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First order LTI systems are characterized by the differential equation + = where τ represents the exponential decay constant and V is a function of time t = (). The right-hand side is the forcing function f(t) describing an external driving function of time, which can be regarded as the system input, to which V(t) is the response, or system output.
The zeroth order transfer constant denotes the ratio of the output to input when all elements are zero-valued (hence the superscript of 0). Using the time constants and transfer constants, all terms of the numerator can be calculated. In particular: =
The transfer function of an electronic filter is the amplitude at the output as a function of the frequency of a constant amplitude sine wave applied to the input. For optical imaging devices, the optical transfer function is the Fourier transform of the point spread function (a function of spatial frequency ).
If the transfer function of a first-order low-pass filter has a zero as well as a pole, the Bode plot flattens out again, at some maximum attenuation of high frequencies; such an effect is caused for example by a little bit of the input leaking around the one-pole filter; this one-pole–one-zero filter is still a first-order low-pass.
The transfer function for a first-order process with dead time is = + (), where k p is the process gain, τ p is the time constant, θ is the dead time, and u(s) is a step change input. Converting this transfer function to the time domain results in
These equations show that a series RC circuit has a time constant, usually denoted τ = RC being the time it takes the voltage across the component to either rise (across the capacitor) or fall (across the resistor) to within 1 / e of its final value. That is, τ is the time it takes V C to reach V(1 − 1 / e ) and V R to reach ...
The transfer constants are calculated under similar zero- and infinite-value conditions of reactive elements used in the Cochran-Grabel (CG) method [2] to calculate time constants, but calculating the low-frequency transfer functions from a defined input source to the output terminal, instead of the resistance seen by the reactive elements.
This closed-loop gain is of the same form as the open-loop gain: a one-pole filter. Its step response is of the same form: an exponential decay toward the new equilibrium value. But the time constant of the closed-loop step function is τ / (1 + β A 0), so it is faster than the forward amplifier's response by a factor of 1 + β A 0: