Abstract:
Distortion control in a push-pull output stage of a speech amplifier of a telephone powered through the telephone line is more effectively and advantageously implemented by independently sensing an eventual state of saturation reached by any of the two output transistors of the amplifier, summing the current signals representative of the sensed state of saturation of either or both output transistors, integrating the resulting sum current signal to produce a DC signal and using the DC signal for activating an AGC loop. The DC signal indiscriminately accounts for any cause of saturation, though virtually representing the level of the amplified AC signal. Distortion may be controlled without penalizing output voltage swing and power consumption.

Description:
This application is a continuation of application Ser. No. 08/339,122, filed Nov. 10, 1994 and now abandoned. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Italian app&#39;n VA/93/A/0027, filed Nov. 12, 1993, which is hereby incorporated reference. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     The present invention relates to an automatic distortion control system for an amplifier, the effectiveness of which is substantially independent from variations of the supply voltage, from the load level and from the process spread, while not requiring any substantial reduction of the output dynamic characteristics of the amplifier. The system of the invention is particularly though not exclusively useful in speech circuits of telephones powered through the telephone line. 
     In many known applications such as in line-powered telephones containing an amplifier that drives a loudspeaker, automatic control of distortion is required, which is commonly implemented by an automatic gain control system (AGC). The ability to deliver the maximum power to the load, compatible with the supply voltage and current, is an important attendant requirement of a power amplifier. 
     In telepowered systems, as for example in common telephones, the functional circuits of the apparatus are powered directly by the telephone (signal carrying) line when the handset is lifted from its cradle, thus closing a supply switch. In this activated condition, a current may therefore flow through the speech circuits of the apparatus. In terms of continuous current (DC), the speech circuit of a telephone may, in a first approximation, be considered similar to a resistance connected in series with a battery. On the other hand, a telephone line may be considered as a series of RC networks, the number of RC cells being a function of the length of the telephone line. As a consequence, the DC current that flows through a remote telephone is a function of the local substation&#39;s battery, of the electrical characteristics of the telephone line as well as of the speech circuit that is powered through the line. This in turn determines a certain supply voltage across the speech circuit of the apparatus. 
     Commonly, the DC current that is absorbed from the telephone line is divided by a dedicated current divider circuit in a priority mode in order to ensure as far as possible correct operation and functionality of the telephone as a whole. In more sophisticated telephones, a part of the current drawn from the telephone line may be used for powering an amplifier that drives a loudspeaker, for implementing so-called &#34;monitor&#34; and &#34;amplified listening&#34; functions. 
     The &#34;monitor&#34; function (monitoring of the state of the line) becomes operative when the user, without unhooking the handset but by pressing a button, closes an activation switch of the telephone and thereafter dials the required telephone number. In these conditions, the amplifier is powered and through the loudspeaker embodied in the telephone, it is possible to acoustically recognize the state of the line. 
     The &#34;amplified&#34; listening function becomes operative when, having already established a connection by lifting the handset from its cradle, it is desired to let bystanders listen to the dialogue taking place between the two telephone users. 
     Of course, along the telephone line, the telephone communication (audio) signals TX and RX are exchanged, superimposed on the DC current. 
     It is evident that in telepowered systems it is particularly important to minimize power consumption while ensuring the maximum possible dynamic swing of the undistorted amplified signal. 
     In order to optimize the dynamic output characteristics of a telepowered amplifier, an output power stage may be used which is composed of a complementary pair of bipolar transistors PNP/NPN, functioning in phase opposition. The pull-up element of the output stage, that is the PNP transistor, notably requires a relatively large area of integration because of its intrinsic low current density of operation. For this reason, the PNP transistor is often replaced, in integrated circuits, by a composite PNP+NPN structure, which can be virtually considered as a PNP transistor. In this case, the output voltage swing is reduced by about a VBE (base emitter voltage), a consequence that is often accepted as a viable compromise. 
