Differential transconductance stage, dynamically controlled by the input signal's amplitude

Abstract: The dynamic range of operation of a transconductance differential input stage is reduced when the amplitude of the input signal decreases, thus reducing the level of the noise that is generated by the input stage. A DC signal representaive of the sensed amplitude of the input signal is employed for either reducing the value of a common, emitter-degenerating resistance (R) or of the bias current that is forced in the two branches of the differential input stage.

Description: The present invention relates to differential amplifiers and more particularly to a transconductance differential input stage wherein the bias conditions are modified upon the varying of the amplitude of the signal which is fed to the input nodes of the differential stage in order to improve overall noise characteristics of the amplifier. A transconductance input differential stage, realised with a pair of bipolar junction transistors, is depicted in Fig. 1. The admittance of the stage may be defined as follows: g m = 1/R, where R represents an emitter degenerating resistance, as shown in the figure. The difference between the currents in the two branches of the differential circuit is given by: Δ I = (I1 - I2) = g m v in = 1/R v in as long as the following condition is verified: 1/R << 1/r e = KT/I where K is the Boltzmann's constant and T is the absolute temperature in Kelvin degrees. The dynamic characteristics of such an input stage, i.e. the maximum amplitude of the input signal which may be handled by the differential stage without causing clipping effects on the output current, are proportional to the product between the emitter degeneration resistance R and the biasing current I that is forced through both branches of the differential circuit by respective current generators. The US patent No. 3,806,823 describes a differential amplifier wherein the non linearity in the input voltage-output current characteristic of the amplifier is compensated by a corresponding non linearity of a two terminal coupling network constituting an emitter degeneration resistance of the input pair transistors. The US patent No. 4,647,839 describes a high precision voltage-to-current converter, particularly for low supply voltages wherein the common emitter terminals of the input pair of transistors are connected to ground through two additinal transistors controlled by the current flowing through the input pair transistors for improving linearity of the output signal represented by the current passing through one of the added transistors. On the other hand, if one considers the noise which is produced by such a transconductance input stage, it may easily be observed that the generated noise is directly proportional to the value of the degeneration resistance R as well as to the level of the biasing current I. As a consequence, a high input dynamic characteristic is accompanied by a relatively high level of noise generated by the input stage. This drawback of transconductance differential stages of the known type, if not eliminated completely, which is practically impossible, is greatly reduced in its effects by making the circuit according to the present invention, which permits to improve the noise figure of the stage. Basically, the present invention as defined in Claim 1 relates to a circuit arrangement capable of modifying the dynamic characteristics of a transconductance input stage in function of the amplitude of the input signal, i.e. capable of reducing the dynamic range of operation when the amplitude of the input signal decreases. According to a first embodiment of the invention, the parameter affecting the dynamic characteristics of the stage that is decreased as the amplitude of the input signal decreases is the biasing current that is forced through the two branches of the differential circuit by the dedicated current generators. According to an alternative embodiment of the invention, the parameter affecting the dynamic characteristics of operation of the stage that is reduced when the amplitude of the input signal decreases is the value of the degenerating resistance. Normally, the first embodiment is particularly suited for improving the noise characteristics of a transconductance input stage employed in a system wherein an automatic gain control loop already exists and therefore where a signal proportional to the amplitude of the input signal is already available, as for example in radio receivers. In this type of application, an operational amplifier driven, by the AGC signal, is employed for driving a pair of "by-pass" transistors capable of diverting a certain amount of the current that is forced by the biasing current generators through the two branches of the differential circuit and therefore reducing the effective biasing current which is actually forced through the two branches of the transconductance input differential stage. According to an alternative embodiment of the invention, the degenerating resistance is made up of a plurality of resistances connected in series, each having a shunting switch for being selectively excluding from the series, as needed. Each shunting switch is driven by a respective comparator of a plurality of comparators having gradually decreasing threshold values and to which a signal representing the amplitude of the signal present at the input of the transconductance differential stage is fed. In this way, when the amplitude of the signal at the input of the stage decreases, one or more resistances are progressively excluded from the series in order to steppingly reduce the effective value of the degenerating resistance and therefore the dynamic characteristics