Variable gain amplifier

Abstract: A temperature-stable variable gain amplifier comprising a differential input stage (Q1,Q2) coupled to a first terminal (+Vc) of a voltage supply generator and to a second terminal (GND) of the voltage supply generator through first (M1) and second (M2) field-effect transistors whose gate terminals are connected into a first common control terminal (VG1). Connected to the differential stage are two bipolar transistors (Q3,Q4) which are also connected between the first terminal (+Vc), through two resistors (R3,R4), and the second (GND) of the voltage supply generator, through third (M3) and fourth (M4) field-effect transistors. The third and fourth transistors have gate terminals connected into a second common control terminal (VG2). The collector terminals of the bipolar transistors form output terminals (OUT+,OUT-) for the amplifier.

Description: This invention relates to variable gain amplifiers (VGA), and in particular to a VGA whose gain is dependent essentially on a control voltage and can be temperature stabilized. In many applications -- e.g. automatic gain control (AGC) systems in audio and TV apparatus -- the signals whose amplitudes may vary within a wide range need to be amplified so that an amplified signal with constant amplitude can be presented at the output. In such automatic gain controls as usually comprise at least one variable gain amplifier in a loop structure, a signal level detector, a control current generator driven by the signal level detector, and a stabilizing capacitor, it is indeed the variable gain amplifier that constitutes the most critical part of the arrangement. For a truly effective control of the gain, the dependence of the variable gain amplifier gain on control voltage should follow an exponential law and this dependence should remain stable as temperature varies. The most widely used arrangement in the art for implementing a variable gain amplifier is to cut down the gain of a balanced stage using a Gilbert multiplier in the manner shown in Figure 1. Such a variable gain amplifier is disclosed in patent document US-A-3 684 974. The signal to be amplified is applied to the input terminals of a differential stage which includes transistors Q1 and Q2. On the contrary, a control voltage Vc is applied to the input terminals of two differential structures Q3, Q4 and Q5, Q6 of a Gilbert multiplier, connected to the legs of the differential stage. Such control voltage determines the amplification gain of the signal that appears at the output terminals formed by one of the leg pair of each of the two differential structures. The problems brought about by this prior approach are the following: the gain dependence on the control voltage is not exponential but rather of the "hyperbolic tangent" type, and the noise introduced by the multiplying cell is high, and a common output mode adjusting circuit must be used. In order to obtain an exponential type dependence from the control voltage for the amplifier gain, a complicated distortion of said control signal must be resorted, so that said exponential type dependence is difficult to stabilize versus temperature changes. Another known method to the skilled men for providing a variable gain amplifier is to use as gain control a field-effect transistor acting as a variable resistor, as shown in Figure 2. In such a case, the gain relation to the control voltage follows a quadratic law, and the drawback is even more serious since a limited range of gain values is available, thereby making the cascade connection of at least two stages necessary to expand that range. A further drawback is a limited input dynamic range. Similar to the previously mentioned method, any possible exponential dependence obtained by skewing the control voltage would be difficult to stabilize versus temperature. The technical problem that underlies this invention is to provide a variable gain amplifier which can provide, as against conventional variable gain amplifiers, a gain dependence on control voltage of the exponential type and stabilized in temperature over a wide range of values of the control voltage, using simpler and more economical circuitry. This problem is solved by the provision of a variable gain amplifier of the type described and characterized in the appended claim 1 to this specification. The features and advantages of an amplifier according to the invention will become apparent from the following description of an embodiment thereof, given by way of example and not of limitation with reference to the accompanying drawings. In the drawings: Figures 1 and 2 show circuit diagrams for variable gain amplifiers of known types; Figure 3 shows a circuit diagram for a variable gain amplifier according to the invention; and Figures 4 and 5 show in circuit diagram form component parts of a control circuit for the amplifier in Figure 3. The circuit diagram of Figure 3 comprises first and second cascade-connected gain stages which constitute the core portion of a variable gain temperature-stabilized amplifier according to the invention. The first gain stage includes a differential input stage coupled to a first