. Electronic apparatus for biological research . Vooi=*Vll Figure requires two inputs, equal and in anti-phase, such as may be obtained from a 'concertina' phase-splitter valve, to be described later in this part. The modulus of the output is Kat all frequencies, and the phase shift is tan~^ wCR, referred to the phase of generator 2. We now come to consider a group of three, rather more elaborate, R-C networks; they are filters. One will pass all frequencies within a 'band', and attenuate those outside it—a 'band-pass' filter. The other two pass all frequencies except a band, which they a
. Electronic apparatus for biological research . Vooi=*Vll Figure requires two inputs, equal and in anti-phase, such as may be obtained from a 'concertina' phase-splitter valve, to be described later in this part. The modulus of the output is Kat all frequencies, and the phase shift is tan~^ wCR, referred to the phase of generator 2. We now come to consider a group of three, rather more elaborate, R-C networks; they are filters. One will pass all frequencies within a 'band', and attenuate those outside it—a 'band-pass' filter. The other two pass all frequencies except a band, which they attenuate; within the attenuation band there is a centre frequency at which the attenuation is infinite, and the networks are hence known as 'null-transmission networks'. These are the Wien bridge and the parallel T network. The R-C band-pass filter This is shown in Figure When presented with a new and complicated network it is a good plan not to plunge at once into calculations, but rather to get some idea what the network is likely to do. This is achieved by con- sidering what happens when a> is very low, when m is very high, and when to is intermediate. 46
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