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LOW SPACE PRINTED BALUN Disclosure Number: IPCOM000008021D
Original Publication Date: 1997-Mar-01
Included in the Prior Art Database: 2002-May-13
Document File: 2 page(s) / 121K

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Jean-Jacques Bouny: AUTHOR


The present invention relates to bahms, that is transformers capable of matching a single ended output to a balanced input and vice versa.

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MOTOROLA Technical Developments


by Jean-Jacques Bouny


  The present invention relates to bahms, that is transformers capable of matching a single ended output to a balanced input and vice versa.

  A convenient and useful method of coupling two transistors is the technique known in the art as "push-pull." Here the drive is shared between a tran- sistor driving current through the load in one direc- tion and a second transistor driving current in the load in the opposite direction. The arrangement is advantageous in terms of impedance matching, since it presents a higher output impedance than the very low impedance of a single transistor or the yet lower impedance of parallel transistors, which high- er impedance is closer to the typical target imped- ance of 50 Ohms. Unfortunately such a push-pull arrangement is an inherently balanced system, that is it produces an output signal which is symmetrical with respect to the common ground of the coupled transistors, A solution to this problem is to provide a transformer between the output of the push-pull pair and the load: by virtue of its isolating properties, a transformer is able to couple the balanced output to the single ended load. Such a transformer is referred to in the art as a balun.

  Many forms of baluns, including transmission line forms, are known in the art. Examples have been described by C. L. Ruthroff in his paper "some Broadband Transformers" (Proceeding of the Institute of Radio Engineers, 1959) page 1337- 1352.


European patent EP-B-0418538 also discloses a balun which is printed on both sides of a PCB in a

loop form, the primary of the transformer being on the bottom side of the PCB and the secondary of transformer being on the top side of the PCB. The advantages of this balun compared to the prior art is that it is printed on the same PCB as the amplifier and the matching networks surrounding the pair of transistors (generally using microstrip techniques)

20 Z

Eg Z

Fig. 1

which means that it does not require any additional components like coaxial cable which is very expen- sive. Reproducibility is also very good, and connec- tions between the matching network and the balun are no longer required which increases reliability. The electrical schematic of such a balun is given in Figure 1. An implementation of such a balun is given in Figure 2 which also specifies the size of the balun of a Teflon glass substrate. In some appli- cations, the space require...