Page 212 - Complete Wireless Design

Amplifier Design

Amplifier Design 211

Figure 3.115 High-frequency transformer coupling between two stages.

Degree of coupling. The degree of coupling between a tuned transformer’s pri-
mary and secondary, which is mainly governed by the distance between the
windings, will affect the signal’s amplitude and bandwidth as it passes
through the transformer. Indeed, as the coefficient of coupling increases (over-
coupling, Fig. 3.116a) or, in other words, as the windings are brought closer
together, more flux lines from the primary will cut the secondary. This will pro-
duce a higher output voltage and a wider bandwidth over that of the loose cou-
pling of Fig. 3.116c. The wider bandwidth is caused by the high capacitance
now present between the closely spaced primary and secondary, while the high
signal amplitude is due to the increased flux lines that cut the secondary.
However, as the coefficient of coupling is decreased toward loose coupling, the
amplitude and the bandwidth of the signal diminish. Nonetheless, loose cou-
pling can be used to lower the capacitive coupling into the next stage, thus
lowering harmonic output, and give the narrower bandwidth that may be
required for certain applications. For typical narrowband uses, optimum cou-
pling (Fig. 3.116b) will be found a good compromise between bandwidth and
amplitude.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.