Abstract:
Mixer circuit  300  receives a single-ended rf voltage signal on terminal  330  and a bias potential on terminal  361 . Transistor  301  functions as a transconductance amplifier and presents a current signal, representative of the input voltage signal, to mixer core  391 . Inductor  310  provides noiseless degeneration in the base-emitter circuit of transistor  301 . Local oscillator driver  393  is configured such that its common mode output impedance is higher than the input impedance of transistors  305  and  306 . In this way, phase splitting is carried out within the mixer core  391  itself and less transistors are needed. Mixer circuit  300  thereby requires less voltage headroom than prior art mixers.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to mixer circuits and in particular to mixer circuits having a single-ended input and a differential output. 
     RF mixers are the key blocks of modern radio systems and their parameters determine the main characteristics of the system in which they are used. The most common mixer circuit configurations are those of the Gilbert cell and the Micromixer, shown in FIGS. 1 and 2 respectively. 
     Each of these mixer circuits receives at its input terminal a single-ended rf input signal and provides at its output a differential signal being the input signal first amplified and subsequently mixed with a signal from a local oscillator. Both of these circuits are easily implemented in IC form and are commonly used in mobile telephones and the like. However, mixers constructed using these circuit configurations exhibit poor noise properties. They also require a supply voltage of 2.7 V or more because each has three transistors in series between supply and ground. This can make them unsuitable for low voltage applications. 
     Referring to FIG. 1, Gilbert cell circuit  100  receives a single-ended input voltage signal at terminal  130  and a differential local oscillator voltage signal at terminals  140  and  141 . Transistors  101 ,  102 , resistors  110 ,  111  and current source  115  form a differential transconductance amplifier  160  whilst transistors  103 - 106  form a mixer core  150 . An increasing input voltage at terminal  130  will cause an increasing signal current to flow from the collector terminal of transistor  101 . Current source  115  and resistors  110 ,  111  ensure that a complementary decreasing current will flow from the collector electrode of transistor  102 . These current signals will be balanced if current source  115  is implemented as a constant current source. 
     Mixer core  150  receives differential local oscillator signals on terminals  140 ,  141 . When the voltage on terminal  140  is positive, the voltage on terminal  141  will be negative causing transistors  104  and  105  to be switched on and transistors  103  and  106  to be switched off. The collector current of transistor  101  will therefore be routed to output electrode  121  whilst the collector current of transistor  102  will be routed to output terminal  120 . The collector currents of transistors  101 ,  102  will be switched to the opposite output terminal  120 ,  121  when terminal  141  receives a higher voltage than terminal  140 . 
     The poor noise properties of this mixer configuration are due largely to the thermal noise of resistors  110  and  111  which produce noise directly in the main current paths. Current source  115  will also introduce noise into the output signal, because it experiences quite large voltage swings across its input and output terminals. A significant amount of noise will appear at output terminals  120 ,  121  as a result of transistors  101  and  102  having their base resistances in series. 
     The micromixer circuit  200  of FIG. 2 receives a single-ended input signal at input terminal  230  and differential local oscillator signals at terminals  240  and  241 . Transistors  201 - 203  and resistors  210 - 212  form a transconductance amplifier  260  whilst transistors  204 - 207  form a mixer core  250 . 
     An increase in voltage at input terminal  230  will cause increased current to flow from the collector electrode of transistor  202  and a decreased current to flow from the collector of transistor  203 . The circuit therefore acts as a transconductance amplifier having a single-ended input and a differential output. The output from amplifier  260  is provided on the collector electrodes of transistors  202  and  203 , as a differential current signal, to mixer core  250 . 
     Mixer core  250  functions in the same manner as mixer core  150  of the FIG. 1 mixer circuit described above. 
     Micromixer circuits have very linear characteristics and large dynamic range at radio frequencies but, due to the large number of resistors used in the main current paths, have even worse noise properties than Gilbert cell circuits. There exists a need for a mixer circuit with improved noise properties and low voltage supply requirements. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided a mixer circuit arrangement comprising a mixer core and a single-ended amplifier stage, in which the mixer core is arranged to receive a single-ended output signal of the amplifier stage on a first main input and to provide a differential output signal in response thereto. 
