Abstract:
A control signal is conditioned such that it produces a conditioned control signal that is the sum of two exponentially varying components. The resulting conditioned control signal, applied to an amplifier circuit, produces a gain that varies linearly in dB with changes in the voltage of the control signal.

Description:
This application claims priority under 35 U.S.C. §§119 and/or 365 to 9930675.5 filed in United Kingdom on Dec. 24, 1999; the entire content of which is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to an electronic circuit, and in particular to a variable gain amplifier circuit. 
     BACKGROUND OF THE INVENTION 
     It is desirable in some situations to provide a variable gain amplifier, in which the gain varies logarithmically with a control signal, to produce an output signal which produces a linear change in output signal level measured in dB, in response to a linear change in input signal level. 
     U.S. Pat. No. 5,572,166 describes a variable gain amplifier, in which a linear change in a gain control current produces an exponential change in gain, thereby providing linear-in-decibel gain control. 
     U.S. Pat. No. 4,816,772 discloses a variable gain amplifier with multiple cascode amplifier stages, connected in cascade. A control voltage is applied to a linearization circuit, the output of which is input to a voltage controlled voltage source, which produces a conditioned control voltage. The conditioned control voltage is fed back to the linearization circuit and is provided to each of the multiple cascode amplifier stages. The conditioned control voltage controls the amplifier stages such that a linear change in the control voltage produces a linear change in dB of the overall gain of the circuit. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the invention, a control signal is conditioned such that it produces a conditioned control signal which is the sum of an exponentially varying component and a constant component. The resulting conditioned control signal, applied to an amplifier circuit, produces a gain which varies linearly in dB with changes in the control voltage. 
     In accordance with a second aspect of the invention, a control signal is applied to an amplifier made up of a plurality of variable gain attenuator stages, the control signal having been conditioned such that it produces a gain which varies linearly in dB with changes in the control voltage. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a schematic circuit diagram of an attenuator stage forming part of an amplifier circuit in accordance with the invention. 
     FIG. 2 is a schematic circuit diagram of an alternative attenuator stage forming part of an amplifier circuit in accordance with the invention. 
     FIG. 3 is a schematic circuit diagram of a conditioning circuit forming part of a circuit in accordance with the invention. 
     FIG. 4 is a schematic circuit diagram of a component of a conditioning circuit forming part of a circuit in accordance with the invention. 
     FIG. 5 is a schematic circuit diagram of a further component of a conditioning circuit forming part of a circuit in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a variable gain attenuator  2 , which is based around a long-tailed pair of bipolar transistors  4 ,  6 , the emitter terminals of which are connected together. A signal current Isignal, represented as a current source  8 , is applied to the emitters of the transistors  4 ,  6 . A control voltage V CONT  is applied to a base terminal of a first of the transistors  4 , while the base terminal of the second transistor  6  is held at a constant reference voltage V REF . The relative sizes of the voltages V CONT , V REF determine what fraction of the signal current Isignal is steered through each of the transistors  4 ,  6 , and the current Iout through the first transistor  4  is taken as the attenuator output current. Thus, the circuit of FIG. 1 acts as a controllable attenuator. 
     FIG. 2 shows a differential current steering attenuator, acting on the same principle as the circuit of FIG.  1 . 
     A first long-tailed pair is made up of transistors  22 ,  24  with their emitter terminals connected together. A positive signal current Isignal+, represented as a current source  26 , is applied to the emitter terminals of the transistors  22 ,  24 . A second long-tailed pair is made up of transistors  28 ,  30  with their emitter terminals connected together. A negative signal current Isignal−, represented as a current source  32 , is applied to the emitter terminals of the transistors  28 ,  30 . 
     A positive control signal Vc+ is applied to the base terminals of the transistors  22 ,  28 , while a negative control signal Vc− is applied to the base terminals of the transistors  24 ,  30 . The collector terminals of the transistors  24 ,  30  are connected together, for example to a positive supply voltage. 
     A fraction of the positive signal current Isignal+ is steered through the transistor  22 , and this fraction is the positive output current Iout+. The size of the fraction is determined by the difference between the positive control signal Vc+ and the negative control signal Vc−. 
     Similarly, a fraction of the negative signal current Isignal− is steered through the transistor  28 , and this fraction is the negative output current Iout−. Again the size of the fraction is determined by the difference between the positive control signal Vc+ and the negative control signal Vc−. 
