistors  17  and  27  are used to obtain, at the midpoint, a potential representative of the common mode value of the differential amplifier outputs O 1  and O 2 . The gate of transistor  4  receives a reference voltage, V CM , which is used to set the common mode stage bias level and which is generally set to V dd /2 to obtain a maximum dispersion output signal at terminals O 1  and O 2 . 
     The gates of transistors  5 ,  11  and  21  are all connected together. The gate and drain of transistor  5  are connected to each other, thus ensuring it operates within the square zone of its characteristic I (V GS ). Thus, the transistors are mounted in current mirror configuration and a same drain current flows through them because, as they are substantially identical, they undergo the same variations of gate-source voltage V GS . 
     The differential pair formed by transistors  10  and  20  is used as a first stage for a second Miller gain stage, which is comprised of a pair of PMOS-type transistors  12  and  22  assembled as a common source. More precisely, transistor  10  has its drain connected to the gate of transistor  12 , the source of which is connected to supply terminal V dd . Similarly, transistor  20  has its drain connected to the gate of transistor  22 , the source of which is connected to supply terminal V dd . Transistor  12  has its drain connected to a current source  14  connected in turn, at the other end, to ground. Similarly, transistor  22  has its drain connected to a current source  24  connected in turn, at the other end, to ground. The transistor  12  has its drain also connected to the output terminal O 2  of the differential amplifier. Similarly, the transistor  22  has its drain also connected to the output terminal O 1  of the differential amplifier. 
     A pair of capacitors  13  and  23  complete the Miller structure. The capacitor  13  is connected between the drain and source of transistor  12 . Similarly, the capacitor  23  is connected between the drain and source of transistor  22 . The capacitors are calibrated to set the gain-bandwidth product of the corresponding Miller stage. It is to be noted that the latter is designed to operate in class A, current sources  14  and  24  will therefore be calibrated accordingly to discharge the current in the amplifier load. 
     Associating the differential pair of transistors  10  and  20  and the Miller gain stage transistors  12  and  22  ensures a particularly high open loop gain for all of the amplifier and further helps to set its gain-bandwidth product. Feedback resistors  15  (R 1 ),  16  (R 2 ),  25  (R 3 ) and  26  (R 4 ) set the open loop gain to the desired value which is R 1 /R 2 =R 3 /R 4 . More precisely, as is shown in FIG. 1, resistors  15  and  16  together form a resistor bridge, the ends of which are connected to output terminal O 1  and input terminal E 1  of the differential amplifier, respectively. The midpoint of the resistor bridge is connected to the gate of transistor  10 . Similarly, resistors  25  and  26  define a bridge, the ends of which are connected to output terminal O 2  and input terminal E 2  respectively, and the midpoint of which is connected to the gate of transistor  20 . 
     As shown in FIG. 1, the common mode supporting circuit allows setting common mode voltages about the reference value, that is to say V CM =V dd /2. Indeed, it can be seen that should the potential of one of the outputs increase for any reason, for instance a circuit temperature rise, the increase would affect the midpoint of the resistive bridge  17  and  27 , causing a corresponding voltage increase in the gate of transistor  3 . A flow of current would then flow through transistor  3  because the additional gate voltage of transistor  4  would still be set to the unchanged value of reference V CM . Currents in transistors  11  and  21  would then be modified to cause the output voltage to go back to the reference value. 
     It is noted that, at start-up, such a circuit can stay locked because of blockage of the current sources embodied by transistors  11  and  21 . As a matter of fact, at start-up, voltages at terminals E 1 , E 2 , O 1  and O 2  are all set to ground, which corresponds to a constant load. In such a case, current I 4  delivered by source  2  is entirely derived only by transistor  4  and no current flows through transistor  3 . All other transistors  5 ,  11 ,  21  are then blocked and the amplifier cannot operate. Similarly, source  1  stays blocked. 
     Even when the voltages at terminals E 1  and E 2  reach a suitable value, for example V dd /2, because of the amplifier preceding stage receiving a steady load, for instance, transistor pair  10  and  20  may become blocked. This can be the case with some configurations of feedback resistor values R 1 , R 2  and R 3 , R 4 , in particular, when the amplifier is used as a voltage attenuator. Therefore, having a common mode potential on both input terminals E 1  and E 2  may not be enough to ensure conduction of transistors  3 ,  5 ,  11  and  21  and allow the amplifier to operate in a linear mode. 