     The systems for dividing the total current that can be absorbed from the telephone line implement a priority system which allocates current according to functional requirements. 
     In most systems for dividing the supply current drawn from the telephone line, the supply voltage of a particular functional circuit (e.g., an audio amplifier that drives a loudspeaker) is established by a shunt regulator to a certain value that is correlated to the DC voltage on the telephone line. The regulated supply voltage is developed across a supply tank capacitor. A system of this type is described in the article: &#34;A single chip BIMOS telephone set&#34; by C. Nguyen et al., 1989 ISSCC DIGEST 254. A particularly suitable circuit for splitting a supply current is described in the European Patent Application 92830498.9, filed on Sep. 28, 1992 by the same applicant. 
     In these telepowered systems, it may occur that in presence of a particularly strong AC signal at the input of the amplifier, the total electrical load (loudspeaker +amplifier) requires a higher current than the fraction of line current that is eventually diverted to the shunt regulator that charges the supply tank capacitor of the amplifier-loudspeaker block. As a consequence, the voltage across the tank capacitor will drop and, for the same gain of the amplifier, a strongly distorted output signal may be produced because of the saturation of one and/or the other transistor of the push-pull output stage of the amplifier, in coincidence with positive and/or negative peaks of the AC signal. 
     Another eventuality is that the transistors of the output stage may saturate (in correspondence with the AC signal peaks) when the voltage across the supply tank capacitor of the amplifier is insufficient for allowing the correct output dynamics (for a given gain of the amplifier and amplitude of the input AC signal), even though the total current absorbed by the amplifier and its load remain lower than the current delivered to the amplifier by the current splitting system of the DC current drawn from the line. 
     In order to control the distortion caused by the saturation of the output stage transistors, an automatic gain control system (AGC) of the amplifier is normally employed. 
     FIG. 1 shows a block diagram of an amplifier provided with an automatic gain control (AGC) according to a known technique. 
     The amplifier shown is essentially constituted by a gain block G1 and by an output power block G2. The automatic gain control, suitable to prevent saturation of the transistors of a push-pull power stage of the G2 block, is implemented by detecting the AC signal at the output of the pre-amplifier G1, comparing the DC voltage developed across an integrating RC network with a reference voltage V REF  and generating a AGC difference signal which is used for modifying the gain of the pre-amplifier G1. 
     The system is capable of preventing saturation of the transistors of the output stage of G2. On the other hand, such a system essentially acts on the AC signal level and is unable to consider other parameters such as variation of the gain of the output block G2, offset voltage, the load level, which in a telepowered system, as mentioned above, may have a direct influence on the supply voltage V LS , as well as the so-called fabrication spread, temperature of operation, etc. In practice, the reference voltage V REF  must be fixed in the design phase, by taking into account a hypothetical concurrence of more factors that could limit the maximum output voltage swing and therefore it must necessarily be fixed conservatively at a value such as to exclude saturation of the output transistors also in one of the worst hypothesis of operating conditions of the amplifier. This inevitably limits the maximum output voltage swing that could be practicable otherwise. 
     There is the need or utility for an automatic distortion control system that will not necessarily require a preventive limitation of the dynamic characteristics of a telepowered amplifier. 
     This objective is fully met by the automatic gain control system of the present invention. 
     Basically the system of the invention consists in independently detecting an eventual state of saturation of any one of the two transistors of a push-pull pair of the output stage of the amplifier and generating a current representative of the state of saturation reached by a respective transistor. The currents thus independently generated are summed and the resultant sum current, suitably processed, is used for charging an integrating RC network. Across the integrating network a DC signal develops, that is virtually representative of the level of the AC signal amplified by the amplifier, whenever one or the other of the output transistors enters saturation. Therefore, a gain regulation loop for reducing the gain of a pre-amplifier section of the amplifier is activated only upon the occurrence of saturation conditions of one or the other or both output transistors of the power stage, irrespectively of the cause that has determined saturation thereof. 