terminal +Vcc of a voltage supply generator through a constant current generator G1 and comprising two transistors Q1, Q2 and two resistors R1, R2. The control terminals of the transistors are the amplifier input terminals IN+ and IN-. The output terminals of the differential stage are coupled to a second terminal, GND, of the voltage supply generator, through two field-effect transistors M1, M2 which function as variable resistance load elements and a first circuit node DC1. The gate terminals of such transistors are connected together into a control terminal VG1. The transistors Q1 and Q2 may be bipolar transistors. The second gain stage comprises third and fourth transistors Q3 and Q4 which may be bipolar, such transistors have control terminals connected to the output terminals of the differential stage. The transistors Q3 and Q4 are connected between the first terminal of the voltage supply generator, through two resistors R3 and R4, and the second terminal of the voltage supply generator, through two field-effect transistors M3 and M4 which function as variable resistance circuit elements, that is, emitter degeneration resistors and through a second circuit node DC2. The gate terminals of said transistors M3 and M4 are connected together into a second control terminal VG2. The amplifier output terminals OUT+ and OUT- consist of two terminals of the transistors Q3 and Q4. The cascade connection of two variable gain stages allows a wide range of gain values to be covered. The circuit embodied as in Figure 3 avoids the disadvantage of having the minimum linearity range for input voltages when gain is at a minimum and the drawback that noise is introduced by a gain control stage. By having the input signal applied to a fully differential structure, the advantage is secured that the effects of second order non-linearity introduced by the field-effect transistors which control the gain can be cancelled. Since such transistors function first as load and then to provide emitter degeneration, linearity can be further improved compared to conventional approaches. A principal portion of a control circuit for the variable gain amplifier according to the invention is depicted in Figure 4. Shown therein are first and second control circuits which respectively comprise a field-effect transistor MCA and a field-effect transistor MCB, which transistors function as a variable resistance load element and a variable resistance circuit element, respectively. The transistor MCA, connected between the first terminal of the voltage supply generator and the first circuit node DC1, is also connected in series with a resistor RA between the inverting and non-inverting input terminals of a first operational amplifier A1. The output terminal of that amplifier is connected to the gate terminal of the transistor MCA and the control terminal VG1, shown in Figure 3. The transistor MCB, connected between the first terminal of the voltage supply generator and the second circuit node DC2, is also connected in series with a resistor RB and a voltage generator VB between the inverting and non-inverting input terminals of a second operational amplifier A2. The output terminal of that amplifier is connected to the gate terminal of the transistor MCB and the control terminal VG2, shown in Figure 3. The feedback loops which include the amplifiers A1 and A2 impose predetermined values of the drain-source resistances of the transistors MCA and MCB. The imposed resistances are, therefore, unaffected by temperature and any process spread. The transistors M1, M2, M3 and M4 which set the amplifier gain operate in the same polarization conditions as the reference transistors MCA and MCB, and accordingly, the control circuits will impose the same drain-source resistance on them too, thereby controlling the amplifier gain. To obtain an exponential dependence of the gain on the control voltage, the circuit shown in Figure 5 should be used to impose the voltage drop across the resistors RA and RB. The control circuit of Figure 5 includes a differential input stage which is connected to an output terminal as well as to an input leg of a current mirror circuit. The other output terminal of the differential stage and the output leg of the current mirror circuit are respectively connected to the inverting input terminal of amplifier A1 and the inverting terminal of amplifier A2. To an input terminal of the differential stage shown in Figure 5, with transistors QX1 and QX2, is applied a reference voltage Vref, whilst the control voltage Vcontrol is applied to the other input terminal. The temperature-wise stability and independence from process spreads of the control element resistances ensure stability for the characteristic curve of the variable gain amplifier versus temperature. The low gain required for the amplifiers A1 and A2 allows of their implementation with very simple structures, thereby limiting the circuit complexity of a variable gain amplifier according to the invention. It will be appreciated that modifications or integrations may be made to the embodiment described above without departing from the protection scope of the appended claims.

