Balanced-to-unbalanced converters, commonly to referred to as "balun"s, are widely used in electronic systems to convert balanced inputs to unbalanced outputs. For example, they may be used in radio communications equipment such as cellular telephones.
In particular, cellular telephones generally use mixers for frequency conversion. A typical radio transceiver included in a cellular telephone may include three mixers, two for the receiver and one for the transmitter. A dual band cellular telephone may include up to six or more mixers.
Because the mixer is a nonlinear device, it may generate signals at other than its desired output frequency. In order to reduce or suppress these signals, many mixers are designed for differential inputs and outputs. These mixers are known as "balanced mixers". When using such a balanced mixer, it is often desirable to convert the output of the balanced mixer into an unbalanced signal. This conversion is performed by a balun. The balun receives the output of the balanced mixer, i.e. a signal and its complement, which is a 180.degree. phase shifted signal of the same amplitude. The balun combines these two signals, one at 0.degree. phase and the other at the same amplitude, but with a 180.degree. relative phase difference. The combination is performed by subtracting the balanced output signals to form a single unbalanced signal with twice the amplitude of either individual signal.
With increasing integration levels, balanced mixers are now generally available in integrated circuit form. These balanced mixers generally include an upper frequency limit within which the device should be used. Thus, a separate mixer is generally used for a particular function in each band. For example, a separate transmit mixer is generally used for each band in a dual band cellular telephone.
Separate mixers may be used in order to simultaneously provide a high impedance load at the mixer output and to combine the differential (balanced) outputs to a single ended (unbalanced) output. A high impedance is often desirable for balanced-to-unbalanced conversion, because the mixer's voltage gain is generally given by A.sub.V =g.sub.c R.sub.L where A.sub.V is the voltage gain of the mixer expressed as a ratio, g.sub.c is the conversion transconductance of the mixer active device, and R.sub.L is the load resistance of the mixer. If the two differential outputs of the mixer are considered as current sources, and the outputs are combined in phase by the balun, there is twice the current available, i.e. an increase in gain of 6dB. Moreover, by combining the two differential outputs, spurious responses that might otherwise degrade performance may be canceled.
FIG. 1 schematically illustrates a first embodiment of a conventional balun. As shown in FIG. 1, the balun 100 includes a balanced input including balanced input terminals 104a and 104b, and an unbalanced output 106. A balun circuit, such as an n:1 turns ratio transformer 108 is used to convert the balanced inputs 104a, 104b to an unbalanced output 106. As shown in FIG. 1, the balanced input 104a, 104b of balun 100 may be connected to the output of a mixer 102. Impedance transformation is also provided. For example, if the load resistance of the mixer 102 is R.sub.L and the output resistance of the balun is R.sub.O, then the impedance is transformed according to the formula R.sub.O =R.sub.L /n.sup.2, where n is the turns ratio of transformer 108.
It will be understood that one or more additional components may be used in the balun of FIG. 1 to cancel reactive components of the impedance at the mixer output. It will also be understood that the transformer implementation of a balun as shown in FIG. 1 may be difficult to implement because the impedance transforming ratio is directly dependent on the turns ratio, which may often lead to custom transformers and relatively high cost. Moreover, the transformer may be large and there may be an inverse relationship between impedance ratio and operational bandwidth.
FIGS. 2A and 2B illustrate second embodiments of a conventional balun. In FIGS. 2A and 2B, three element baluns are shown, including two capacitors 110a and 110b and an inductor 112. The balun 100' of FIG. 2A uses two discrete capacitors 110a and 110b and an inductor 112. In the balun 110" of FIG. 2B, the second capacitor 110b is implemented as a third and fourth capacitor 110c and 110d, which are serially connected to provide an unbalanced output 106' of lower impedance therebetween. The balun 100' of FIG. 2A provides balanced-to-unbalanced conversion. The balun 110" of FIG. 2B provides balanced-to-unbalanced conversion and simultaneous impedance transformation to provide a low impedance output 106'.