RF Circuit Design, 2nd Edition

Chapter 15

RF Balun

15.1 Introduction

A balun is a transformation block between a single-ended stage and a differential pair.

A balun transforming a signal from a single-ended stage to a differential pair splits a single-ended signal into a pair of differential signals with the same magnitude but 180^° phase difference. It is called splitter as shown in Figure 15.1(a). A differential pair plays some special functions in the transport or manipulation of a signal. For instance, the capability of common mode rejection potentially exists in a differential pair but not in a single-ended stage.

Figure 15.1 A balun, which is a block between a single-ended block and a differential pair. (a) From single-ended block to differential pair. (b) From differential pair to single-ended block.

A balun transforming a signal from a differential pair to a single-ended stage combines a pair of differential signals with the same magnitude but 180^° phase difference as a single-ended signal. It is called combiner as shown in Figure 15.1(b). The conversion from a differential pair to a single-ended stage simplifies both simulation and testing of a block.

The word “balun” is a portmanteau formed from the words “balanced” and “unbalanced,” in which balanced implies a differential configuration and unbalanced represents a single-ended configuration. It would seem reasonable to rename the balun in Figure 15.1 ...