     Thus, a differential output current is produced, the magnitude of which is determined by the magnitude of the differential input current. The relationship between these two, that is, the degree of attenuation introduced by the circuit, is determined by the magnitude of the differential control voltage. 
     In fact, the gain factor Y(x) of each variable gain attenuator circuit is related to the magnitude of the control voltage x by a relationship:          Y        (   x   )       =     1     1   +              -   q     ·   x     kT                                  
     where (kT/q) is the thermal voltage of the transistors. 
     In order to produce a gain factor which varies linearly in dB with changes in the control voltage, the control voltage x is preprocessed to produce a preprocessed control voltage signal V(x), which is a function of x. As discussed, it is desired that the gain factor Y(V(x)) produced by the preprocessed control voltage varies linearly in dB with changes in the control voltage. Thus, it is desired that:            20   ·   log                     (     1     1   +              -   q     ·   V     kT           )       ∝   x                          
     Put differently:                    x            [       20   ·   log                     (     1     1   +            -   qV     kT           )       ]       =     const   .                            
     Solving this equation for V gives the result that the desired form of preprocessing is that which gives:          V        (   x   )       =           -   kT     q     ·   ln                     (       3.35   ·            -   21.87     ·   x         -   1     )                              
     FIG. 3 is a schematic circuit diagram of the preprocessing circuit used to produce a preprocessed control voltage V from the input x which has this desired form, and can therefore be used to control the variable gain atttenuator circuit FIG.  2 . 
     In fact, in order to make the circuit more easily realisable in bipolar technology, the circuit of FIG. 3 preprocesses the input signal x to produce a preprocessed signal V which is of the form:          V        (   x   )       =         kT   q     ·   ln                     (         I   b     ·            -     (       (     Vb   +     A   ·   x       )     ·   q     )       kf         -       I   b     ·            -     (       (     Vb   +     A   ·   Vref       )     ·   q     )       kT           )                              
     The circuit of FIG. 3 receives the input linear control signal x at an input terminal  50 . The circuit also receives a differential bias voltage Vb (=Vb 1 −Vb 2 ) at input terminals  52 ,  54  respectively. 
     FIG. 4 shows a circuit for generating the voltages Vb 1  and Vb 2 . A known reference voltage is applied to the base of a transistor  90 , the emitter terminal of which is connected to ground through a resistor  92 , and the collector terminal of which is connected to a positive supply through a PNP transistor  94 , which forms part of a current mirror with a second PNP transistor  96 . The collector terminal of the second PNP transistor  96  is connected to ground through two resistors  98 ,  100 . Thus, a known current is drawn through these resistors, and the voltages Vb 1 , Vb 2  can be taken from the terminals of the resistor  98 , by appropriate selection of component values. 
     The circuit of FIG. 3 also receives a reference voltage Vref at input terminal  56 . 
     FIG. 5 shows a circuit for generating the voltage Vref. A known reference voltage is applied to the base of a transistor  102 , the emitter terminal of which is connected to ground through a resistor  104 , and the collector terminal of which is connected to a positive supply through a PNP transistor  106 , which forms part of a current mirror with a further PNP transistor  108 . The collector terminal of the further PNP transistor  108  is connected to ground through a resistor  110 . Thus, a known current is drawn through this resistor, and the reference voltage Vref can be taken from the junction of this resistor with the collector terminal of the transistor  108 , by appropriate selection of component values. 
     Returning to the circuit of FIG. 3, the control voltage x is attenuated by resistors  58 ,  60 , having values R 1 , R 2  respectively. These resistors provide an attenuation factor A, such that the proportion of the input voltage which is applied to the base of transistor Q 0  is A.x. The attenuation ensures that the transistor Q 0  does not move into the saturated region of operation. 
     Transistor Q 0  forms a long-tailed pair with a transistor Q 1 , their emitters being connected together, and connected to a negative supply through a resistor  62 . The voltages Vb 1 , Vb 2 , taken from the terminals of the resistor  98  and applied to input terminals  52 ,  54 , as discussed earlier, are applied to base terminals of transistors Q 0 , Q 1  respectively through resistors  60 ,  64 . 
