Active only filters

Active filters have always received attention as an essential building block of many analog signal processing applications. One of the important factors in realization of active filters is the passive elements used in filter design. From this point of view, in the filter circuits called as active-C and active-R, the compansated opamp transfer function is used as an ideal integrator circuit. In the large part of active-C filter, filter parameters can be adjusted by using dc voltage or current values. One of the most important restrictions in this type of filter circuits is that passive elements occupy too much space on the chip area. On the other hand, active filter topologies are obtained also by using resistive voltage divider in the feedback paths of active-R filter. As it is known that, the important restriction in active-R filter is that circuit parameters can not be set electronically. The first introduced active-only filters in the literature are based on using opamp as an integrator such as active-R filters. Voltage divider function is aimed to realize by using tansconductance element (OTA) instead of using resistances in the feedback paths. In this way, it is possible to adjust circuit parameters electronically by transconductance gains of OTAs in active only filters. On the other hand, it should be also noted that, the compansation capacitance of opamp used in active only filters is a passive element and the integration design realized by using this capacitance is based on Miller theory. Therefore, these circuits should not be defined as active only filters actually. These circuits, which should be considered as pseudo active-ony filters entail the basic restrictions, e.g. limited highfrequency performance, large chip area, limitations in slew rate capability. In this thesis, in order to tackle these problems, a "real" active-only integrator and a "real" active-only filter based on this integrator are presented. The integrator pole comes from the intrinsic capacitances and resistances of a current controlled current conveyor (CCCII), although the active element could be replaced with any transconductance amplifiers offering electronically tunable parameter. This type of filters offers two obvious advantages over the conventional active-R filters: i) The filter is free from slew-rate limitation, hence offers better highfrequency performance compared to the conventional active-only filters which employs opamps. ii) The removal of external capacitors significantly reduces the chip area occupied by the filter. However, this approach also entails some practical implementation issues: i) the time constants turn out to be very large, making the filter design challenging at relatively lower frequencies, ii) some of the intrinsic capacitances due to involved active devices suffer from large tolerances, iii) some of the intrinsic capacitances are nonlinear. In order to address these issues, we considered and discussed possible circuit design techniques, which, in turn, have lead us to a new active-only integrator capable of operating at high-frequencies and providing electronically adjustable time constant with wide tuning range. The integration frequency of the proposed integrator is determined by the intrinsic xterminal resistance and z-terminal capacitor of a current controlled current conveyor (CCCII), the multiplication factor of a capacitance multiplier and the loss factor of a voltage divider. To illustrate the usefulness of the proposed integrator, a secondorder filter based on the well-known two-integrator loop filter topology is considered and simulation results using SPECTRE in the CADENCE design tool verifying the proper operation of the circuit are provided. In this thesis, filters called as MOS-only filters, which do not employ any external passive components, neither capacitors nor inductors, but rather which exploit the intrinsic capacitors of the MOS transistors are also studied. These circuits provide very useful advantages from IC realization viewpoint such as: i)low occupied chip area as a result of using as reactive component MOS capacitances with higher capacitance density compared to those of intentional, external capacitors. ii) these filters are capable of operating at high frequencies since most significant parasitic capacitances are taken into account during the synthesis procedure. iii) electronic tuning property of the important filter parameters, since the design of these circuits relies on the device transconductances which can be controlled by bias currents. On the other hand, allpass filters are very useful blocks for realizing phase shifters, phase equalizers, high-Q bandpass filters, quadratic oscillators with improved frequency stability and even for implementing arbitrary magnitude filters designed with classical approximations such as Butterworth, Chebyshev and Elliptic characteristics. While many active RC allpass filters are proposed in the literature, to the best of author's knowledge, only one single MOS-only allpass filter realization exists in the literature. However, this circuit realizes only first-order transfer functions, thus lacks the possibility to realize complex poles and zeros In this thesis, a simple filter topology that can be used to implement both first- and second-order MOS-only allpass filters is proposed. The filters derived from this topology provide all the advantages mentioned above. Unlike to the other MOS-only allpass filter, the proposed first- and second-order filters can also be considered as building blocks for the realization of nth-order allpass filters, since filters of any order can be realized by cascading first- and second-order stages. Moreover, the proposed MOS-only allpass filters offer inherently very accurate magnitude and phase characteristics at very high frequencies. Simulation results performed using Spectre simulator in Cadence design environment and experimental results verifying the proper operations of the allpass circuits are provided. The second-order effects, as well as some remedies for these are also discussed. In this thesis, a simple MOS-only second-order filter which simultaneously realizes bandpass and lowpass type characteristics is also proposed. The filter is based on a general RLC prototype circuit and provides all the advantages mentioned above. The main nonidealities of the filter are studied and its proper operation is verified trough Spectre simulation results. However, MOS-only active filters suffer from an inherent low frequency limitation. This fact can be intuitively justified by noting that filters' natural frequencies are proportional to gm/Cgs, where gm is the device transconductance and Cgs is the gate source capacitance. Therefore, for achieving low natural frequencies, either the value of gm, i.e. MOS biasing current should be decreased or the value of Cgs should be increased. However, MOS biasing current cannot be decreased below a specific value at which the transistor enters the subthreshold regions where filter suffers from large distortion due to the governing transistor's exponential nonlinearity. On the other hand, to increase the value of Cgs implies the use of large devices that increase the chip area occupied by the circuits. In order to address this issue, two modification techniques allowing the derivation of MOS-only filter which allow extension of the operating frequency towards lower frequency region are proposed. The first method, based on using cross coupled MOS transistor pairs, while the second method relies on modifying the given filter by using capacitance multipliers. Detailed simulations results of the filters modified according to the proposed techniques are also provided in order to verify the usefulness of the theoretical approach. Finally, in order to demonstrate the usefullness of the approach, a 4MHz first-order allpass filter is first realized using a classical MOSFET-C filter topology with integrated capacitors and then using a MOSFET-only filter, that is obtained according the proposed methods. From the filters' layouts, it is found that an area saving of 80% is achieved using the proposed technique.