Biasing network for use with field effect transistor ring mixer
BACKGROUND OF THE INVENTION 
     The present invention relates to field effect transistors and more specifically to biasing circuits for use in conjunction with mixers which utilize field effect transistors. 
     Mixers which use field effect transistors in radio frequency applications are subject to conversion loss problems which are a function of temperature and process conditions. As shown in FIG. 4 by curves 5 and 7 representing different local oscillator power levels, changes in the gate bias voltage which commonly occur as a result of temperature and process variations have a substantial effect on the amount of conversion loss in the mixer and seriously limit mixer performance. Minimum conversion loss is achieved when the field effect transistors are biased at or near their threshold or pinch-off voltages. However, since the threshhold voltage which is characteristic of a particular field effect transistor changes as a function of temperature and process conditions it is desirable to provide some form of dynamic biasing which responds to such changes in order to maintain bias levels in proximity to threshhold voltages. 
     It is therefore an object of the present invention to provide a biasing circuit for use with field effect transistors which dynamically biases the transistors in response to process and temperature changes. 
     It is a further object of the present invention to provide a field effect transistor ring mixer having a biasing circuit which responds to process and temperature changes to maintain the bias level at or near the threshhold voltages for the transistors in the mixer. 
     It is further object of the present invention to provide a biasing circuit for a radio frequency mixer employing field effect transistors which provides minimum conversion loss over a range of temperature and process conditions. 
     SUMMARY OF THE INVENTION 
     The present invention constitutes a biasing network or circuit for use in conjunction with a radio frequency mixer employing field effect transistors which provides for minimum conversion loss over a range of process and temperature conditions. The biasing circuit includes a first field effect transistor having a small gate periphery, or channel width, compared to the gate peripheries of the transistors in the mixer which are actually used in performing the mixing functions and a second or &#34;reference&#34; field effect transistor having a gate periphery substantially equal to the gate peripheries of the transistors in the mixer which are used in performing the mixing functions. The first field effect transistor is configured as a current source providing a limited amount of current to the second field effect transistor. The second field effect transistor is interconnected with a voltage divider which helps define its operating conditions. The gate of the second field effect transistor is coupled to the gates of the field effect transistors in the mixer for providing the required DC biasing to these transistors. 
     In operation, the first field effect transistor provides only a small amount of current to the second field effect transistor for controlling its operating conditions and setting the gate to source voltage of the second or reference field effect transistor in proximity to its threshhold voltage. A DC biasing voltages are thereby generated near the threshhold voltage for the similar field effect transistors in the mixer which is supplied to these transistors for maintaining optimum conversion loss conditions. In the preferred embodiment, the biasing network and mixer and implemented on a single integrated circuit chip with the mixer constituting a ring mixer comprised of four interconnected field effect transistors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 provides a block diagram showing a biasing circuit in accordance with the present invention implemented on a single integrated circuit chip with a mixer employing field effect transistors to which the biasing circuit provides a DC biasing voltage. 
     FIG. 2 provides a schematic diagram of a ring mixer employing field effect transistors for which the biasing circuit of the preferred embodiment of the present invention is specially designed for providing a DC biasing voltage. 
     FIG. 3 provides a schematic of a biasing circuit in accordance with the present invention showing the details of the circuit components making up the present invention and their interconnections. 
     FIG. 4 provides a graph of changes in gain as a function of gate voltage at different local oscillator (&#34;LO&#34;) power levels for a typical field effect transistor of the type which might be used in a ring mixer. 
     FIG. 5 provides a graph of the characteristic curves and transfer curve of a typical field effect transistor and includes a load line of a second transistor configured as a current source for setting the operating points and operative voltage levels of the first transistor. 
     FIG. 6 provides a graph showing characteristic curves and transfer curves for a typical &#34;reference&#34; field effect transistor in which load lines, operating points and operative voltages are shown for three different sets of process and temperature conditions in accordance with the principles of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, the present invention comprises a biasing network or circuit 12 which is specifically designed for use in conjunction with field effect transistors (or &#34;FETs&#34;) which are utilized as components in a mixer 10. The mixer 10 receives a radio frequency signal at terminal 11 and a local oscillator signal at terminal 13 as inputs and combines these signals to provide an intermediate frequency output signal at terminal 15. The biasing circuit 12 insures that proper DC operating conditions are maintained for the field effect transistors making up the mixer 10. The biasing circuit 12 adjusts biasing levels to correct for temperature and process perturbations in order to maintain the gain G C  of the mixer 10 at optimum levels. As intended to be shown by the dashed line 17 the biasing circuit 12 and mixer 10 are linked in that the biasing circuit 12 senses and corrects for changes in temperature and process conditions affecting the mixer 10. The biasing circuit 12 and mixer 10 are implemented on a single monolithic circuit chip 14 with the biasing circuit 12 and mixer 10 being positioned close enough together (e.g. within 1000 microns) to be affected by the same temperature and process conditions. 
     Referring now to FIG. 2, design details are shown for a FET ring mixer 10 employing four interconnected field effect transistors 20, 22, 24 and 26 which perform mixing operations. The transistors 20, 22, 24 and 26 are connected together in a ring structure whereby the gates of oppositely positioned transistors are connected together and the sources and drains of adjacent transistors are connected together. The gates 20A and 24A of the transistors 20 and 24 are connected together as well as the gates 22A and 26A of the transistors 22 and 26. The sources 20C and 22C of the transistors 20 and 22 are connected together as well as the sources 24C and 26C of the transistors 24 and 26. The drains 22B and 24B of the transistors 22 and 24 are connected together as well as the drains 20B and 26B of the transistors 20 and 26. The biasing circuit 12 is connected to the gates 20A, 22A, 24A and 26A of the transistors 20, 22, 24 and 26 by way of the resistors 30 and 32 which isolate the biasing circuit 12 from the FET ring mixer 10. 
