Abstract:
A circuit particularly useful in AGC systems, produces an output current which is proportional to the difference between a signal voltage and a reference voltage which is practically independent of temperature. By being a function of a ratio among actual values of integrated resistances and of a ratio among substantially temperature-stable voltages. The effects of temperature dependent value of integrated resistances and of temperature-dependent electrical characteristics of integrated semiconductor devices are compensated in order to produce the desired temperature-independent output current which may usefully be utilized for implementing an automatic gain control.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 07/806,905, filed Dec. 12, 1991, now abandoned. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an integrated circuit for the generation of a current proportional to the difference between a signal voltage and a reference voltage, and independent of temperature and of variations in the integrated resistors. The circuit is particularly useful, although not exclusively, for effecting automatic gain control in signal processing systems. 
     BACKGROUND OF THE INVENTION 
     The input signal of a signal processing system, especially if this comes from magnetic transducers, mechanical transducers, tuners, etc., can be subject to large amplitude variations. Variations of 30-60 dB can occur under various circumstances. It is therefore useful to equip the system for processing the signal with a device for automatic gain control, commonly denoted by the acronym AGC, which determines a variable amplification of the input signal so as to obtain an amplified signal with constant amplitude at the output. 
     Some integrated circuit components have highly temperature-dependent intrinsic electrical characteristics. Notably, the junction V BE  in silicon is inversely proportional to temperature, while the value of an integrated resistor is directly proportional to temperature. 
     In AGC systems and in analogous integrated circuits these variations in the electrical characteristics relative to the nominal design values, can often determine intolerable inaccuracies in the operation of these very sensitive circuits. 
     SUMMARY OF THE INVENTION 
     Hence it is a main aim of the present invention to provide an integrated circuit capable of generating a current proportional to the difference between a signal voltage and a reference voltage, and which is substantially independent of temperature variations. 
     A further aim of the invention is to provide an improved AGC circuit. 
     The circuit of the invention for generating a current proportional to the difference between a signal voltage and a reference voltage, and independent of temperature, comprises a differential input circuit having a first input terminal to which is applied the signal voltage and a second input terminal to which is applied a reference voltage stable relative to temperature. The circuit is formed from a pair of transistors connected in a common-emitter configuration, and each transistor is biased by means of an individual current generator connected between a first circuit supply node and the emitter of the relevant transistor. The two current generators are substantially identical and essentially deliver a current inversely proportional to a certain integrated resistor value (i.e., inversely proportional to the temperature coefficient of the integrated resistors) and the emitters of the two transistors are connected across an integrated resistor in order to raise the value of the trigger threshold of the differential, thereby increasing the zone of linearity in order to secure the conversion of the potential difference between the signal voltage and the reference voltage into a difference between the currents flowing through the two branches of the differential circuit. 
     Between a second (virtual) supply node and the respective collectors of the two transistors of the differential input circuit, are connected two identical, forward-biased diodes which respectively function as load for the respective transistor. In this way, a differential voltage, which represents a current/voltage conversion, in accordance with a logarithmic law, of the currents which flow through the two branches of the differential circuit, is obtained between the output (collector) nodes of the differential input circuit. 
     This differential voltage is applied to the inputs of a first differential stage, across which is forced a bias current generated by a circuit capable of generating a current directly proportional to temperature and inversely proportional to the value of at least one integrated resistor. 
     Any one of many suitable circuits for generating a current proportional to temperature and inversely proportional to the value of an integrated resistor are acceptable. According to a preferred embodiment, a bias current with these characteristics can conveniently be derived from a &#34;band-gap&#34; circuit which is normally present in integrated circuits for signal processing. A band-gap circuit per se is known to those of ordinary skill in the art, being widely used as source of a constant voltage with value substantially independent of temperature variations and of the supply voltage; however, such a band-gap is modified and used to provide a temperature-dependent current output signal. 
     The differential output voltage produced by such a first differential stage is applied to the inputs of a second differential stage able to carry out a voltage/current conversion, in accordance with an exponential law, in order to generate the desired output current across an output terminal, as a function of the differential voltage applied to the individual inputs. This second differential stage is biased by means of a generator of constant current which is essentially independent of temperature variations. 
