Abstract:
A transconductance cell has first and second transistors, each transistor having a control terminal and first and second terminals. A signal is output from the second transistor in response to a voltage input applied to the control terminal of the first transistor. The transconductance cell includes a linear element coupled between the first terminal of the first transistor and the first terminal of the second transistor. A tank circuit is coupled between a reference potential and a node between the linear element and the first terminal of the second transistor.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to transconductance cells, in particular, to linearized transconductance cells for use in electronic circuits. 
     A transconductance cell is an electronic building block used to build more complex electronic circuits. It is widely used in RF circuits such as low noise amplifiers and Gilbert cell mixers. 
     The transconductance cell performs the function of converting a voltage input into a current output. The characteristics of a desirable transconductance cell are high bandwidth, low noise, low power consumption, high output impedance, low distortion and good common mode rejection. 
     FIG. 1 shows a schematic diagram of a prior art transconductance cell  20 . Other parts of the RF circuit (e.g., the Gilbert cell mixer) to which the transconductance cell  20  is connected are not shown. The prior art transconductance cell  20  for RF applications includes a first bipolar transistor  22 , a second bipolar transistor  32 , a first inductor  24 , a second inductor  30 , a resistor  26  and a capacitor  34 . A voltage input  21  is coupled to the base of the first transistor  22 . The collector of the first transistor  22  is coupled to another part of the circuit. The emitter of the first transistor  22  is coupled to one end of the first inductor  24 . The opposing end of the first inductor  24  is coupled to one end of the second inductor  30 . One end of the resistor  26  is coupled to a node between the first and second inductors  24  and  30 . The other end of the resistor  26  is coupled to a node  28  which may be a ground. 
     The opposing end of the second inductor  30  is coupled to the emitter of the second transistor  32 . The collector of the second transistor is coupled to another part of the circuit. The base of the second transistor  32  is coupled to the capacitor  34  and is biased at a constant voltage. 
     In operation, a bias voltage is applied to the transconductance cell  20  at the bases and the collectors of the first and second transistors  22  and  32  to bias the first and second transistors  22  and  32  for operation. In response to the bias voltage, DC currents flow through the first and second transistors  22  and  32  and exit through the resistor  26 . The bias voltage typically ranges between 2.7 volts and 5.5 volts for RF circuits. Much of the bias voltage is dropped across the resistor  26  which is designed to have high impedance, as explained below. The resistor  26  also can be implemented as a transistor which also cause a voltage drop. 
     Once the bias voltage has been applied, the voltage input  21  is applied to the base of the first transistor  22  to output a signal from the emitter of the first transistor  22 . The signal travels through the first and second inductors  24  and  30  and is output from the collector of the second transistor  32  to another part of the circuit. The signal output by the first transistor  22  may be directed from the first inductor  24  to the second inductor  30  without significant signal dissipation through the resistor  26  by using a resistor that has a high impedance value as the resistor  26 . 
     One problem associated with the prior art transconductance cell  20  is that this requisite high impedance of the resistor  26  makes it difficult to operate the circuit at a low voltage. 
     Another problem associated the prior art transconductance cell  20  that the noise factor (NF) and the third-order input intercept point (IIP 3 ) are degraded due to signal loss in the resistor  26 . The noise contribution of the resistor  26  lowers the NF as well. 
     SUMMARY OF THE INVENTION 
     In one aspect, the invention features a linearized transconductance cell. The invention includes a first transistor having a control terminal and first and second terminals; a second transistor having a control terminal and first and second terminals, wherein a signal is output from the second terminal of the second transistor in response to an input voltage applied to the control terminal of the first transistor; a linear element coupled between the first terminal of the first transistor and the first terminal of the second transistor; and a tank circuit coupled between a reference voltage and a node between the linear element and the first terminal of the second transistor. 
     In another aspect, the invention features an electronic circuit including a transconductance cell. The transconductance cell includes a first transistor having a control terminal and first and second terminals; a second transistor having a control terminal and first and second terminals, wherein a signal is output from the second terminal of the second transistor in response to an input voltage applied to the control terminal of the first transistor; a linear element having first and second ends coupled between the first terminal of the first transistor and the first terminal of the second transistor; and a tank circuit having a first end coupled to a reference voltage and a second end coupled to a node between the linear element and the first terminal of the second transistor. 
     In another aspect, the invention features a transconductance cell having a single-ended input. The invention includes a first transistor having a control terminal and first and second terminals; a second transistor having a control terminal and first and second terminals, wherein a current flows between the first and second terminals of the second transistor in response to an input voltage applied to the control terminal of the first transistor; a first linear element having first and second ends, the first end coupled to the first terminal of the first transistor; a second linear element having first and second ends coupled between the second end of the first linear element and the first terminal of the second transistor; and a tank circuit coupled between a reference voltage and a node between the first and second linear elements. 