     Referring now to FIG. 3, the biasing network or circuit 12 comprises a pair of field effect transistors 40 and 42 which are coupled together so that the current supplied by the transistor 40 effectively controls the operation of the transistor 42. The field effect transistor 42 is of identical or very similar construction to the field effect transistors 20, 22, 24 and 26 making up the ring mixer 10. The field effect transistor 40 is also of identical or very similar construction to the transistors 20, 22, 24 and 26 making up the ring mixer 10 except that the transistor 40 has a very small gate periphery as compared to the transistors making up the ring mixer. For example, the gate peripheries, or channel width, of the transistors 20, 22, 24 and 26 might typically be in the range of 150 to 200 microns while the gate periphery of the transistor 40 would typically be on the order of 5 to 10 microns. The drain 40C of the transistor 40 is connected to a voltage source providing a voltage V DD  at terminal 60. The gate 40A of the transistor 40 is directly connected by the line 44 to its source 40B so as to configure this transistor for operation as a current source. The source 40B of the transistor 40 is also connected to the drain 42C of the transistor 42 while the source of 42B of the transistor 42 is connected to ground at terminal 64. The gate 42A of the transistor 42 is connected by way of the line 46 to the resistors 30 and 32 which interface the circuit to the ring mixer 10. A voltage divider 50 is interconnected between the drain 42C and the gate 42A of the transistor 42 and a voltage source providing a voltage V ss  at terminal 62. More specifically, the resistor 52 of the voltage divider 50 is connected between the drain 42C and the gate 42A of the transistor 42 while the resistor 54 is connected between the gate 42A and the terminal 62. The resistors 52 and 54 are selected to provide a voltage drop of approximately V DD  /2 at the drain 42C of the transistor 42 and are of sufficiently large value to prevent any significant amounts of current from being diverted away from the transistor 42. The resistors 52 and 54 of the voltage divider 50 are sized with respect to each other so as to bias the gate 42A of the transistor 42 slightly above but in close proximity to the threshhold (or pinch-off) voltage for the transistor 42 under nominal operating conditions. Typical values for the resistors 52 and 54 might be 13 K and 10 K ohms, respectively. In operation, the voltage divider 50 couples the drain to source voltage V DS  of the transistor 42 to its gate to source voltage V GS  thereby providing for dynamic biasing in accordance with the operating point of the transistor. 
     Referring now to FIG. 5, the curves 70 represent characteristic curves for the field effect transistor 42 while the curve 76 represents the corresponding transfer curve for this device. The line 72 corresponds to the load line of the transistor 42 as defined by the transistor 40 operating as a current source with the point 74 representing a nominal operating point determining a drain to source voltage V 1  for the transistor 42. The operating point 74 also determines a drain to source current I 1  for the transistor 42 which in turn corresponds to a point 78 on the transfer curve 76 determining a gate to source voltage V 2  for the transistor 42. The gate to source voltage V 2  corresponds to the desired DC bias voltage (in proximity to the threshold voltage V P1 ) which is supplied onto the gates of the transistors 20, 22, 24 and 26 of the FET ring mixer 10 by way of the resistors 30 and 32. 
     In operation, as process and temperature variations perturb the nominal drain currents of both the transistors 40 and 42 the ratio of the drain current perturbations in these transistors is proportional to their gate peripheries. Since the transistor 40 functions as a current source and defines the operating conditions for the transistor 42, the fact that the current perturbations are not equal causes a change in V DS  of the transistor 42 and a corresponding change in I DS  which leads to the establishment of a new operating point on the transfer curve for the transistor 42 which is always in proximity to the threshhold voltage for the device. The biasing circuit 12 acts to set a relative location for the operating point on the transfer curve of the transistor 42 which is maintained proportionately in proximity t the threshhold voltage despite process and temperature variations. The biasing circuit 12 thereby provides a bias level which is dynamically variable resulting in optimum conversion loss levels that are more consistent over process and temperature variations. 
     Referring now to FIG. 6, the operation of the biasing circuit 12 is illustrated in greater detail under three different sets of operating conditions. The curves 80A, 80B and 80C represent operating characteristics for the device 42 under nominal operating conditions. The curves 82A, 82B and 82C correspond to conditions under which drain to source currents have been perturbed to increase. The curves 84A, 84B and 84C correspond to conditions under which drain to source currents have perturbed to decrease. Different operating points 86-92 are established for each set of operating conditions in accordance with the process and temperature perturbations affecting the devices resulting in different drain to source voltages V 3  -V 5  and corresponding gate to source voltages V 6  -V 8 . However, due to the ratio between the gate peripheries of the transistors 40 and 42, the operating points vary in such a way as to shift the drain to source voltages by amounts ΔV DS1  and ΔV DS2  so as to determine gate to source voltages V 6 , V 7  and V 8  which are always proportionately in proximity to the threshhold voltages for the transistors regardless of operating conditions. The dynamic interaction between the field effect transistor 40 operating as a current source, the field effect transistor 42 functioning as a &#34;reference&#34; device and the voltage divider 50 maintains the operating point on the transfer curve of the device 42 in relative proximity to the threshhold voltage for this transistor and the transistors 20, 22, 24 and 26 regardless of the process and temperature conditions. Conversion loss is thereby more consistently maintained at optimum levels over the range of expected process and temperature conditions. 
     While particular embodiments of the present invention have been shown and described, it should be clear that changes and modifications may be made to such embodiments without departing from the true scope and spirit of the invention. It is intended that the appended claims cover all such changes and modifications.