Claims: A variable gain amplifier comprising first and second gain stages cascade connected between first (+Vcc) and second (GND) terminals of a voltage supply generator, wherein the first gain stage comprises a differential stage coupled to the first terminal of the voltage supply generator and having first (IN+) and second (IN-) input terminals which form first and second input terminals for the amplifier, as well as first and second output terminals coupled to the second terminal (GND) of the voltage supply generator, and wherein the second gain stage comprises first (Q3) and second (Q4) transistors each having first and second terminals and a control terminal, characterized in that said first and second output terminals of the differential stage are coupled to said second terminal (GND) of the voltage supply generator respectively through first (M1) and second (M2) variable resistance load elements having a first common control terminal (VG1), and in that the control terminals of said first and second transistors are respectively connected to the first and second output terminals of the differential stage, the first terminals of said first and second transistors being both coupled to the first terminal (+Vcc) of the voltage supply generator and forming respectively a first output terminal (OUT+) and a second output terminal (OUT-) for the amplifier, the second terminals of the first and second transistors being both coupled to the second terminal (GND) of the voltage supply generator through respective first (M3) and second (M4) variable resistance circuit elements which are both connected to a second common control terminal (VG2). A variable gain amplifier according to Claim 1, characterized in that it comprises: first and second control circuits, of which the first control circuit comprising a third variable resistance load element (MCA) connected between the first terminal (+Vcc) of the voltage supply generator and a first circuit node (DC1) between the first and the second variable resistance load element and having a control terminal; and a first operational amplifier (A1) having an inverting input terminal, a non-inverting input terminal, and an output terminal, the inverting input terminal being connected, through a first resistive element (RA), to a first end of the third variable resistance load element (MCA), the non-inverting input terminal being connected to a second end of the third variable resistance load element, and the output terminal being connected to both the control terminal of the third variable resistance load element and the first common control terminal (VG1); and the second control circuit comprises a third variable resistance circuit element (MCB) connected between the first terminal (+Vcc) of the voltage supply generator and a second circuit node (DC2) between the first and the second variable resistance load elements and having a control terminal, and a second operational amplifier (A2) having an inverting input terminal, a non-inverting input terminal, and an output terminal, the inverting input terminal being connected, through a second resistive element (RB) and a voltage generator (VB), to a first end of the third variable resistance circuit element (MCB), the non-inverting input terminal being connected to a second end of the third variable resistance circuit element, and the output terminal being connected to both the control terminal of the third variable resistance circuit element and the second common control terminal (VG2). A variable gain amplifier according to Claim 2, characterized in that it comprises a control circuit including a differential input stage (QX1,QX2) having first and second input terminals, and first and second output terminals, and a current mirror circuit (QX3,QX4) having an input leg and an output leg, the input leg of said current mirror circuit being connected to the second output terminal of the differential stage, the first output terminal of the differential stage and the output leg of the current mirror circuit being respectively connected to the inverting input terminal of the first operational amplifier (A1) and the inverting input terminal of the second operational amplifier (A2). A variable gain amplifier according to any of the preceding claims, characterized in that the first and second variable resistance load elements (M1,M2) are field-effect transistors with interconnected gate terminals. A variable gain amplifier according to any of the preceding claims, characterized in that the first and second variable resistance circuit elements (M3,M4) are field-effect transistors with interconnected gate terminals. A variable gain amplifier according to either of Claims 2 and 3, characterized in that the third variable resistance load element (MCA) and third variable resistance circuit element (MCB) are field-effect transistors and the control terminals are gate terminals.

Inventors: DEMICHELI MARCO, DEMICHELI, MARCO
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1993
Main classification: H03G1
IPC codes: H03G5, H03F1, H03G3, H03G1, H03F3