Systems electronicsjalil

Linear time-invariant (LTI)
Linearity means that if you have two inputs and two corresponding outputs, if you take a linear combination of those two inputs you will get a linear combination of the outputs. An example of a linear system is a first order low-pass or high-pass filter. Linear systems are made out of analog devices that demonstrate linear properties. These devices don't have to be entirely linear, but must have a region of operation that is linear. An operational amplifier is a non-linear device, but has a region of operation that is linear, so it can be modeled as linear within that region of operation. Time-invariance means it doesn't matter when you start a system, the same output will result. For example, if you have a system and put an input into it today, you would get the same output if you started the system tomorrow instead. There aren't any real systems that are LTI, but many systems can be modeled as LTI for simplicity in determining what their output will be. All systems have some dependence on things like temperature, signal level or other factors that cause them to be non-linear or non-time-invariant, but most are stable enough to model as LTI. Linearity and time-invariance are important because they are the only types of systems that can be easily solved using conventional analog signal processing methods. Once a system becomes non-linear or non-time-invariant, it becomes a non-linear differential equations problem, and there are very few of those that can actually be solved. (Haykin & Van Veen 2003)
Common systems
Some common systems used in everyday life are filters, AM/FM radio, electric guitars and musical instrument amplifiers. Filters are used in almost everything that has electronic circuitry. Radio and television are good examples of everyday uses of filters. When a channel is changed on an analog television set or radio, an analog filter is used to pick out the carrier frequency on the input signal. Once it's isolated, the television or radio information being broadcast is used to form the picture and/or sound. Another common analog system is an electric guitar and its amplifier. The guitar uses a magnet with a coil wrapped around it (inductor) to turn the vibration of the strings into a small electric current. The current is then filtered, amplified and sent to a speaker in the amplifier. Most amplifiers are analog because it's easier and cheaper than making a digital amplifier. There are also many analog guitar effects pedals, although a large number of pedals are now digital (they turn the input current into a digitized value, perform an operation on it, then convert it back into an analog signal).