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## Patrick Magee and Mark Tooley

Print publication date: 2011

Print ISBN-13: 9780199595150

Published to Oxford Scholarship Online: November 2020

DOI: 10.1093/oso/9780199595150.001.0001

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PRINTED FROM OXFORD SCHOLARSHIP ONLINE (oxford.universitypressscholarship.com). (c) Copyright Oxford University Press, 2021. All Rights Reserved. An individual user may print out a PDF of a single chapter of a monograph in OSO for personal use. date: 16 June 2021

# Electronics and Biological Signal Processing

Chapter:
Chapter 5 Electronics and Biological Signal Processing
Source:
The Physics, Clinical Measurement and Equipment of Anaesthetic Practice for the FRCA
Publisher:
Oxford University Press
DOI:10.1093/oso/9780199595150.003.0009

This chapter continues the discussion of electricity but looks at the effect of connecting components together and briefly looks at the operational amplifier and active circuits (circuits discussed in the previous chapter have been passive ones, which involve no electronic circuits). It will then describe how the circuits can be used to process biological signals. If a resistor, an inductor and a capacitor are joined as in Figure 5.1, the magnitude of the voltage across the resistor (Vout) will vary as the input frequency, Vin, changes, because of the properties of the capacitor and inductor. The resistive action of inductor and capacitor oppose each other, and at a certain resonant frequency (r in the figure), the total AC resistance (impedance) will be at a minimum. The graph of reactance against frequency demonstrates this, and shows the minimum reactance at the resonant frequency. As the LC and R are forming a voltage divider, the voltage across R will be maximum at this frequency. At other input frequencies, the output voltage will be low. This simple circuit forms the basics of the passive band-pass filter, where the filter passes, or lets through, a certain band of frequencies (around the resonant frequency in this circuit). Frequencies lower or higher than the band-pass will be attenuated. Normally operational amplifiers (discussed in the next section) or digital filters are more effective and are used to achieve the same effect. Resonance is important for the understanding of the behaviour of transducers and this is discussed in Chapter 12. If a square wave is applied to the resistor capacitor network as shown in Figure 5.2, the capacitor charges up on the rising edge of the input with a time constant equal to the product of the resistance (Ω) and the capacitance (F), RC, and the output voltage will be Vouput = Vinput (1 − e−t /RC ). When the square wave is in the off state, or zero volts, the voltage will fall exponentially with the same time constant. It can be seen from the diagram that, with the appropriate values of R and C, the output from the network is a filtered version of the input.

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