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In the design of mixers, push-pull amplifiers, baluns are used to link a symmetrical (balanced)
circuit to a asymmetrical (unbalanced) circuit.
Baluns are designed to have a precise 180-degree phase shift, with minimum loss and equal
balanced impedances. In power amplifiers loss of symmetry will degrade efficiency and the
symmetrical port must be well isolated from ground to eliminate parasitic oscillations.
The basic construction/design of a balun consists of two 90-degree phasing lines that provide
the required 180-degree split, and this involves the use of λ/4 and λ/2.
A wire-wound transformer provides an excellent balun. Miniature wirewound transformers are
commercially available covering frequencies from low kHz to beyond 2GHz. They are often
realised with a centre-tapped secondary winding, if grounded this provides a short circuit to
even-mode (common-mode) signals whilst having no effect on the differential (odd-mode)
Wire-wound transformers are more expensive than the printed or lumped element baluns
described below, which find greater adoption in practical mixer designs. It should be noted
that most of these lumped element and printed baluns do not provide the centre-tapped
ground to even mode signals and this fact must be accounted for in the mixer design.
(1) L-C Balun
This design is essentially a bridge and is known as a ‘lattice-type’ balun. It consists of two
capacitors and two inductors, which produce the ± 90 degree phase shifts. The diagram
below (Figure 1) shows the circuit diagram of the Balun.
Figure 1 Schematic diagram of a L-C lumped balun.
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When designing this circuit make sure that the operating frequency is well below the self-
resonant frequencies of the components and take account of pad capacitances.
One of the