     To avoid a current source I 1  blockage, which would impede the overall operation of the amplifier, a very simple and efficient unblocking device is incorporated which will now be described with reference to FIG.  2 . For the sake of clarity, elements with common numerals from FIG. 1, will keep the same numerals in FIG.  2 . The amplifier again comprises a differential pair of transistors  10  and  20 , common mode supporting stage  3  and  4 , and a Miller gain stage including transistors  12  and  22 . 
     To ensure conduction of transistors  12  and  22 , the circuit has an additional pair of NMOS-type transistors connected between the midpoint of the resistive bridges  17  and  27  to follow the level of output potentials O 1  and O 2  in common mode and, the transistor gates of the Miller gain stage. As a consequence, at start-up, the transistor gates enable conduction of the Miller stage. For that purpose, as shown in FIG. 2, a first NMOS-type transistor  18  is provided, the drain of which is connected to the gate of transistor  12 , the source of which is connected to the midpoint of bridge resistors  17  and  27  tapping the common mode voltage. 
     A second NMOS-type transistor  28  having a drain connected to the gate of transistor  22  and a source connected to the midpoint of bridge resistors  17  and  27 , measuring the common mode value, is also provided. The gates of both transistors  18  and  28  are common and receive the reference voltage V CM . So when potentials of output terminals O 1  and O 2  are abnormally low, which is the case during start-up, NMOS-type transistors  18  and  28  are conductive and thus set corresponding Miller stage transistors  12  and  22  respectively to conduction. Therefore, a rise of the common mode potential is ensured. Consequently, a start-up current for transistor  5  and current mirrors formed by transistors  11  and  21  and finally in the differential pair of transistors  10  and  20 , is also achieved. 
     The disclosed unlocking circuit is found to be particularly efficient and, as is shown in FIG. 2, only needs two additional components. As a consequence, it can readily be incorporated in a semiconductor circuit. Further, it is noted that unlocking transistors  18  and  28  can be very small-sized transistors as a resistance amount of a few ohms is enough to ensure the Miller stage unlocking. Every differential amplifier can thus be provided easily and at a low cost with such a device, which automatically and simply self-inhibits as soon as a steady-state linear load is established. This represents an advantage over known unlocking systems. 
     Indeed it is noted that once the common mode potential reaches the reference value V CM , both transistors  18  and  28  are blocked thus allowing the amplifier to operate in linear mode. Here, no additional inhibiting circuit such as those in known systems is necessary. This unlocking device is especially adapted to the realisation of differential amplifiers and to incorporation thereof in integrated circuits. 
     FIG. 3 shows another embodiment for a differential amplifier structure further comprising a cascade circuit forming an impedance adapter between a differential pair of transistors  10  and  20  and a Miller gain stage. In this embodiment, the transistor  10  drain electrode is not connected directly to the transistor  11  drain electrode. Rather, an NPN-type bipolar transistor  19  is interposed between transistors  10  and  11 . More precisely, the transmitter and collector terminals of the NPN-type bipolar transistor  19  are connected to the transistor  10  drain terminal and the transistor  11  drain terminal, respectively. 
     Similarly, a bipolar transistor  29 , also of NPN-type, is interposed between transistor  20  and transistor  21 . More precisely, the transmitter and collector terminals of transistor  29  are connected to the transistor  20  drain terminal and the transistor  21  drain terminal, respectively. The bases of both transistors are connected to a resistor  7  dropping back to supply voltage V dd  and to a current source  8 , the opposite end of which is connected to a ground. 
     As will be apparent to people qualified in the art, the advantage of the cascade circuit is to provide large impedance at the first stage comprised of the pair of transistors  10  and  20  to further increase the amplifier&#39;s open loop gain. As can be seen, this disclosed amplifier circuit is perfectly adapted to bi-CMOS technology. Furthermore, note that any person qualified in the art could very easily adapt the structure in FIG. 3 to a cascade circuit comprised of NMOS-type transistors instead of bipolar transistors.