Abstract:
An amplifier with programmable gain and input linearity at high frequency allows an increase in the gain without effecting input linearity and without significantly increasing current consumption. The amplifier includes an input stage which receives a voltage signal for performing a current conversion thereof with compression. An output stage is connected to the input stage and decompresses the signal provided by the input stage for producing gain amplification thereof. The amplifier further includes at least one current amplifier stage interposed between the input stage and the output stage. The at least one current amplifier includes at least one bipolar transistor series-connected to a load diode and to a current source. A reduction in the transconductance of the load diode is provided in the at least one amplifier stage to determine a programmable gain factor for the amplifier.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of electronics, and, more particularly, to an amplifier. 
     BACKGROUND OF THE INVENTION 
     Signal processing for digital video disk (DVD) applications, for example, conventionally requires amplifiers which can be programmed with a wide gain range, and are high performance in terms of the operating frequency. A data read channel is an example in which signal processing requires a gain-programmable amplifier. 
     FIG. 1 illustrates four signals A, B, C and D which have different amplitudes and are voltage-added in a node 1, and then amplified by an amplifier circuit 2 which sets the gain to obtain a sum signal. The sum signal is sent to an equalizer circuit 3 and finally to a buffer 4. The intended result is a dB linear gain variation with a linear variation of the current. 
     A gain setting circuit of the prior art is shown in FIG. 2, in which a differential input stage, designated by the reference numeral 10, is connected to a differential output stage, designated by the reference numeral 11. The gain A v  is determined by a resistance ratio multiplied by a current ratio. The resistance ratio is determined by the ratio between the resistances of the output stage with respect to those of the input stage. The current ratio is the ratio between the current of the output stage and the current of the input stage. Accordingly, the gain A v  is determined by the following relation: ##EQU1## 
     The above-described circuit is effective for gain programming since the gain can be changed not only by varying the resistive ratio, but also by primarily varying the current ratio. Varying the resistive ratio is difficult to implement, and in any case, requires a large circuit area. Varying the current ratio can be achieved easily with a modest use of the physical area of the device. Although the circuit of FIG. 2 is efficient for gain programmability, it is affected by drawbacks due to the high noise present at the output. The noise is amplified by the current ratio between the output stage and the input stage, and this is unacceptable whenever noise is a negative factor. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an amplifier with programmable gain and input linearity usable in high-frequency lines, while increasing the gain without altering input linearity and without excessively increasing current consumption. 
     Another object of the present invention is to provide an amplifier with programmable gain and input linearity usable in high-frequency lines, wherein the gain of the amplifier is closely correlated to current variation. 
     A further object of the present invention is to provide an amplifier with programmable gain and input linearity usable in high-frequency lines, wherein the amplifier has high gain and high gain precision. 
     Yet another object of the present invention is to provide an amplifier with programmable gain and input linearity usable in high-frequency lines, wherein the amplifier is highly reliable, relatively easy to implement, and is produced at competitive costs. 
     These objects and others, which will become apparent hereinafter, are achieved by an amplifier with programmable gain and input linearity. The amplifier comprises an input stage for receiving a voltage signal and for performing current conversion thereof with compression. An output stage is connected to the input stage and decompresses the signal from the input stage, and produces gain amplification thereof. The amplifier further comprises at least one current amplifier stage interposed between the input stage and the output stage. The at least one current amplifier stage includes at least one bipolar transistor series-connected to a load diode and to a current source. A programmable circuit for reducing a transconductance of the load diode is provided in the at least one amplifier stage to determine a programmable gain factor of the amplifier. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further characteristics and advantages of the invention will become apparent from the description of the preferred, but not exclusive, embodiments of the programmable amplifier according to the invention, illustrated only by way of non-limitative examples in the accompanying drawings, wherein: 
     FIG. 1 is a conceptual diagram of a data read channel according to the prior art; 
     FIG. 2 is a circuit diagram of a programmable amplifier according to the prior art; 
     FIG. 3 is a circuit diagram of a gain-programmable amplifier according to the present invention; 
     FIG. 4 is a circuit diagram of a second embodiment of a gain-programmable amplifier according to the present invention; and 
     FIG. 5 is a circuit diagram of a third embodiment of the gain-programmable amplifier according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The programmable amplifier according to the present invention is described in detail with reference to FIGS. 3 to 5. Referring to FIG. 3, the circuit according to the invention comprises a first differential input stage 10 which is similar to the differential input stage shown in FIG. 2. The first differential input stage 10 comprises a first bipolar transistor 15 and a second bipolar transistor 16, which are arranged so that their emitter terminals are connected to respective resistors R E  and their collector terminals are connected to respective diodes 17 and 18. A current source 2I 1  is connected to a common node between the resistors R E . 