     The output current is equal to the product of the value of said constant bias current of the second differential stage, and which is independent of temperature variations, and of an exponential function of a ratio between integrated resistors (which is therefore substantially invariable with respect to temperature), of the logarithm of a pure number and of the ratio between voltages which are substantially temperature-stable, as will be demonstrated in more detail later. 
    
    
     The various aspects and advantages of the circuit which is the subject of the present invention will become clearer through the following description and reference to the attached drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified block diagram of a system for automatic gain control (AGC), as is known in the prior art; 
     FIG. 2 is a functional block diagram of the circuit of the invention; 
     FIG. 3 is a circuit diagram according to a preferred embodiment of the circuit of FIG. 2; 
     FIG. 4 is a circuit diagram of one alternative embodiment of a circuit generating a current proportional to temperature and inversely proportional to the value of an integrated resistor, alternatively usable in the circuit of the invention of FIG. 3. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A typical simplified diagram of a prior art system furnished with AGC is shown in FIG. 1. The signal at the input to the AGC circuit is a DC voltage V AGC , which is proportional to the amplitude of the signal V X . The AGC circuit supplies an output current I out  which regulates the gain of the amplifier G, thereby keeping the amplitude of the signal V X  constant independently of the amplitude of the input signal V IN . 
     With reference to FIG. 2, the circuit of the invention operationally comprises a first block A which performs a linear/logarithmic conversion of the differential voltage V AGC  -V R  into currents I 1  +x and I 1  -x. 
     The block B performs a current/voltage conversion in accordance with a logarithmic law of the currents I 1  +x and I 1  -x into the differential voltage V d . 
     The block C is a first differential stage across which is driven by a bias current I 2  generated by circuit BG. The circuit BG is able to generate a current I 2  directly proportional to temperature and inversely proportional to the value of at least one integrated resistor. The block C operationally performs a voltage/voltage conversion, in accordance with an exponential law, of the differential voltage V d  applied to the inputs, into a differential output voltage V D . 
     The block D is a second differential stage, across which is driven a bias current I EE , which is essentially independent of temperature. Operationally, the block D performs a voltage/current conversion, in accordance with an exponential law, between the differential voltage V D  applied to the individual inputs, into an output current I OUT . The Current I OUT  provided by the inventive circuit is a function of the difference between the voltages V AGC  and V R  and is thus independent of temperature variations, as will now be explained. 
     A preferred embodiment of the invention is represented by the circuit of FIG. 3, in which the relevant circuit blocks A, B, C, D and BG are identified by means of dashed lines. 
     One circuit that is usable for circuit BG is a modified band-gap circuit. A typical, known band-gap circuit (BG) consists only of the transistors T6, T7, T8, T9, T10 and T11 and of the resistors R A , R B , R C  and R D . This circuit of six transistors and four resistors is commonly present in integrated circuits and is widely used to produce, on the respective output terminal, a constant voltage V G , which is extremely temperature-stable and independent of variations in supply voltage. 
     This common circuit is modified by adding a transistor T5 and, optionally, three resistors R as shown in FIG. 3. The current across the transistors T6 and T7 of the band-gap circuit can be mirrored, by means of a transistor T5, and it can be demonstrated that this current I2 is given by the relationship: ##EQU1## where K is Boltzman&#39;s constant, q is the electron charge, T is the temperature in degrees Kelvin and A is the area of the transistor T8. The resistors R connected between the emitters of the transistors T6, T7 and T5 can optionally be introduced and values selected with the aim of increasing the precision of the mirroring ratios, in accordance with circuit design considerations well known to those of ordinary skill in the art. 
     According to this preferred embodiment, the circuit BG is a modified band-gap circuit used to generate a bias current I 2  essentially proportional to temperature (T) and inversely proportional to the value of at least one integrated resistor (R A ), which current is forced across a first differential stage (block C) formed by the transistors T3 and T4 and by the relevant load resistors R 2 . 
