Abstract:
A current-controlled resistor comprises a first input terminal configured to receive an input signal and a second input terminal configured to receive a current control signal. The resistor comprises a first stage configured to receive the current control signal; the first stage includes first and second PN diodes having first terminals of a first type and second terminals of a second type. The first terminals of the first and second PN diodes are coupled each other and a second terminal of the first PN diode is coupled to the first input terminal. The resistor comprises a second stage configured to receive the current control signal; the second stage includes a third PN diode having first and second terminals of the first and second types, the second terminal of the third PN diode being coupled to the second terminal of the second PN diode.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present disclosure relates to the electronic field. More in particular, the disclosure concerns a current-controlled resistor. 
         [0003]    2. Description of the Related Art 
         [0004]    An attenuator is an electronic device commonly used in wireless applications at high frequencies, for example at radio-frequencies (RF) or microwaves (more in general a frequency in the range between 10 Mhz and 2 Ghz); the attenuation of the attenuator is variable in order to control automatically the voltage level of a high-frequency input signal. 
         [0005]    It is known that forward biased Positive-Intrinsic-Negative (PIN) diodes are used in the design of radio-frequencies or microwaves attenuators having a variable attenuation, wherein at least one PIN diode is used for controlling automatically the voltage level of an input RF signal. In fact, at high frequencies a PIN diode is equivalent to a current-controlled resistor, that is, it operates as a variable resistor having a resistance value controlled by the forward bias current flowing in the PIN diode, the resistance value being inversely proportional to the forward bias current; for example, the minimum resistance value of the PIN diode is 10 Ohm and the maximum resistance value is 1500 Ohm. 
         [0006]      FIG. 1  shows a current-controlled resistor  1  according to the prior art. The current-controlled resistor  1  comprises: 
         [0007]    an input terminal I 101  for receiving a RF input signal; 
         [0008]    a control input terminal I 103  for receiving from a current source IE 102  a variable bias control current I for controlling the small signal resistance value between the input terminal I 101  and ground and between the input terminal I 101  and the control input terminal I 103 ; 
         [0009]    a series connection of two forward biased PIN diodes D 100 , D 101  connected between the current source IE 102  and ground GND (PIN diodes D 100 , D 101  are enclosed in  FIG. 1  by a square for distinguishing with respect to a PN diode). 
         [0010]    The series connection of the two forward biased PIN diodes D 100 , D 101  implement two variable resistors, each one having a variable resistance value controlled by the bias control current I. 
         [0011]    The current-controlled resistors according to the prior art have the disadvantage to require the use of at least one PIN diode, which is a device which can&#39;t be simply implemented into an integrated circuit, because additional process steps are necessary for the formation of the intrinsic layer of the PIN diode in standard technologies (such as Bipolar, CMOS or BiCMOS technologies). 
       BRIEF SUMMARY 
       [0012]    According to a first aspect, the present disclosure provides a current-controlled resistor comprising a first input terminal configured to receive an input signal and comprising a second input terminal configured to receive a current control signal. The current-controlled resistor comprises a first stage configured to receive at least part of the current control signal; the first stage includes first and second PN diodes having respective first terminals of a first type and respective second terminals of a second type, wherein the first terminals of the first and second PN diodes are coupled each other and wherein a second terminal of the first PN diode is coupled to the first input terminal. The current-controlled resistor further comprises a second stage configured to receive at least part of the current control signal; the second stage includes a third PN diode having first and second terminals of the first and second types, respectively, the second terminal of the third PN diode being coupled to the second terminal of the second PN diode of the first stage. 
         [0013]    Preferably, the current-controlled resistor includes a first current mirror interposed between the second input terminal and the first terminals of the first, second and third PN diodes; the first current mirror is configured to transmit part of the current control signal to the first stage and part to the second stage. 
         [0014]    Preferably, the first terminals of the first and second PN diodes are anodes coupled each other and to the first current mirror, the second terminal of the first PN diode is a cathode coupled to the first input terminal, the first terminal of the third PN diode is an anode coupled to the first current mirror, the second terminal of the second PN diode is a cathode and the second terminal of the third PN diode is a cathode coupled to the cathode of the second PN diode. 
