Integrated circuit for generating a temperature independent current proportional to the voltage difference between a signal and a reference voltage

Abstract: A circuit particularly useful in AGC systems, produces an output current (I OUT ) which is proportional to the difference between a signal voltage (V AGC ) and a reference voltage (V R ) 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: BACKGROUND OF THE INVENTION 1. Field of the invention 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. Examples of gain controlled amplifiers having low temperature coefficients can be found in US-A-4 413 235 and in US-A-4 101 841. 2. Description of the prior art 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. A typical simplified diagram of a 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 . On the other hand, 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. OBJECT AND SUMMARY OF THE INVENTION Hence it is a main aim of the present invention as defined in the appended claims, 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 circuit means which are able to generate a current directly proportional to temperature and inversely proportional to the value of at least one integrated resistor. These circuit means for generating a current proportional to temperature and inversely proportional to the value of an integrated resistor can be various. According to a preferred embodiment, a bias current with these characteristics can conveniently be derived from a common "band-gap" circuit which is normally present in integrated circuits for signal processing, being widely used as source of a constant voltage with value substantially independent of temperature variations and of the supply voltage, as is well known to the expert. 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. BRIEF DESCRIPTION OF THE DRAWINGS 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 in which: Fig. 1 is, as already described, a simplified block diagram of a system for automatic gain control (AGC); 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 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. DESCRIPTION OF THE PREFERRED EMBODIMENT 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 I₁ + x and I₁ - x. The block B performs a current/voltage conversion in accordance with a logarithmic law of the currents I₁ + x and I₁ - x into the differential voltage V d . The block C is a first differential stage across which is driven a bias current I₂ generated by a circuit able to generate such a current I₂ directly proportional to temperature and inversely proportional to the value of at least one integrated resistor. This circuit can be a "band-gap" circuit (B.G.). 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 which is a function of the difference between the voltages V AGC and V R and independent of temperature variations. 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 B.G. are identified by means of frames executed with dashed lines. A typical "band-gap" circuit (B.G.) consists 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 is commonly present in integrated circuits in that it 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. 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: I₂ = KT qRA ln A where K is Boltzman'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 with the aim of increasing the precision of the mirroring ratios, in accordance with an art well known moreover to the expert. According to this preferred embodiment, a band-gap circuit is therefore exploited in order to generate a bias current I₂ 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 R₂. 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 literature. The work entitled "Analysis and Design of Analog Integrated Circuits" by P.R. Gray, R.G. Meyer, Publisher J. Wiley & Sons 1984, second edition, contains, appendix A/3 on pp. 284-296, a description of numerous temperature-independent current and voltage generating circuits, which description is incorporated here for express reference. Other circuits of this type are moreover well known to the expert 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 R₁ connected between the emitters of the two transistors and of the pair of current I₁ generators able to supply a current given by the following relationship: I₁ = V REF R E where V REF is a constant, temperature-independent voltage and R E is an integrated resistor, 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 band-gap circuit (B.G.). 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 sole 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 can be derived from the constant voltage V G supplied by the same band-gap circuit) with a signal voltage V AGC , and if the condition: R₁ I₁ >> V T (where V T = KT q ) is valid, it is possible to write: V AGC - V R R₁ = x 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: V d = V BE1 - V BE2 = V T ln I₁ + x I₁ - x The equation which links the current y to the differential input voltage of the differential V d is as follows: y = I₂ 2 tanh V d 2V T Substituting (4) into (5) gives: y = I₂ 2 V AGC - V R R₁ I₁ Then substituting equation (1) for I₂ and equation (2) for I₁, gives: The following equation: I OUT = I EE 1 1 + exp V D V T is valid for the output current I OUT produced by the second differential stage T12-T13 (block D), and if V D >> V T : I OUT = I EE exp (- V D V T ) is obtained. Substituting equation (7) into equation (9): 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. The necessary condition for forcing 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 of the circuit of the invention, can also be satisfied using circuits different from the band-gap circuit (block B.G.). 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 I₂ will in fact be given, to a good approximation, by the following equation: I₂ = KT q 1 RA Hence, this, like other circuits, will be able to be used to generate the current I₂ to be forced across the first differential stage T3-T4 of the circuit of the invention, as will be evident to the expert. Moreover, it remains advantageous in integrated circuits where a band-gap circuit is already present, to add a transistor (T5) to drive the necessary current I₂ across the first differential stage.