of the stage. In both cases, a reduction of the biasing current (I) and/or of the degenerating resistance (R) produces also a reduction of the noise that is generated by the input stage. In this way, the level of noise that is produced by the stage may be advantageously reduced whenever the amplitude of the input signal becomes smaller than a maximum design value. Overall, the input stage of the invention has a remarkably reduced noise figure. The different aspects and advantages of the invention will become more evident through the following description of several embodiments thereof and by referring to the annexed drawings, wherein: Fig. 1 is a basic circuit diagram of a transconductance differential stage; Fig. 2 is a basic circuit diagram of a transconductance differential stage with controlled dynamic characteristics, according to a first embodiment of the invention. Fig. 3 is a basic circuit diagram of a transconductance differential stage with controlled dynamic characteristics, according to an alternative embodiment of the invention. Fig. 4 is a functional diagram of an analog AC signal compressor stage employing a dynamically controlled transconductance stage of the present invention. A first embodiment of the invention is functionally depicted in Fig. 2. For simplicity's sake it is assumed that an AGC (Automatic Gain Control) signal be available, i.e. a signal proportional to the amplitude of the input signal v in which is applied to the input nodes of the transconductance differential stage. An AGC signal is normally present in analog amplifiers designed for amplifying signals of extremely variable amplitude, as for example in radio receivers. According to this first embodiment, the circuit for controlling the dynamic characteristics of the differential transconductance stage comprises a pair of transistors Q3 and Q4, functionally connected between the generators I of the biasing current of the two branches of the differential circuit (Q1 and Q2), respectively, and a common ground node (or virtual ground node). The "by-pass" transistors Q3 and Q4 are driven by a driving buffer OP, to a first input of which an AGC signal is fed. In practice, the OP amplifier and the pair of transistors Q3 and Q4 constitute a voltage-to-current converter circuit. When the level of the AGC signal (i.e. the amplitude of the input signal v in ) decreases, the "conductivity" of the by-pass transistors Q3 and Q4 decreases and so does the control current Ic that is added to the current I generated by the respective current generators in each branch of the differential circuit. In this way, the actual biasing current and therefore the dynamic range of operation of the transconductance input stage is decreased, thus adapting it to the real amplitude of the input signal. By so reducing the level of the biasing current an attendant decrease of the level of the noise which is generated by the input stage is achieved as desired. An alternative embodiment of the invention is schematically depicted in Fig. 3. According to this alternative embodiment, the parameter that is acted upon for reducing the level of the noise which is generated and the dynamic range of operation of the input stage is no longer the biasing current as in the prior embodiment, but the value of a common emitter degenerating resistance R. This is obtained by replacing a single degenerating resistance R with an equivalent series composed of a plurality of resistances (R1, R2, R3, and R4) connected in series. Each single resistance of the series is provided with a shunting switch (S1, S2, S3 and S4) driven by a respective threshold comparator (C1, C2, C3 and C4), belonging to a plurality of comparators, functionally having a different threshold which is made increasingly smaller. Each threshold may be established by applying a respective reference voltage (Vrefl, Vref2, Vref3 and Vref4) to a first input of each comparator. To the other input of the comparators a signal having an amplitude proportional to the amplitude of the input signal v in is applied. In this case, when the amplitude of the input signal v in decreases, the triggering of a first and thereafter sequentially of the other comparators will occur, thus closing the relative shunting switches and reducing progressively the effective value of the degenerating resistance R of the transconductance stage. This will reduce the dynamic range of operation of the stage as well as also the level of the noise that is generated, as desired. In the example shown in Fig. 3, the amplitude of the input signal v in is monitored as a DC voltage of variable level produced on the node A, by employing for this purpose a mirroring system for the currents I1 and I2 flowing in the two branches of the differential circuit, as shown in Fig. 3. Of course, other methods and means for producing a signal that is proportional to the amplitude of the input signal v in may be employed. An AGC signal may be used whenever available. An operational amplifier OP drives the various comparators having different thresholds (C1, C2, C3 and C4) in function of the signal present on the node A. The switches S1, S2, S3 and S4 may be made of as many field effect transistors, as shown. The central circuit of the invention is particularly suited and simple to implement in systems provided with automatic gain control, such as for example analog compressors, where a DC signal proportional to the amplitude of the input signal is available. A functional circuit diagram of an analog signal compression circuit for AC signals of varying amplitudes, incorporating a control circuit of the present invention, is depicted in Fig. 4. In this application, the circuit which performs a control on the level of the biasing