         [0015]    Preferably, the first stage includes fourth and fifth PN diodes. The anode of the fourth PN diode is coupled to the first input terminal and to the cathode of the first PN diode and the anode of the fifth PN diode is coupled to the cathode of the second PN diode. The cathode of the fourth PN diode is coupled to the cathode of the fifth PN diode. The second stage includes a sixth PN diode having an anode coupled to the cathode of the third PN diode and coupled to the anode of the fifth PN diode. The current-controlled resistor includes a second current mirror coupled to a cathode of the sixth PN diode and coupled to the cathodes of the fourth and fifth PN diodes. 
         [0016]    According to a second aspect, the present disclosure provides an electronic receiver including an amplifier configured to receive an input signal and to generate therefrom an amplified signal and a current control signal. The electronic receiver further includes a current-controlled resistor comprising a first input terminal configured to receive the input signal and comprising a second input terminal configured to receive the current control signal. The current-controlled resistor comprises a first stage configured to receive at least part of the current control signal; the first stage includes first and second PN diodes having respective first terminals of a first type and respective second terminals of a second type, wherein the first terminals of the first and second PN diodes are coupled each other and wherein a second terminal of the first PN diode is coupled to the first input terminal. The current-controlled resistor further comprises a second stage configured to receive at least part of the current control signal; the second stage includes a third PN diode having first and second terminals of the first and second types, respectively, the second terminal of the third PN diode being coupled to the second terminal of the second PN diode of the first stage. 
         [0017]    Preferably, the amplifier is configured to change the value of the current control signal to keep the voltage level of the input signal within a range of values. 
         [0018]    Preferably, the amplifier is configured to change its gain value to maintain the amplified signal level smaller or equal than a value. 
         [0019]    The Applicant has recognized that the current-controlled resistor according to the present disclosure has the advantage that it can be implemented into an integrated circuit in standard bipolar or BiCMOS technologies, keeping at the same time electric properties (conductance, linearity, noise) which are substantially equivalent to current-controlled resistors implemented by PIN diodes. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0020]      FIG. 1  schematically shows a current-controlled resistor according to the prior art. 
           [0021]      FIG. 2  schematically shows an electronic receiver according to the present disclosure. 
           [0022]      FIG. 3  schematically shows a current-controlled resistor according to the present disclosure. 
           [0023]      FIG. 4  schematically shows the graph of the small signal conductance of the current-controlled resistor as a function of the voltage excursion around the bias point. 
           [0024]      FIG. 5  schematically shows the graph of the small signal conductance of the current-controlled resistor as a function of the bias current. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    Referring to  FIG. 2 , it shows an electronic receiver  50  according to the present disclosure. 
         [0026]    The electronic receiver  50  includes a RF amplifier  51 , a current-controlled resistor  1 , an input capacitor C in  and a decoupling capacitor C out . At RF frequencies typically employed for communications, for example 10 Mhz-2 Ghz, both the input capacitor C in  and the decoupling capacitor C out  are substantially equivalent to a short circuit. 
         [0027]      FIG. 2  further shows an antenna  52  for receiving a RF received signal S rx  from a communication channel and shows a source impedance Z s  which is the direct or transformed impedance of the antenna  52 . 
         [0028]    The RF amplifier  51  includes an input terminal for receiving an RF input signal S in , a first output terminal for generating an amplified signal S amp , obtained by amplification of the radio-frequency signal S in , and a second output terminal for generating a variable current control signal I b . 
         [0029]    The current-controlled resistor  1  includes a first input terminal I 1  for receiving the RF input signal S in  and includes a second input terminal I ctrl  for receiving the variable current control signal I b . 
         [0030]    The operation of the RF amplifier  51  is such to change the value of the current control signal I b  provided to the current-controlled resistor  1  in order to control the RF input signal S in , for example by keeping the voltage level of the RF input signal S in  within a range of values: in this example, the RF amplifier  51  and the current-controlled resistor  1  are such to operate as an automatic gain controlled (AGC) loop, wherein the gain control is achieved by a voltage divider which is composed of the impedance Z s  and of the current-controlled resistor  1  having a small signal resistance r 1  between the first input terminal I 1  and ground. 
         [0031]    It will be described hereinafter the operation of the electronic receiver  50 . 