     The first differential input stage 10 is connected to at least one current amplifier stage, designated by the reference numeral 20. The at least one current amplifier stage 20 is interposed between the differential input stage 10 and a differential output stage 30. A differential stage is shown, but the circuit according to the invention is equally valid if a single-input stage or a single-output output stage is used. 
     The current amplifier stage 20 is formed by a first bipolar transistor 21 and by a second bipolar transistor 22 having common-connected emitter terminals. A current source 2I 2  is connected to the emitter terminals. The base terminals of the bipolar transistors 21 and 22 are respectively connected to the collector terminals of the transistors 15 and 16. Voltage signals V +   and V -   are applied to the base terminals of the transistors 15 and 16 of the first differential input stage. The collector terminals of the transistors 21 and 22 are connected, respectively, to diodes 23 and 24, wherein the anode terminal is connected to the collector terminal of the respective transistors. Respective current sources I 2  * are parallel-connected to the diodes 23 and 24. 
     Finally, the second stage, i.e., the current amplifier stage 20, is connected to the output stage. The current amplifier stage 20 is also of the differential type, and the same considerations previously discussed equally apply. The differential output stage 30 is formed by two transistors 31 and 32. The emitter terminals are common-connected, including connection to a current source 2I 3 . The collector terminals of the transistors 31 and 32 are respectively connected to resistors R L . The base terminals of the bipolar transistors 31 and 32 are respectively connected to the collector terminals of the transistors 21 and 22 of the current amplifier stage 20. 
     The above-described circuit has a gain which is given by: ##EQU2## 
     The input stage 10 receives at an input a air of voltage signals and converts them into current signals, and compresses them. The signals are sent to the second current amplifier stage 20, which amplifies the gain and then sends the amplified signals to the differential output stage 30. The differential output stage 30 converts the amplified signals back to voltage signals to provide a voltage output. It is also possible to have a current output, which is useful, for example, when the output is applied to other filters. 
     The transconductance of the diodes 23 and 24 of the second current amplifier stage 20 can be modulated by varying the static current supplied by the sources I 2  *, which is independent of the current of the input stage 10. Thus, by significantly increasing the static current, it is possible to increase the gain because the transconductance decreases. Accordingly, a further degree of freedom to increase the gain is obtained in addition to the one provided by varying the currents of the differential input and output stages, i.e., currents I 1  and I 3 . 
     The current sources I 2  * therefore subtract static current from the diodes 23 and 24, which allows a reduction in the transconductances of the diodes 23 and 24. It is thus possible to mutually cascade-connect a plurality of current amplifier stages 20 between the differential input stage 10 and the differential output stage 30 to have a cascade of a plurality of gain factors provided by a current ratio in which the static current of the sources I 2  * is present. 
     In the above defined gain equation, gm1, gm2, gm3 and gm2* are, respectively, the transconductances of diodes 17 and 18, of transistors 21 and 22, of diodes 23 and 24, and of transistors 31 and 32. Therefore, by varying the currents included in the gain equation, it is possible to change the gain. To achieve a high gain, it is possible to have a small current I 1 . However, a limit is set by input linearity, which is determined by 2I 1  R E . An alternative would be to increase the current I 3 . But in this case, the power dissipated by the circuit also increases. 