current in the transconductance, input, differential stage (Q1-Q2) is represented in practice by a voltage-to-current converter, composed of the operational amplifier OPAMP 2 and by the pair of transistors Q3-Q4, similar to the circuit described in relation to Fig. 2. The resistance Rg is a current limiting resistance toward a common ground node of the circuit. The function of analog compression of an AC input signal v in is performed by the operational amplifier OPAMP 1, the pair of comparators COMP1 and COMP2, the logic gate OR and the RC circuit, which comprises the capacitor C and the resistances R att and R rec ,respectively of charge (attack) and of discharge (recovery) of the capacitors C. The switch S A , driven by the output signal of the logic OR gate and the variable resistance R*, the value of which is regulated by the voltage V2, shown schematically in Fig. 4, complete the functional diagram of the compressor circuit. Under conditions of low input signal amplitude, the value of the variable resistance R* is maximum, therefore the voltage gain of the stage is given by: v1/v in = R* max /R* min . Moreover, by supposing that there are no other limitations in the management of the input dynamic characteristics, when the amplitude of v1 increases and becomes greater than the high threshold (V thH ), a high ("1") signal is produced at the output of the logic OR gate, which by closing the switch S A , causes a rising of the voltage V2 across the RC group. Commonly, the rise time constant of the voltage V2 (i.e. charge of C), regulated by the product R att C (also referred to as attack RC), is greater than the fall time constant of the voltage V2 (i.e. discharge of C), that is when the output of the OR gate is low ("0"), and which is given by the product R rec C (also referred to as recovery RC). In any case, to a rising value of the voltage V2 corresponds a decreasing value of the resistance R* and therefore an overall attenuation of the signal v1 which is generated at the output of the operational amplifier OPAMP1. Therefore the system compresses relatively large signals by scaling them down to a maximum amplitude that is equal to the difference between the two threshold values: V thH and V thL . By assuming that the input signal has a peak value substantially lower than the product between the biasing current I and the emitter degenerating resistance R, under these conditions clipping of the signal will not occur because the reference voltage V thH > IR. By increasing the amplitude of the input signal, when IR > V thH , an increase of the voltage V2 occurs and consequently also the biasing current through the two branches of the transconductance input differential stage increases. This modifies (increases) the dynamic characteristics of operation of the input stage, thus adapting it to the increased level of the input signal. It may be said that when the input signal has a relatively low amplitude, the so-called "tail" current is low and therefore the noise which is generated by the input stage is reduced. When the amplitude of the input signal increases, the biasing current (tail current) as well as the noise generated increase. In conclusion, the operation characteristics of the input stage are optimized for what pertains to the noise characteristics thereof. Regardless of the fact that the circuit of the invention has been described as made with bipolar junction transistors, it is also possible to make the circuit with field effect transistors, for example in MOS technology. Also in this case the circuit arrangement of the invention will improve the noise characteristics of the input stage.

Claims: A differential transconductance stage comprising a pair of transistors (Q1, Q2) each functionally connected in series with a respective bias current generator (I) between two supply nodes (Vcc, GDN) and having a common degenerating resistance (R) connected between respective source or emitter terminals of said transistor pair, an input signal (v in ) being applied across respective control terminals of said two transistors (Q1, Q2), characterized by comprising means controlling the difference between the currents flowing through said pair of transistors (Q1, Q2) for reducing the dynamic range of operation of said differential transconductance stage upon a decrease of the amplitude of said input signal. The differential transconductance stage of claim 1, characterized in that said means comprise a pair of transistors (Q3, Q4) each having a collector or drain connected to the emitter or source nodes of said first pair of transistors (Q1, Q2), respectively, an emitter or source connected in common to a ground node and a control terminal connected in common and to the output of a controlling amplifier (OP) to an input of which a signal (AGC) representative of the amplitude of said input signal (V in ) is fed. The differential transconductance stage of claim 1 characterized in that said means comprise a plurality of short-circuiting switches (S1, S2, S3, S4) functionally connected in parallel to fractionary portions (R1, R2, R3, R4) electrically connected in series with each other of said common degenerating resistance (R) and a number of comparators (C1, C2, C3, C4) having an output connected to the control terminal of a corresponding switch of said plurality of switches, each comparator comparing a signal representative of the amplitude of said input signal (v in ) with a certain reference voltage (V ref1 , V ref2 , V ref3 , V ref4 ) different from each other.

Inventors: ONETTI ANDREA MARIO, ROSSI DOMENICO, ONETTI, ANDREA MARIO, ROSSI, DOMENICO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1992
Main classification: H03F1
IPC codes: H03G3, H03F3, H03F1, H03G5