     In another aspect, the invention features a transconductance cell having a single-ended input; a first transistor having a control terminal and first and second terminals; a second transistor having a control terminal and first and second terminals, where a current flows between the first and second terminals of the second transistor in response to an input voltage applied to the control terminal of the first transistor; a linear element having first and second ends coupled between the first terminal of the first transistor and the first terminal of the second transistor; and an inductor coupled between a reference voltage and a node between the linear element and the first terminal of the second transistor. 
     Among the advantages of the invention are that the invention: (1) virtually eliminates consumption of the bias voltage and significantly improves the headroom; (2) significantly improves the noise factor (NF) since an inductor, a noiseless device, is used rather than a resistor; (3) significantly improves the third-order input intercept point (IIP3) when compared to a standard differential pair; and (4) allows for the tuned transfer characteristic. 
     For fuller understanding of the nature and further advantages of the invention, reference should be made to the detailed description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 shows a schematic diagram of a prior art transconductance cell  20 . 
     FIG. 2 shows a schematic diagram of a linearized transconductance cell  100  in accordance with the invention. 
     FIG. 3 shows a frequency response curve of a tank circuit of the invention which includes an inductor and a capacitor. 
     FIG. 4 shows a transconductance cell of the invention having a tank circuit which includes an inductor and a varactor diode. 
     FIG. 5 shows a transconductance cell of the invention having a tank circuit which includes an inductor, a capacitor and a resistor. 
     FIG. 6 shows frequency response curves of a tank circuit of the invention as the resistance of a resistor is varied. 
     FIG. 7 shows a linearizing resistor coupled to a base of each of the two transistors in a transconductance cell of the invention. 
     FIG. 8 shows a Gilbert cell mixer using a transconductance cell according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2 shows a schematic diagram of a linearized transconductance cell  100  in accordance with the present invention. Other parts of the circuit (e.g., Gilbert cell mixer) to which the transconductance cell  100  is connected are not shown. The transconductance cell  100  includes a first transistor  102 , a second transistor  106 , a linear element, such as an inductor  104 , a tank circuit  120 , and a first capacitor  108 . Although the first and second transistors  102  and  106  may be field effect transistors, they are illustrated as bipolar transistors for purposes of the discussion herein. Similarly, although the linear element  104  may be a resistor or other form of linear device, the preferred embodiment is an inductor and referred to as “a first inductor  104 ” for discussion herein. 
     A voltage input  101  is coupled to the base of the first transistor  102  and controls the output signal on the collector of the second transistor  106 . The collector of the first transistor  102  is coupled to another part of the circuit. The emitter of the first transistor  102  is coupled to one end of the first inductor  104 . The opposing end of the first inductor  104  is coupled to the emitter of the second transistor  106 . The first inductor  104  is used to improve the linearity of the first and second transistors  102  and  106 . The collector of the second transistor  106  is coupled to another part of the circuit. The base of the second transistor  106  is coupled to the first capacitor  108  and is biased at a constant voltage. Therefore, the signal output from the collector of the second transistor  106  is determined solely by the voltage input  101  applied to the base of the first transistor  102 . 
     One end of the tank circuit  120  is coupled to a node between the first inductor  104  and the emitter of the second transistor  106 . The opposing end of the tank circuit  120  is coupled to a node  129  which may be a ground. The tank circuit  120  generally includes an inductor (i.e., a second inductor  122 ) and a capacitor (i.e., a second capacitor  124 ) coupled in parallel. Other forms of the tank circuit  120  will be discussed later. 
     In operation, a bias voltage is applied to the transconductance cell  100  at the bases and the collectors of the first and second transistors  102  and  106  to bias the first and second transistors  102  and  106  for operation. In response to this bias voltage, DC currents flow through the first and second transistors  102  and  106  and exit through the first inductor  122 . Since an inductor behaves like a short circuit to a DC current, the bias voltage does not experience any voltage drop across the first inductor  122 . This enables the transconductance cell  100  to be operated at a relatively low voltage and also increases the headroom for the circuit. 
     Once the bias voltage has been applied at the bases and collectors of the first and second transistors  102  and  106 , the voltage input  101  is applied to the base of the first transistor  102  to output a signal at the emitter of the first transistor  102 . The signal flows through the first inductor  104  and is received at the emitter of the second transistor  106 . The signal is output from the collector of the second transistor  106  to another part of the circuit. 
     The signal output by the first transistor  102  may be directed to the second transistor  106  without significant signal dissipation through the tank circuit  120  by appropriately choosing the inductance value of the second inductor  122  and the capacitance value of the second capacitor  124 , as explained below. The tank circuit  120  may be seen as a band-pass filter. The frequency response curve of the tank circuit  120  is shown in FIG.  3 . The resonant frequency f o  of the tank circuit is determined by the equation:          f   o     =     1       L                 C                                
     where L is the inductance of the second inductor  122  of the tank circuit  120  and C is the capacitance of the second capacitor  124  of the tank circuit  120 . 