Claims: An integrated circuit for the generation of a current proportional to the difference between a signal voltage (V AGC ) and a reference voltage (V R ) and independent of temperature, which comprises first means (B.G.) able to drive a bias current directly proportional to temperature and inversely proportional to the value of at least one integrated resistor across a first differential stage (C) having a first input and a second input and a first and a second output, which is able to carry out a voltage/voltage conversion, in accordance with an exponential law, of a differential input voltage (V d ) applied to said inputs; a current (I EE ) generator able to drive a constant bias current essentially independent of temperature variations across a second differential stage (D) having a first input connected to said first output of said first differential stage and a second input connected to said second output of said first differential stage and which is able to carry out a voltage/current conversion, in accordance with an exponential law, in order to generate said output current (I OUT ) across an output terminal, as a function of the differential voltage (V D ) applied to said inputs; a differential input circuit (A) having a first input terminal to which is applied said signal voltage (V AGC ) and a second input terminal to which is applied said reference voltage (V R ), essentially independent of temperature, and formed by a pair of transistors (T1, T2) connected in a common-emitter configuration, respectively biased by means of two identical generators of current (I₁) respectively connected between a first supply node of the circuit and the emitters of said transistors and which are able to generate a current inversely proportional to the value of at least one integrated resistor, which emitters are interconnected across an integrated resistor (R₁), which transistors have identical, forward-biased diodes (D1, D2) as respective loads, respectively connected between the collectors of said transistors and a second virtual supply node of the circuit, the differential output voltage (V d ) across the collector nodes of said pair of transistors representing a current/voltage conversion, in accordance with a logarithmic law, of the respective currents (I₁ + x and I₁ - x) flowing through the transistors (T1, T2) of said pair and being applied to the inputs of said first differential stage (A), which currents represent a voltage/current conversion, in accordance with a linear/logarithmic law of the difference between said signal voltage (V AGC ) and said reference voltage (V R ). The integrated circuit as claimed in claim 1, wherein said first means able to generate a bias current directly proportional to temperature and inversely proportional to the value of at least one integrated resistor are a band-gap circuit and a transistor (T5) having a collector connected to a common emitter node of a pair of input transistors (T3, T4) of said first differential stage (C), an emitter substantially connected to a supply node and a base connected to the bases of a first pair of transistors (T6, T7) of said band-gap circuit which have their respective emmitters connected to said supply node. The integrated circuit as claimed in claim 1, wherein said two identical generators of current (I₁) for biasing the transistors (T1, T2) of said pair of transistors of the differential input circuit (A) generate a current with value equal to a ratio between a constant, temperature-stable voltage and an integrated resistor. An integrated circuit for the generation of a direct current (I OUT ) for control of the gain of an amplifier comprising a circuit as claimed in Claim 1. The integrated circuit as claimed in claim 4, wherein said first means able to generate a bias current directly proportional to temperature and inversely proportional to the value of at least one integrated resistor are a band -gap circuit and a transistor (T5) having a collector connected to a common emitter node of a pair of input transistors (T3, T4) of said first differential stage, an emitter substantially connected to a supply node and a base connected to the bases of a first pair of transistors (T6, T7) of said band-gap circuit which have their respective emmitters connected to said supply node. The integrated circuit as claimed in claim 4, wherein said two identical generators of current (I₁) for biasing the transistors (T1, T2) of said pair of transistors of the differential input circuit (A) generate a current with value equal to a ratio between a constant, temperature-stable voltage and an integrated resistor.

Inventors: MOLONEY DAVID, SACCHI FABRIZIO, VAI GIANFRANCO, ZUFFADA MAURIZIO, MOLONEY, DAVID, SACCHI, FABRIZIO, VAI, GIANFRANCO, ZUFFADA, MAURIZIO
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1990
Main classification: H03G1
IPC codes: G05F3, H03F3, H03F1, H03G1