     The advantages that are achieved with the novel system of the invention are evident. Eminently, the output dynamic characteristics of the amplifier are maximized under any phase of operation, with a direct positive effect on power consumption. In practice, all variables that may directly or indirectly cause distortion are effectively counter-balanced upon the occurrence of a still negligible distorting effect by the particular automatic gain control system object of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The different aspects and advantages of the invention will become more evident through the following description of several important embodiments and by referring to the annexed drawings, wherein: 
     FIG. 1 is a functional block diagram of an amplifier provided with an AGC system, according to a known technique as described above; 
     FIG. 2 is a functional block diagram of an automatic gain control system of an amplifier according to the present invention; 
     FIGS. 3A and 3B are diagrams of a circuit for the automatic control of distortion according to the present invention. 
     FIG. 4 is a circuit diagram for an adaptation of the circuit to MOS power output. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The numerous innovative teachings of the present application will be described with particular reference to the presently preferred embodiment (by way of example, and not of limitation), in which: 
     With reference to the block diagram of FIG. 2, an amplifier is composed of a gain stage G1 and an output stage G2. The inventive system for controlling distortion in the amplifier is based on detecting saturation of the transistors of the amplifier&#39;s push-pull output power stage, and generating a current representative of an eventual state of saturation reached by either or both transistors. 
     The generated currents I SH  and I SL  are summed in the block Σ and the resulting sum current I SENS  is fed to an automatic gain control block AGC. The current I SENS , suitably processed, determines the charge through a current I EA , which may be constant, of an integrating RC network. Across the integrating network a DC signal develops, used by the AGC block as a signal virtually representing the level of the AC signal amplified by the amplifier. In practice, the voltage across the integrating RC network is compared with a reference voltage V REF , in order to produce a signal &#34;difference&#34; or AGC signal, which is used for modifying the gain of the gain stage G1. 
     In other words, the system of the invention intervenes whenever one or the other or both the transistors of the push-pull output power stage enters a stage of saturation thus determining, through an AGC control loop, a correlating reduction of the total gain of the amplifier, sufficient to re-establish linear operating conditions of the output transistors. 
     Of course, the time of response of the self-regulating loop may be extremely fast so as not to produce detectable (audible) effects. Response times, as well as other operating parameters may be regulated either by modifying the gain of the gain stages of the current I SENS  and/or by modifying the time constant determined by the integrating RC network and/or by modifying the reference voltage V REF  of the AGC regulating loop. 
     The circuit diagram of a typical amplifier output stage, suitable for driving a load that may be represented by a loudspeaker, is shown in FIGS. 3A and 3B. Commonly the output stage may comprise a driving stage capable of driving a pair of complementary power transistors PW H  and PW L , according to a common push-pull scheme. 
     In the given example, the upper or &#34;pull-up&#34; transistor PW H  is in fact realized in integrated form as a composite structure that comprises a PNP transistor and an NPN transistor, as depicted in the Figure. Electrically, the composite structure behaves as an &#34;equivalent PNP&#34; transistor, the respective emitter (E), base (B) and collector (C) nodes of which are identified in the Figure. This integrated circuit solution, which sensibly reduces the integration area on silicon, implies the use of a bias current generator IP for the PNP of the composite structure, as well as of a driving stage for the power NPN, constituted by the transistor Q 1  and by the relative bias generator IQ1. 
     The diagram also shows base resistance, R bH  and R bL  that may be advantageously added in order to enhance the rising of the collector voltage above the base voltage for the PNP transistor of the equivalent composite structure of PW H , and the fall of the collector voltage below the base voltage for the NPN transistor PW L , upon the reaching by the relative transistor of a saturation condition. 
