Variable Gain Amplifier

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 FIG. 1.

The signal to be amplified is applied to the input terminals of a differential stage which includes transistors Q1 and Q2.

Further, a control voltage Vcontrol is applied to the input terminals of two differential structures Q3, Q4 and QS, Q6 of a Gilbert multiplier, connected to the legs of the differential stage. The control voltage Vcontrol 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 the control signal must be resorted to, so that the exponential type dependence is difficult to stabilize when temperature changes.

Another method known to those skilled in the art for providing a variable gain amplifier is to use as gain control a field-effect (FET) M.sub.10 transistor acting as a variable resistor, in conjunction with load resistors R.sub.8, R.sub.9, transistors Q.sub.11 and Q.sub.12, and current generators G.sub.12 and G.sub.13, as shown in FIG. 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 for temperature changes.

The technical problem is solved by this invention by providing a variable gain amplifier which can provide a gain dependence on control voltage of the exponential type and which is stabilized for temperature changes over a wide range of values of the control voltage, using simpler and more economical circuitry.

In particular, a variable gain amplifier of the type described and characterized in the appended claims to this specification is disclosed herein.

For example, a variable gain amplifier is disclosed, which has a first input terminal and a second input terminal, as well as a first output terminal and a second output terminal. The variable gain amplifier comprises a first gain stage comprising a differential stage coupled to a voltage supply generator having a first terminal and a second terminal. The differential stage includes a first variable resistance load element coupling the first input terminal of the amplifier to the second terminal of the voltage supply generator. The first variable resistance load element has a control terminal. The differential stage also includes a second variable resistance load element coupling the second input terminal of the amplifier to the second terminal of the voltage supply generator. The second variable resistance load has a control terminal coupled to the control terminal of the first variable resistance load element. Further, the differential stage also includes a first output terminal positioned between the first input terminal of the amplifier and the first variable resistance load element, and a second output terminal positioned between the second input terminal of the amplifier and the second variable resistance load element. The variable gain amplifier also comprises a second gain stage cascade connected with the first gain stage between the first terminal and the second terminal of the voltage supply generator. The second stage comprises a first transistor having a first terminal coupled to the first terminal of the voltage supply generator and forming a first-second stage output terminal, a second terminal, and a control terminal, the control terminal of the transistor being coupled to the first output terminal of the differential stage. The second gain stage also comprises a second transistor having a terminal coupled to the first terminal of the voltage supply generator and forming a second-second stage output terminal, a second terminal, and a control terminal. The control terminal of the second transistor is coupled to the second output terminal of the differential stage. The second gain stage further comprises a third variable resistance load element coupling the second terminal of the first transistor to the second terminal of the voltage supply generator. The third variable resistance load element has a control terminal. Finally, the second gain stage comprises a fourth variable resistance load element coupling the second terminal of the second transistor to the ground terminal of the voltage supply generator, the fourth variable resistance load element has a control terminal coupled to the control terminal of the third variable resistance load element.

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 limited by the accompanying drawings.

In the drawings:

FIGS. 1 and 2 show circuit diagrams for variable gain amplifiers of known types;

FIG. 3 shows a circuit diagram for a variable gain amplifier according to the present invention; and

FIGS. 4A, 4B, and 5 show in circuit diagram form component parts of a control circuit for the amplifier in FIG. 3.

The circuit diagram of FIG. 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 Q21, Q22 and two resistors R1,

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 which is connected to a voltage source H.sub.I.

The gate terminals of such transistors are connected together into a control terminal VG1.

The transistors Q21 and Q22 may be bipolar transistors.

The second gain stage comprises third and fourth transistors Q23 and Q24 which may be bipolar, such transistors have control terminals connected to the output terminals of the differential stage.

The transistors Q23 and Q24 are connected between the first terminal of the voltage supply generator, through two resistors R3 and R4, and the second terminal GND 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, which is connected to current source G.sub.14.

The gate terminals of the 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 Q23 and Q24.

The cascade connection of two variable gain stages allows a wide range of gain values to be covered.

The circuit embodiment as shown in FIG. 3 avoids both the disadvantage of having the minimum linearity range for input voltages when gain is at a minimum and the drawback of noise being introduced by a gain control stage.

By having the input signal applied to a fully differential structure, the advantage of canceling the effects of a second order non-linearity introduced by the field-effect transistors, which control the gain, and this technical result (in claim) can be obtained.

Since such transistors function first as load and then to provide emitter degeneration, linearity can be further improved compared to conventional approaches.

An example of a control circuit for the variable gain amplifier according to the invention is depicted in FIGS. 4A and 4B. Shown in FIG. 4A is a first control circuit and shown in FIG. 4B is a second control circuit 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 MC A and the control terminal VG1, which is shown in FIG. 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, which is shown in FIG. 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 FIG. 5 should be used to impose the voltage drop across the resistors RA and RB.

The control circuit of FIG. 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 and has resistors R.sub.5 and R.sub.6. The other output terminal 20 of the differential stage and the output leg 21 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 FIG. 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 for 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 scope of the appended claims.

Although specific embodiments of the invention have been described for purposes of illustration, various modifications may be made without departing from the spirit and scope of the invention, as is known by those skilled in the art. Accordingly, the invention is not limited by the disclosure, but instead its scope is to be determined entirely by reference to the following claims.