         [0032]    It is supposed that for RF communication frequencies both the input capacitor C in  and the output capacitor C out  are equivalent to a short circuit. It is also supposed that the RF amplifier  51  generates the current control signal I b  having a direct current value 
         [0033]    The antenna  52  receives from the communication channel a RF received signal S rx , which generates a voltage at the terminal of the source impedance Z s . It is supposed that the input impedance of the RF amplifier  51  is high with respect to the impedance Z 1  of the current-controlled resistor  1 : in this case, the RF input signal S in  is a voltage signal which is calculated by the voltage divider composed of the impedance Z s  and of the current-controlled resistor  1  having the small signal resistance r 1  between the first input terminal I 1  and ground. Therefore the RF input voltage signal S in  is calculated according to the following formula: S in =S rx *r 1 /(Z s +r 1 ) 
         [0034]    The RF amplifier  51  receives the RF input voltage signal S in  and generates the amplified signal S ampl  which is a voltage signal obtained by amplification of the RF input voltage signal S in  according to the actual gain value of the RF amplifier  51 . 
         [0035]    Moreover, the RF amplifier  51  receives the RF input voltage signal S in , detects that the value of the RF input voltage signal S in  is outside the ranges of values and generates the current control signal I b  having a current value i b2  which has a small variation with respect to the direct current value I b1 . 
         [0036]    The current-controlled resistor  1  receives at the second input terminal I ctrl  the current control signal I b  having the current value i b2 , which modifies the small signal resistance between the first input terminal I 1  and ground of the current-controlled resistor  1  from the value r 1  to the value r 1 ′; therefore the RF input voltage signal S in  changes to the following value: S in ′=S rx *r 1 ′/(Z s +r 1 ′) 
         [0037]    The RF amplifier  51  receives the RF input voltage signal S in ′ and detects that the value of the RF input voltage signal S in ′ is within the ranges of values: the RF amplifier  51  keeps the actual value i b2  of the current control signal I b  until the RF amplifier  51  detects that the RF input voltage signal S in  changes to a new value S in ″ which is outside the range of values. 
         [0038]    Advantageously, the gain of the RF amplifier  51  is controlled by an automatic gain controlled loop including an AGC RF amplifier  51  and the current-controlled resistor  1 : this allows to extend the range of the RF received signal S rx  for which the automatic gain controlled amplifier  51  can maintain a defined maximum signal level of the amplified signal S ampl . 
         [0039]    Referring to  FIG. 3 , it shows more in detail the current-controlled resistor  1  of  FIG. 2 . 
         [0040]    The current-controlled resistor  1  includes: 
         [0041]    the first input terminal for receiving an input signal; 
         [0042]    the second input terminal I ctrl  for receiving a variable current control signal I b ; 
         [0043]    a first stage  30 ; 
         [0044]    a second stage  40  connected to the first stage  30  into the node  13 ; 
         [0045]    a first current mirror  31 . 
         [0046]    The first stage  30  includes a first PN diode D 1  and a second PN diode D 2 ; the anode  5  of the first PN diode D 1  is connected to the anode  6  of the second PN diode D 2  and the anodes  5 ,  6  are connected to the first current mirror  31 . 
         [0047]    The first current mirror  31  is interposed between the second input terminal I ctrl  and the anodes  5 ,  6  of the first and second PN diodes D 1 , D 2  of the first stage  30  and is interposed between the second input terminal I ctrl  and the second stage  40 . The first current mirror is such to transmit part of the current control signal I b  to the first stage  30  and part to the second stage  40 . 
         [0048]    The cathode  17  of the first PN diode D 1  is connected to the first input terminal I 1 . 
         [0049]    The second stage  40  includes a PN diode D 5  having the anode  12  connected to the first current mirror  31  and having the cathode  8  connected to the cathode  7  of the second PN diode D 2 . 
         [0050]    Preferably, the first stage  30  further includes a third PN diode D 3  having the anode  18  connected to the cathode  17  of the first PN diode and includes a fourth PN diode D 4  having the anode  4  connected to the cathode  7  of the second PN diode D 2 ; moreover, the cathode  9  of the third PN diode D 3  is connected to the cathode  10  of the fourth PN diode D 4 . The second stage  40  further includes another PN diode D 6  having the anode  11  connected to the cathode  8  of the PN diode D 5 . 
         [0051]    Preferably, the current-controlled resistor  1  further includes a second current mirror  32  interposed between the third and fourth diodes D 3 , D 4  of the first stage  30  and ground and is interposed between the other PN diode D 6  of the second stage  40  and ground. Specifically, the second current mirror  32  is connected from one side to the cathodes  9 ,  10  of the third and fourth diodes D 3 , D 4  of the first stage  30  and to the cathode  14  of the other PN diode D 6  of the second stage  40  and is connected from another side to ground. 
