pev17i5p238

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A simple phase shifting circuit This article has been downloaded from IOPscience. Please scroll down to see the full text article. 1982 Phys. Educ. 17 238 (http://iopscience.iop.org/0031-9120/17/5/412) Download details: IP Address: 70.185.109.91 The article was downloaded on 29/07/2011 at 00:10 Please note that terms and conditions apply. View the table of contents for this issue, or go to the journal homepage for more Home Search Collections Journals About Contact us My IOPscience
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Figure 9 Induced EMF in the secondary coil E, is out of phase with the applied to the primary. Note the coils are wound in the same sense in phase whilst V, was 180' out of phase with V' and v,. An example of a textbook that deals with the phase relationship and gets it right is Muncaster's A-level Physics (1981). He says that the EMF induced in the secondary coil (E,) is completely out of phase with the EMF applied to primary (Ep). Sadly, however, it is unlikely that any student would fully understand this since thediagram he gives doesnotmake clearthe direction in which he is measuring AC voltages. In figure 9 it should be noticed that the coils are wound in the same sense. A student would scarcely guess that show phase difference between one must connect CRO with the ground terminal of input to A and terminal of the other to B. The problem is clearly a topological one. A SIMPLE PHASE SHIFTING CIRCUIT J S LOPES and A A MELO Department, Cniversity Lisbon, Portugal V S OLIVEIRA Filipa de Lencastre Secondarq. School, Lisbon, When displaying Lissajous' figures of two signals it is very instructive continuously vary the phase of one of them. The may be shifted by a passive CR filter network but this procedure is inconvenient because it also changes the signal amplitude. A circuit produces phase shifts from 0 to n whilst keeping the amplitude constant may be easily built around an integratedoperational amplifier (op.amp.).Here we describe such a circuit and give details about its implementation performance. For this purpose even a low cost op. amp. like the pA 741 may be considered as an ideal amplifier. We recall that the ideal op. amp. has infinite differential voltage gain, infinite input impedance zero output impedance. This ensures properties a feedback amplifier built around depend entirely on the feedback network. My conclusion is that teach either must do it properly (which would take a great deal of time) or avoid it completely in this context stop making true, effectively meaningless statements like 'The EMF applied a transformer is 180' out of
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