     The gain, output over input, of the tank circuit  120  is greatest at the resonant frequency f o . That is, a signal having the resonant frequency flows from the first inductor  104  to the emitter of the second transistor  106  without experiencing significant signal dissipation through the tank circuit  120 , which is placed between the first inductor  104  and the emitter of the second transistor  106 . The tank circuit  120  with appropriately selected inductance L and capacitance C would, therefore, behave as a resistor having a high impedance value against a signal having the resonant frequency. 
     FIG. 4 shows the transconductance cell  100  having the tank circuit  120  which includes a second inductor  122  and a varactor diode  126 . The varactor diode acts as a capacitor whose capacitance changes according to an input voltage across it. The tank circuit  120  of FIG. 4 allows for tunable response to changes in the frequency of the signal across the varactor diode. For example, if the signal frequency is lowered, then the capacitance of the varactor diode  126  may be increased by adjusting the input voltage across the varactor diode  126 . 
     FIG. 5 shows the transconductance cell  100  having a tank circuit  120  which includes a second inductor  122 , a second capacitor  124  and a resistor  128 . The transconductance cell  100  of FIG. 5 may be used for broadband applications, such as TV tuners, which generally operates at a bandwidth greater than 100 MHz. 
     The bandwidth of a frequency response curve derived from the tank circuit  120  depends upon the total resistance across the tank circuit  120 . Since there are no perfect conductors, inductors inherently have some resistive value. Inductors generally have a very low resistance so the bandwidth of the frequency response of the tank circuit  120  of FIG. 2, having only one inductor and one capacitor tends to be narrow, for example, about 200 MHz or less. Such a narrow bandwidth is appropriate for narrow bandwidth applications such as cellular phones or cordless telephones. However, such tank circuits are not appropriate for broadband applications. The transconductance cell  100  of FIG. 2 may be adopted for broadband applications by coupling a resistor, such as the resistor  128  shown in FIG. 5, in parallel with the second inductor  122  and the second capacitor  124 . The resistance of the resistor  128  may be selected according to the desired bandwidth. 
     FIG. 6 shows frequency response curves  50 ,  52  and  54  as the resistance of the resistor  128  in FIG. 5 is progressively increased. The frequency response curve  50  illustrates the use of a resistor  128  having the lowest resistance of the three, and the frequency response curve  54  is for a resistor  128  having the highest resistance. 
     FIG. 7 shows a schematic diagram of a linearized transconductance cell  150  according to another embodiment of the present invention. The circuit arrangement of the transconductance cell  150  is substantially similar to that of the transconductance cell  100  shown in FIG.  2 . The transconductance cell  150  additionally includes first and second linearizing resistors  110  and  112  and capacitors  114  and  116 . 
     One end of the first linearizing resistor  110  is coupled to the base of the first transistor  102 . The opposing end of the first linearizing resistor  110  is coupled to one end of the capacitor  116 . The opposing end of the capacitor  116  is coupled to a node which may be a ground. The collector of the first transistor  102  is coupled to another part of the circuit. The emitter of the first transistor  102  is coupled to one end of the first inductor  104 . The opposing end of the first inductor  104  is coupled to the emitter of the second transistor  106 . The collector of the second transistor  106  is coupled to another part of the circuit. The base of the second transistor  106  is coupled to one end of the first capacitor  108  and one end of the second linearizing resistor  112 . The opposing end of the first capacitor  108  is coupled to a node which may be a ground. The opposing end of the second linearizing resistor  112  is coupled to a node between the first linearizing resistor  110  and the capacitor  116 . One end of the capacitor  114  is coupled to a node between the first linearizing resistor  110  and the base of the first transistor  102 . The opposing end of the capacitor  114  is coupled to a node which may be a ground. 
     One end of the tank circuit  120  is coupled to a node between the first inductor  104  and the emitter of the second transistor  106 . The opposing end of the tank circuit  120  is coupled to a node which may be a ground. The tank circuit  120  includes a second inductor  122  and a second capacitor  124  coupled in parallel. 
     The resistive values of the first and second linearizing resistors  110  and  112  may be selected to improve the linearity of the transconductance cell  150 . Generally, if the resistive value of the linearizing resistor  110  or  112  is decreased, the linearity of the transconductance cell  150  improves. However, such improvement in linearity is obtained at the cost of less gain and more noise. 
     Various forms of the transconductance cells described above may be used in any number of electronic circuits. One common use for the transconductance cell is in a Gilbert cell mixer. FIG. 8 shows a Gilbert cell mixer  300  using the transconductance cell  150  shown in FIG.  7 . Alternatively, the Gilbert cell  300  may use the transconductance cell  100  shown in FIGS. 2,  4  and  5 . 
     The present invention has been described in terms of a preferred embodiment. The invention, however, is not limited to the embodiment depicted and described. Rather, the scope of the invention is defined by the appended claims.