     The intermediate current amplifier stage 20 allows a gain increase without changing input linearity and without excessively increasing current absorption. The term ##EQU3## can be varied continuously or stepwise by changing the current I 2  *. The limit of the above-described structure prevents the term I 2  -I 2  * from becoming smaller than 0. In this case, the load diodes of the second stage 20, i.e., the diodes 23 and 24, switch off and the linear region is avoided. As mentioned, it is possible to introduce a plurality of stages like the one illustrated above to further increase the gain. 
     FIG. 4 is a partial circuit diagram in which only one branch of the corresponding differential circuits is illustrated, with the second branch being omitted. The diagram illustrates a circuit in which two current amplifier stages are interposed between the differential input stage 10 and the differential output stage 30. The amplifier stages are now designated by the reference numerals 20&#39; and 20&#34;, and are provided, according to the invention, like the stage 20 shown in FIG. 3. In this case, gm4 is the transconductance of the bipolar transistors of the output stage 30, while gm3 and gm3* are, respectively, the transconductances of the bipolar transistors and of the diodes of the second current amplifier stage 20&#34;. Therefore, according to the circuit of FIG. 4, the gain of the circuit is: ##EQU4## 
     If the currents are chosen so that their value is not too small, the output impedance of each stage is low. Therefore, the entire structure operates like a single-pole amplifier having a transfer function equal to: ##EQU5## The secondary poles are due to the impedance of the diodes and to the parasitic capacitors that occur on the respective nodes, and are thus localized at high frequencies. This is true if the transconductance of the diodes is kept relatively high. 
     FIG. 4 illustrates a capacitor C L  representing a load capacitance due to a load connected to the amplifier according to the invention. If the current I 4  of the differential output stage 30 is changed exponentially, it is possible to achieve dB linear gain control. The above-described circuit has also been optimally used at high frequencies. 
     The circuit shown in FIG. 5 shows the introduction of a control current I cont  in the differential output stage. This allows an exponential variation of the current 2I 3  by varying the control current I cont  in a linear fashion. Accordingly, the gain of the amplifier also varies exponentially in this case. The differential output stage 30 is formed by a modified current mirror, as shown in FIG. 5, in which an additional bipolar transistor 35 is provided. The transistor 35 is connected by its base terminal to the collector terminal of a transistor 36 which receives, on its emitter terminal, the control current I cont . The transistor 36 is connected to the transistor 37 by its base terminal. 
     If the current I 3  varies to change the gain, the common-mode voltage at the output from the stage 30 changes. Accordingly, it can be useful to have a common-mode circuit, as designated by the reference numeral 50 and as shown in FIG. 3. The common-mode circuit 50 is an amplifier having a reference voltage and receives at an input the voltages received on the collectors of the transistors 31 and 32, and provides feedback control at an output of current sources 45 and 46. The amplifier thus controls the half-sum of the inputs provided by the collectors of the transistors 31 and 32, and compares the half-sum with the reference voltage to generate an error signal, and to control the current sources 45 and 46. The half-sum of the voltages at an input to the common-mode circuit 50 must be equal to the reference voltage. In this case, the circuit 50 does not operate using the current sources 45 and 46. 
     The programmable amplifier according to the invention fully achieves the objects since the gain can be programmed while maintaining input linearity. It is also possible to achieve a high gain amplifier with high precision of the gain simultaneously with high performance in terms of the operating frequency. This is due to the fact that frequency compensation is simpler than a circuit solution in which a cascade of voltage-gain stages is present. All of the secondary poles are associated with low impedances, and therefore with a high frequency. Low impedances can be achieved because it is not necessary to provide voltage gain in the internal nodes of the circuit. 
     Finally, the circuit according to the invention achieves low power dissipation and provides a current output which is useful for applying, for example, to the other filters. Numerous modifications and variations can be made to the amplifier, all of which are within the scope of the invention. All the details may also be replaced with other technically equivalent elements. The materials used may be any according to requirements and to the state of the art, so long as they are compatible with the specific use.