         [0052]    The group of devices including the first current mirror  31 , the first PN diode D 1 , the second PN diode D 2  and the PN diode D 5  of the second stage performs the function of a first variable resistor between the first input terminal I 1  and the second input terminal I ctrl , wherein the resistance value of the first variable resistor is controlled by the bias current I b  received at the second input terminal I ctrl . 
         [0053]    The group of devices including the third PN diode D 3 , the fourth PN diode D 4 , the PN diode D 6  of the second stage and the second current mirror  32  performs the function of a second variable resistor between the first input terminal I 1  and ground, wherein the resistance value of the second variable resistor is controlled by the bias current I b  received at the second input terminal I ctrl . 
         [0054]    Preferably, the current-controlled resistor  1  further includes a diode TD 4  connected between the second input terminal I ctrl  and the anodes  5 ,  6  of the first and second PN diodes D 1 , D 2 . Advantageously, the diode TD 4  is a trans-diode implemented with a pnp (positive-negative-positive) BJT (Bipolar Junction Transistor) having the base terminal b connected to the collector terminal c with a short circuit, having the emitter terminal e connected to the second input terminal I ctrl  and having the collector terminal c connected to the anodes  5 ,  6  of the first and second PN diodes D 1 , D 2  respectively. The trans-diode TD 4  has the advantage to improve the linearity of the conductance of the current-controlled resistor  1  as a function of the current control signal I b , as it will be explained more in detail afterwards. 
         [0055]    Preferably, the current-controlled resistor  1  further includes another diode TD 1  connected between the anodes  9 ,  10  of the third and fourth PN diodes D 3 , D 4  and ground. Advantageously, the diode TD 1  is a trans-diode implemented with a npn (negative-positive-negative) BJT having the base terminal b connected to the collector terminal c with a short circuit, having the emitter terminal e connected to ground and having the collector terminal c connected to the cathodes  9 ,  10  of the third and fourth PN diodes D 3 , D 4  respectively. The trans-diode TD 1  has the advantage to improve the linearity of the conductance of the current-controlled resistor  1  as a function of the current control signal I b , as it will be explained more in detail afterwards. 
         [0056]    Preferably, the current mirrors  31  and  32  are implemented with BJT. Specifically, the first current mirror  31  includes a first pnp BJT TD 6  and a second pnp BJT T 5 ; the base terminal b of the first pnp BJT TD 6  is connected to the base terminal b of the second pnp BJT T 5 , the emitter terminals e of the first pnp BJT TD 6  and of the second pnp BJT T 5  are connected each other and to the second input terminal I ctrl , the collector terminal c of the second pnp BJT T 5  is connected to the anode  6  of the second PN diode and the collector terminal c of the first pnp BJT TD 6  is connected to its base terminal b and to the anode  12  of the PN diode D 5  of the second stage. The second current mirror  32  includes a third npn BJT TD 3  and a fourth npn BJT T 2 ; the base terminal b of the third BJT TD 3  is connected to the base terminal b of the fourth npn BJT T 2 , the emitter terminals e of the third and fourth npn BJTs are connected each other and to ground, the collector terminal c of the fourth npn BJT T 2  is connected to the anode  10  of the fourth PN diode D 4  and the collector terminal c of the third npn BJT TD 3  is connected to its base terminal b and to the anode  14  of said other PN diode D 6  of the second stage. 
         [0057]    Alternatively, the current mirrors  31  and  32  are implemented with MOSFET (Metal-Oxide-Semiconductor Field Effect Transistors), by replacing the pnp BJTs with p-channel MOSFETs and replacing the npn BJTs with n-channel MOSFETs. 
         [0058]    Alternatively, the current mirrors  31  and  32  can be implemented with a circuit different from the one shown in  FIG. 3 . 
         [0059]    It is worth noting that the current-voltage characteristic of the first stage  30  between a current signal injected into the first input terminal I 1  and the voltage drop between the first input terminal I 1  and the node  13  is substantially according to the hyperbolic tangent function, and the current-voltage characteristic of the second stage  40  between the current signal injected into the first input terminal I 1  and the voltage drop between the node  13  and ground is substantially according to the hyperbolic sine function. The combination of the hyperbolic tangent current-voltage characteristic with the hyperbolic sine current-voltage characteristic allows to achieve a graph of the small signal conductance of the current-controlled resistor  1  (that is, the small signal relationship between the current flowing into the first input terminal I 1  and a voltage signal applied to the first input terminal I 1  as a function of the same voltage signal) which is substantially constant over a large range of voltage values around the bias point, as shown schematically in  FIG. 4  with line  61  (or with lines  62  or  63 ). Moreover,  FIG. 4  shows three lines  61 ,  62 ,  63  of the small signal conductance corresponding to bias current I b =5 mA, 3 mA, 1 mA respectively: each of the three lines  61 ,  62 ,  63  shows that the small signal conductance is substantially constant as a function of the voltage applied to the first input terminal I i . 