     The reaching of a saturation condition of a respective power transistor of the output push-pull stage of the amplifier is independently detected by the transistor Q 3  (for the transistor PW L ) and by the transistor Q 2  (for the equivalent transistor structure PW H ). Both detecting transistors Q 2  and Q 3  are in an OFF condition as long as the respective transistor of the complementary output pair remains in a linear operating condition; being kept in an off condition by reverse biasing their base-emitter junction. 
     When one of the power transistors enters saturation, the voltage difference between base and collector thereof inverts its sign, thus bringing to a conducting state the respective transistor (Q 2  or Q 3 ). The currents that are eventually generated by the saturation detecting transistors Q 2  and Q 3  are summed on the sum node d, by mirroring through Q 4  and Q 5  the current of Q 3  on the summation node d. 
     Therefore, the resulting sum current I SENS  constitutes therefore an information on the saturation state eventually reached by either or both output transistors in coincidence with positive and/or negative peaks of an AC signal fed to the input of the amplifier. Such an information (I SENS ) may therefore be assumed to correspond to an information on the level of the AC signal and, as such, may be input to an automatic gain control block AGC. In practice, the signal I SENS  enables, through the transistor Q 6  and Q 7 , the propagation of the current IQ7 through the current mirrors Q 8  -Q 9  and Q 10  -Q 11 , in order to produce a current I EA  that can be used for charging an integrating RC network suitable to convert the current signal into a DC voltage signal (developed across the integrating RC network). Such a signal can be compared with a reference voltage V REF  purposely input to the AGC block (see the functional block diagram of FIG. 2). 
     FIG. 3B shows the presence of information coming from a shunt regulator that fixes the V LS  voltage. I SENS  is an indicator that will be present every time the final stage tends to saturate causing a strong distortion of the signal (clamp) and therefore the circuit intervenes to reduce the output dynamic. Such a distortion could be present when the voltage across the buffer capacitor is insufficient to allow, for the same gain of the amplifier and amplitude of the input AC signal, a correct undistorted output dynamic. 
     Reduction of the output dynamic is also necessary when the current in the shunt regulator tends to reduce itself to zero due to the effect of the total load (a speaker plus amplifier). In this case, through the I EA  pin an indication of &#34;shunt current sensing&#34; is provided and results in a reduction of the output dynamic, with a consequent reduced absorption from V LS . 
     In a typical embodiment of the invention in a telephone set, such as depicted in FIGS. 3A and 3B, the local supply voltage V LS  may vary between 2V MIN  up to a voltage compatible with the fabrication technology of the integrated circuit. 
     With normal fabrication technologies, the current absorption by the amplifier may be minimized to about 300 μA by using a class AB mode of operation of the output stage. The current through the load (loudspeaker) depends on the current availability from the telephone line, normally corresponding to about a maximum current absorption of 100 mA. 
     The maximum output voltage swing of the amplifier when a composite integrated structure PNP-NPN is used for implementing an equivalent PNP transistor PW H  (FIG. 2), is given by 
     
         V.sub.LS -(1V.sub.BE +2V.sub.CESAT). 
    
     Total distortion is lower than 1%. 
     The total distortion is independent of process spread, temperature, loading impedance, and intrinsic gain changes, as well as from a lowering of the supply voltage V LS  and/or a reduced availability of DC current from the telephone line. 
     FIG. 4 discloses an alternative embodiment of the invention, utilizing a MOS power output stage. In this instance, the circuits for detecting the state of saturation are of course modified with comparator circuits which activate SW1/SW2 every time the V drain-source voltage of the power MOS drops below V ref1  /V ref2 . The activation of the switches SW1/SW2 permits the generation of I SENS  which will then be processed as in the case of a bipolar implementation. 
     As will be recognized by those skilled in the art, the innovative concepts described in the present application can be modified and varied over a tremendous range of applications, and accordingly the scope of patented subject matter is not limited by any of the specific exemplary teachings given. For example, as will be obvious to those of ordinary skill in the art, other circuit elements can be added to, or substituted into, the specific circuit topologies shown.