     In accordance with another aspect of the present invention there is provided a mixer circuit arrangement for providing differential output signals in response to an input signal applied thereto, comprising a mixer core having first and second current signal inputs and first and second local oscillator inputs, a single-ended amplifier stage for applying a current signal to said first signal input of said mixer core in response to said input signal, and bias means having a low ac impedance for applying a bias current to said second signal input of said mixer core. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, of which; 
     FIG. 1 shows a prior art Gilbert cell mixer circuit; 
     FIG. 2 shows a prior art Micromixer circuit; 
     FIG. 3 shows a mixer circuit in accordance with the present invention, and 
     FIG. 4 shows a local oscillator driver circuit suitable for use in the mixer circuit of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings, FIG. 3 shows a double-balanced mixer circuit arrangement in accordance with the present invention. 
     Mixer circuit  300  is made up of amplifier  390 , bias arrangement  392 , mixer core  391  and local oscillator driver  393 . In operation, differential local oscillator signals are applied to terminals  340  and  341 , bias potentials are applied to terminals  360  and  361 , a single-ended input signal is applied to terminal  330  and a differential output signal is obtained at terminals  320 ,  321 . 
     Amplifier  390  is centred around transistor  301 . The base electrode of transistor  301  is connected to terminal  360  by resistor  311  and to terminal  330  by capacitor  350 . Inductor  310  is connected between the emitter electrode of transistor  301  and ground potential. The collector electrode of transistor  301  forms the output current path to mixer core  391 . 
     Current bias arrangement  392  comprises transistor  302  which has its emitter electrode connected directly to ground potential, its base electrode connected to terminal  361  by resistor  312  and provides a current signal to mixer core  391  from its collector electrode. Capacitor  351  is connected between the collector electrode of transistor  302  and ground potential. 
     Mixer core  391  comprises four transistors  303 - 306 . Transistors  303  and  304  have their emitter electrodes coupled together and receive the output current signal of amplifier  390 . Transistors  305  and  306  have their emitter electrodes coupled together and receive the current provided by bias arrangement  392 . The collector electrodes of transistors  303  and  305  are connected together and to output terminal  320  whilst the collector electrodes of transistors  304  and  306  are connected together and to the other output terminal  321 . Mixer core  391  is arranged to be controlled by local oscillator driver  393  by the connection of the base electrodes of transistors  303 - 306  to the differential outputs of local oscillator driver  393 . Transistors  303  and  306  have their base electrodes connected together and to a first output of local oscillator driver  393  whilst transistors  304  and  305  have their base electrodes connected together and to the other output of local oscillator driver  393 . 
     Amplifier  390  receives an RF voltage signal at terminal  330  and translates it to a current signal which is provided at the collector electrode of transistor  301 . Amplifier  390  thus forms a high impedance current source. Capacitor  350  acts to block any de component of the input signal. DC biasing of transistor  301  is achieved by way of resistor  311  and the bias potential applied to terminal  360 . Due to the complex value of the common emitter current gain β of the transistor  301  at radio frequencies, the inductor  310  effects series negative feedback in the base-emitter circuit of transistor  301 . 
     Inductor  310  is a noiseless component which provides frequency independent degeneration over a particular frequency range. This range is dependent on the value of inductor  310  and the base-emitter resistance of transistor  301  at the desired frequency. The value of inductor  310  also affects the gain of amplifier  390  and its linearity. Although a resistor could be used in place of inductor  310 , amplifier  390  has much more linear characteristics and better noise properties when inductor  310  is used. 
     Inductor  310  can be implemented, in whole or in part, with the parasitic inductance of IC packaging, bonding wires and/or connecting pins. 
     Transistor  301  is preferably fabricated with a large emitter area to minimise the noise produced by its base-emitter resistance. However, a larger area transistor will also have higher parasitic capacitances, and hence leakage, and a lower current gain β because of a lower current density. A trade-off therefore needs to be made between noise figure and gain when choosing what transistor area and what bias current should be incorporated into a particular mixer circuit design. 
     The input impedance of mixer circuit  300  is determined by the value of inductor  310  and by unity current gain-frequency f T  of transistor  301 . 