     The differential voltage applied to the base terminals of the transistors ensures that the long-tailed pair is kept switched such that a very much larger fraction of the total current flows through transistor Q 0  than through Q 1 . As a result, the current I Q1  through the transistor Q 1  has a negative exponential relationship with the control voltage x. In other words:          I   Q1     ∝          -   x                              
     This provides the required negative exponential relationship between V and x discussed above. The required relationship between V and x also includes a constant current component, which is obtained as follows. 
     The reference voltage Vref, obtained from the circuit of FIG. 5 as discussed earlier, is attenuated by resistors  66 ,  68 , having the same values as resistors  58 ,  60 , namely R 1  and R 2  respectively. These resistors provide an attenuation factor A, such that the proportion of the reference voltage which is applied to the base of transistor Q 2  is A.Vref. 
     Transistor Q 2  forms a long-tailed pair with a transistor Q 3 , their emitters being connected together, and connected to a negative supply through a resistor  70 . The voltages Vb 1 , Vb 2 , taken from the terminals of the resistor  98  and applied to input terminals  52 ,  54 , as discussed earlier, are applied to base terminals of transistors Q 2 , Q 3  respectively through resistors  68 ,  72 . Resistor  68  has a resistance value R 2  matching that of resistor  60 , while resistor  72  has a resistance value R 3  matching that of resistor  64 . Transistors Q 2 , Q 3  match transistors Q 0 , Q 1 . 
     The differential voltage applied to the base terminals of the transistors Q 2  and Q 3  ensures that the long-tailed pair is kept switched such that a very much larger fraction of the total current flows through transistor Q 2  than through Q 3 . As a result, the current I Q3  through the transistor Q 3  has a negative exponential relationship with the constant reference voltage Vref. The matching of the long-tailed pair Q 2 /Q 3  with the long-tailed pair Q 0 /Q 1  ensures that the two exponential relationships track each other. Specifically, although the currents in the resistors  62 ,  70  each have small positive temperature coefficients, those currents track each other. Resistors  62 ,  70  could be replaced by constant current sources, if desired. 
     The negative exponential relationship with the constant reference voltage Vref provides the required constant current component discussed above. 
     The collector terminal of transistor Q 3  is connected to the collector terminal of a PNP transistor Q 5 , which is connected to PNP transistor Q 4  to form a current mirror circuit, such that I Q3  flows in Q 4  also. The collector terminal of transistor Q 4  is connected to the collector terminal of transistor Q 1 . A diode connected transistor Q 6 , that is, having its base and collector terminals connected together and connected to a positive supply, has its emitter terminal connected to the collector terminals of transistors Q 1  and Q 4 . Thus, the current I Q6  flowing in the transistor Q 6  is the difference between the currents flowing in transistors Q 1  and Q 4 . Thus, I Q6 =I Q1 −I Q3 . 
     Moreover, the base emitter voltage of the transistor Q 6 , namely the voltage V 74  at the node  74  between the collector terminals of transistors Q 1  and Q 4 , has a logarithmic relationship to the current flowing in Q 6 . Thus:        V74   =         kT   q     ·   ln                     (     I   Q6     )                              
     This voltage V 74  therefore has the required relationship to the input x. 
     In fact, as discussed above, the currents I Q1  and I Q3 , and hence I Q6  are relatively small, at least compared to the total currents flowing in the long-tailed pair resistors  62 ,  70 . Therefore, in order to provide sufficient drive to the transistors of the long-tailed pair or pairs in the variable gain attenuator, a buffer amplifier  76  is provided, with its first input connected to the node  74 . The buffer amplifier can also be used to convert the single-ended voltage at node  74  into a differential voltage for use in the differential variable gain attenuator of FIG.  2 . In that case, the second input of the buffer amplifier  76  is connected to a node  78  between the collector terminals of transistors Q 3 , Q 5 . Since the two long-tailed pair subcircuits track each other, as discussed above, the DC levels at the nodes  74 ,  78  therefore also track each other, and the difference between the voltage levels there provides the required differential voltage. 
     At the output of the buffer amplifier  76 , there are provided the required positive control signal Vc+ and the negative control signal Vc− for supply to the variable gain attenuator of FIG.  2 . Capacitors C 0  and C 1 , connected between the respective output lines and the negative supply, limit the noise supplied to the variable gain attenuator circuit. 
     There is thus provided a circuit for preprocessing a control voltage such that, when the preprocessed control voltage is applied to a variable gain attenuator, the gain in dB varies linearly with the control voltage.