Environmental Engineering Reference
In-Depth Information
Circuits with internally controlled switches. The switches in these
circuits are controlled by internal network variables of the circuits.
Rectifiers and switching voltage regulators where switches take the
form of diodes and thyristors are representative examples in this cat-
egory.
Circuits with both internally controlled and externally clocked switches.
Examples in this category include switched capacitor and switched
current over-sampled sigma-delta modulators where the integrators
are implemented using switched capacitor or switched current net-
works and the output of the quantizer of the modulators is set by the
input of the quantizer.
2. Switched Capacitor Techniques
Early applications of switched capacitor networks are the implemen-
tation of active filters where switched capacitors are used to replace
monolithic diffusion resistors to overcome the following difficulties en-
countered in realization of these resistors :
Large variation of resistance -
Although diffusion resistors have a
large resistance value, the absolute value of these resistors is heavily
affected by the variation of the fabrication process by which the re-
sistors are fabricated and has a large variance, usually in the range
of ±30~40%. Such a large error significantly affects the performance
of
filters.
Large parasitic capacitance -
diffusion resistors have a large parasitic
junction capacitance that exists between the n-diffusion that forms
the resistors and the p-substrate upon which the resistors are fabri-
cated. This junction capacitance is not only nonlinear but also varies
greatly with the voltages at the terminals of the resistors [1].
Strongly nonlinear characteristics -
the resistance of diffusion resistors
is a strongly nonlinear function of the terminal voltages of the resistor,
mainly due to the dependence of the effective area of the cross-section
of the resistors on the voltages at the terminals of the resistors [2].
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