     The second differential stage, or differential output stage (block D), is composed of the transistors T12 and T13 and of the relevant bias current I EE  generator which is essentially independent of temperature. Numerous temperature-independent current generating circuits are fully described in the prior art. Any one of many suitable circuits are acceptable. The work entitled &#34;Analysis and Design of Analog Integrated Circuits&#34; by P. R. Gray, R. G. Meyer, Publisher J. Wiley &amp; Sons, contains, on pp. 248-259, a description of numerous suitable temperature-independent current and voltage-generating circuits for use as block D, which description is incorporated herein by express reference. Other circuits of this type are moreover well known to those of ordinary skill in the art and hence a repeated description of these circuits is superfluous. 
     The differential input circuit (block A) is composed of the pair of transistors T1 and T2, of the integrated resistor R 1  connected between the emitters of the two transistors and of the pair of current I 1  generators able to supply a current given by the following relationship: ##EQU2## where V REF  is a constant, temperature-independent voltage, V R , and R E  is an integrated resistor R 1 , the value of which is hence subject to temperature variations. As will be clear, a suitable voltage V REF  will be able to be conveniently derived from the voltage V G  available, within the integrated circuit, on the respective terminal of the circuit BG if a modified band-gap circuit is used. That is, V G  is provided as an input to block A as V R . 
     The block B represents the respective loads of the two transistors T1 and T2 of the differential input circuit, D1 and D2, respectively. The two diodes D1 and D2 are respectively connected to the respective collectors of the two transistors T1 and T2 and to a virtual supply node of the circuit. The resistor R V  has the effect of producing a voltage drop sufficient to maintain the pair of transistors T3 and T4 of the first differential stage (block C) in an appropriate zone of the dynamic operating characteristic. 
     ANALYSIS OF THE OPERATION OF THE CIRCUIT 
     In comparing a temperature-stable reference voltage V R , which, in one embodiment is derived from the constant voltage V G  supplied by the band-gap circuit of BG with a signal voltage V AGC , and if the condition: ##EQU3## is valid, it is possible to write: 
     The differential voltage V d  at the input of the first differential stage T3-T4 (block C) is given by the difference between the V BE  of the diodes D1 and D2: ##EQU4## 
     The equation which links the current y to the differential input voltage of the differential V d  is as follows: ##EQU5## 
     Substituting (4) into (5) gives: ##EQU6## 
     Then substituting equation (1) for I 2  and equation (2) for I 1 , gives: ##EQU7## 
     The following equation: ##EQU8## is valid for the output current I OUT  produced by the second differential stage T12-T13 (block D), and if V D  &gt;&gt;V T  : ##EQU9## is obtained. 
     Substituting equation (7) into equation (9): ##EQU10## is obtained. 
     As can be seen, the output current I OUT  is given by the product of the constant bias current I EE  of the second differential stage (block D), which is intrinsically temperature-stable, with the exponential function of a ratio between integrated resistors, which ratio is hence insensitive to temperature variations, of a logarithm of a pure number and of a ratio between substantially temperature-stable voltages. 
     The current I OUT  is therefore particularly suitable for exercising control of the gain of an amplifier in order to effect an extremely accurate system for automatic gain control (AGC), being insensitive to temperature variations. 
     FIG. 4 is an alternative circuit for block BG for providing a current proportional to temperature and inversely proportional to the value of at least one integrated resistor across the first differential stage (block C), in order to permit the desired temperature compensation. This circuit BG thus can be provided using circuits different from the band-gap circuit of FIG. 3. FIG. 4 shows a circuit alternatively suited to generating a current proportional to temperature and inversely proportional to the value of the temperature coefficient of the integrated resistors. The output current I 2  will in fact be given, to a good approximation, by the following equation: ##EQU11## 
     Hence, this, like other circuits, will be able to be used to generate the current I 2  to be forced across the first differential stage T3-T4 of the circuit of the invention, as will be evident to those of ordinary skill in the art. The reference voltage, V R , would be provided by some other acceptable circuit. In integrated circuits where a band-gap circuit is already present, it is advantageous to add a transistor (T5) to drive the necessary current I 2  across the first differential stage. However, the drive current I 2  can be provided by other circuits if desired.