integrated-circuit filter[¦int·i‚grād·əd ′sər·kət ‚fil·tər]
An electronic filter implemented as an integrated circuit, as contrasted with filters made by interconnecting discrete electrical components. The design of an integrated-circuit filter (also called simply an integrated filter) is constrained by the unavailability of certain types of components, such as piezoelectric resonators, that are often valuable in filtering. However, integrated filters can benefit from small size, close integration with other parts of a system, and the low cost of manufacturing very complex integrated circuits.
Filters have many applications. An important one is to smooth signal waveforms sufficiently to allow accurate sampling or to interpolate smoothly between given samples of a signal. Since the analog-to-digital converters that sample signals are usually made as integrated circuits (chips), it is often convenient to put the associated filters on the same chip. See Analog-to-digital converter
Passive filters, made by interconnecting inductors and capacitors, are not easily integrated because integrated-circuit inductors are usually of poor quality. This problem is less serious at frequencies above 1 GHz, so microwave filters can be passive. See Microwave filter
Because amplifiers are very cheap in integrated-circuit technology, active filters are widely implemented. The five main types of active filter—active-RC, MOSFET-C, transconductance-C, switched-capacitor (or switched-C), and active-RLC— are distinguished by their frequency-sensitive components. The switched-capacitor filters operate on samples of signals, while the other types operate without sampling (in continuous time). There is also a trend toward digital filters, which are easily integrated but require that analog signals be converted to digital form, which in turn requires filtering. See Digital filter
Discrete active-RC filters were widely used in the 1970s, and modern integrated filters are derived from them. The frequency-sensitive mechanism in active-RC filters is the charging of a capacitor C through a resistor R, giving a characteristic frequency ω0 = 1/RC radians per second, at which the impedances of the resistor and capacitor are equal. Unfortunately, integrated-circuit manufacturing techniques do not control the product RC at all accurately, with variations of 20–50% being possible. This limits active-RC filtering to those applications where accuracy is unimportant, where external passive components are tolerable, or where tuning circuitry is available.
MOSFET-C filters replace the resistors of an active-RC filter with metal-oxide-semiconductor (MOS) transistors, in which a conducting channel along the surface can be enhanced or depleted by applying an electrical field from a gate electrode, thereby changing the resistance of the channel. The result is a tunable variant of an active-RC filter. See Transistor
Transconductance-C filters combine the functions of the amplifier and the simulated resistor into a transconductance amplifier, whose output current (rather than output voltage) is proportional to its input voltage. Transconductance amplifiers can be very simple and hence are capable of high-frequency operation (up to approximately 1 GHz) but tend to have poor linearity when designed for high speeds.
A technique known as active-RLC filtering combines the ideas of active filtering with the use of physical inductors (made as spirals of metallization on the top layer of the chip). In this method, amplifiers, connected to simulate negative resistors, are used to enhance the performance of the inductors, whose losses can be modeled (to a first approximation) as being caused by a parallel positive resistance.
The primary advantage of switched-capacitor filters is that they can be very accurate, since critical frequencies are determined by the product of a clock frequency and a ratio of capacitors (rather than a single capacitor). Switched-capacitor filters are probably the most prevalent integrated filters. Most telephone systems, for example, use them to smooth signals before sampling them for digital transmission. See Integrated circuits