         [0060]    Moreover, the small signal conductance of the current-controlled resistor  1  depends only on the current control signal I b , that is the graph of the small signal conductance at a defined bias current I b  is substantially flat as a function of the voltage applied to the first input terminal I 1  for a large voltage range around the bias point, while the absolute value of the small signal conductance in the flat region is proportional to the current control signal I b , as shown schematically in  FIG. 5  with line  64 . 
         [0061]    In one embodiment, the junction areas of the first PN diode D 1 , of the second PN diode D 2  and of the PN diode D 5  are substantially equal each other, the emitter area of the first pnp BJT TD 6  is about 5 times the junction area of the trans-diode TD 4  and the emitter area of the second pnp BJT T 5  is about 9 times the junction area of the trans-diode TD 4 . These specific values allows to optimize the flatness of the graph of the small signal conductance of the current-controlled resistor  1  as a function of the voltage excursion around the bias point at the first input terminal I 1 , because the flatness is achieved by adjusting the weight of the hyperbolic tangent current-voltage characteristic with respect to the weight of the hyperbolic sine current-voltage characteristic. 
         [0062]    In one embodiment, the junction areas of the third PN diode D 3 , of the fourth PN diode D 4  and of the PN diode D 6  are substantially equal each other, the emitter area of the third npn BJT TD 3  is about 5 times the junction area of the other trans-diode TD 1  and the emitter area of the fourth npn BJT T 2  is about 9 times the junction area of the other trans-diode TD 1 . These specific values allows to optimize the flatness of the graph of the small signal conductance of the current-controlled resistor  1  as a function of the voltage excursion around the bias point at the first input terminal I 1 , because the flatness is achieved by adjusting the weight of the hyperbolic tangent current-voltage characteristic with respect to the weight of the hyperbolic sine current-voltage characteristic. 
         [0063]    Preferably, the junction areas of the first, second, third, fourth PN diodes D 1 , D 2 , D 3 , D 4  and of the PN diodes D 5 , D 6  are substantially equal each other, the emitter areas of the trans-diodes TD 4  and TD 1  are equal each other, the emitter areas of the first pnp BJT TD 6  and of the third npn BJT TD 3  are equal each other and are about 5 times the junction area of the trans-diode TD 4 , and the emitter areas of the second pnp BJT T 5  and of the fourth npn BJT T 2  are equal each other and are about 9 times the junction area of the trans-diode TD 4 . 
         [0064]    The above indicated values of the ratio between the area of the emitters and the junction area are only an example: a similar flatness of the graph of the small signal conductance can be achieved with different values of the ratio between the area of the emitters and the junctions area. 
         [0065]    It will be described hereinafter the polarization of the electronic circuit implementing the current-controlled resistor  1 . 
         [0066]    The current-controlled resistor  1  receives at the second input terminal I ctrl  the current control signal I b  having a direct current value I b1 , which flows partly in the first current mirror  31  and partly in the trans-diode TD 4 . The transistors T 5  and T 2  are biased in the forward active region, the transistors TD 1 , TD 3 , TD 4 , TD 6  are also biased in the forward active region (thus operating as forward biased diodes) and the diodes D 1 -D 6  are forward biased. 
         [0067]    It is supposed that the voltage drop at the ends of the forward biased diodes D 1 -D 6  is 0.7 volts, that the voltage drop between base and emitter of transistors TD 1 , TD 3 , TD 4 , TD 6  is 0.7 volts and that the voltage drop between base and emitter of transistors T 2 , T 5  is 0.7 volts. 
         [0068]    Therefore the voltage of nodes  9 ,  10 ,  14  is 0.7 volts, the voltage of nodes  18 ,  17 ,  4 ,  7 ,  13 ,  11 ,  8  is 1.4 volts, the voltage of nodes  5 ,  6 ,  12  is 2.1 volts and the voltage of the second input terminal I ctrl  is 2.8 volts. 
         [0069]    The electronic receiver  50  of the present disclosure can be implemented into a tuner for a AM/FM car radio; preferably, the tuner is implemented into an integrated circuit. 
         [0070]    The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.