     Bias arrangement  392  operates to provide a biasing current to transistors  305 ,  306  of mixer core  391  from the collector electrode of transistor  302 . Resistor  312  connects the base electrode of transistor  302  to terminal  361 , to which a biasing potential is applied. Capacitor  351  provides low impedance grounding of the ac component of the signal present on the collector electrode of transistor  302 . The dc component of this current signal will remain reasonably constant. 
     The requirements of local oscillator driver  393  are that it needs to provide translation of the voltage signal applied to its input terminals to its output terminals and to present a high common mode output impedance with respect to the ground potential to which the input signal is referred. The reasons for this will become apparent on reading the description of the operation of mixer core  391  below. 
     Local oscillator driver  393  could be implemented as a transformer. In the case where local oscillator driver  393  has to be integrated on the same chip as the rest of mixer circuit  300 , it can be implemented as the local oscillator driver circuit  493  in FIG.  4 . 
     The driver circuit  493  comprises a long-tailed pair of transistors  401 ,  402  having their base electrodes connected to respective local oscillator signal input terminals  340 ,  341 . Resistors  410  and  411  connect the collector electrodes of transistors  401  and  402  respectively to a supply voltage terminal  440 . Resistor  412  is connected between the emitter electrodes of transistors  401  and  402 . Local oscillator driver circuit output terminals  430 ,  431  are connected to the collector electrodes of transistors  402 ,  401  respectively. These terminals  430 ,  431  form the connections to the base electrodes of the transistors  303 - 306  of mixer core  391  of FIG.  3 . 
     Local oscillator driver circuit  493  is controlled by a local oscillator signal applied to local oscillator input terminals  340 ,  341 . Transistors  401  and  402  are “hard-switched” by the local oscillator signal such that they conduct alternately and thus provide a positive voltage alternately on terminals  430  and  431 . This voltage switches on transistors  304  and  305  and transistors  303  and  306  alternately. 
     In the case where transistors  304  and  305  are switched on, the collector current of transistor  301  passes through the emitter and into the base and collector electrodes of transistor  304 . The collector current of transistor  304 , which is passed to output terminal  321 , will be proportional to the base current, scaled up by a factor of the current gain of that transistor, β. Provided that the input impedance of transistor  305  is low compared with that of the common mode output impedance of local oscillator driver circuit  493 , signal currents from the base electrode of transistor  304  will flow primarily to the base electrode of transistor  305  and that transistor will have a collector current that complements the collector current of transistor  304 . If the common mode output impedance, with reference to the input signal to ground, is sufficiently greater than the input impedance of transistor  305 , a balanced output will be provided at differential output terminals  320 ,  321 . 
     Balancing of the mixer core output can be further controlled by virtue of the independent biasing of transistors  303 ,  306  and  304 ,  305 , the control provided by varying the potentials applied to terminals  360 ,  361 . 
     In the case where transistors  303  and  306  are switched on, the collector current of transistor  301  will be passed to the opposite differential output terminal  320  and its complement passed to the other terminal  321 . 
     Thus the conversion of the single-ended input signal into a differential output signal is carried out within the mixer core  391  itself, allowing fewer transistors to be used in the mixer circuit implementation and thereby allowing a lower supply voltage to be used. 
     The complementary current of transistors  305  and  306  can be increased by forming transistors  303  and  304  with larger emitter areas than transistors  305  and  306  (for example in the ratio of 3:2 or 2:1 depending on the frequencies involved). This will cause a higher base current in transistors  303  and  304  to compensate for losses due to the parasitic capacitances of the mixer core transistors. 
     The common mode output impedance of local oscillator driver circuit  493  is determined by resistors  410  and  411 . The values of these resistors should be as high as is possible consistent with proper operation of driver circuit  493 . Resistors  410  and  411  would usually be much larger than resistor  412 , which resistor determines the differential output impedance of local oscillator driver circuit  493 . Resistors  410  and  411  could equally be substituted with suitable inductors to achieve substantially the same effect. 
     Although the embodiments have been described solely with regard to npn bipolar transistors, the invention is not restricted to such and could equally be effected with pnp bipolar transistors or with field effect transistors. The collector and emitter electrodes referred to would correspond to the drain and source electrodes as the first and second main electrodes of a field effect transistor.