# The contributions of all the noise generators in the

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components with the same amplitude density. The contributions of all the noise generators in the circuit are calculated to determine each one’s magnitude at the output. The 'output noise' is formed from the square root of the sum of the squares (rms value) of these individual contributions. To correctly account for the noise contribution of operational amplifiers, you should use models made by manufacturers by choosing devices from the Spice Macro Models toolbar. Open the macro and check the noise model. Some manufacturer models have special purpose noise models, or no noise modelling whatsoever. TINA’s ideal, standard and linear opamp models (on the Semiconductor toolbar) do not include a noise model. Since the transfer functions of the circuit are known, the equivalent input noise can be calculated from the output noise. Expressing all the circuit noise in terms of an equivalent input noise makes it possible to compare the levels of real signals presented to the input to the circuit noise referred to the input. A circuit expected to amplify small input signals, signals approaching the magnitude of the equivalent input noise, will not be very useful. Resistors generate resistive (thermal) noise which is computed as the following source current: R T k i = 4 2 where k = 1.38e-23 (Wsec/K) Boltzmann constant T = temperature in K R = resistance in ohms Semiconductors generate not only thermal noise (similar to resistors), but flicker noise (1/f noise) as well. The spectral density of flicker noise decreases in inverse proportion to the frequency, hence the name 1/f noise. TINA calculates the Input and Output noise and expresses the noise as the noise voltage per Hz of bandwidth. TINA can also present noise as a curve of Total noise, at the output, as a function of frequency. In computing this curve, TINA sums all the noise from the start frequency up to the specified maximum frequency value and presents it as total noise. Naturally, the curve rises monotonically. For example, if you set the start frequency to 20Hz and the end frequency to 20kHz, you will learn the total output noise over the typical audio bandwidth. Another measure of noise, the “signal to noise ratio,” takes the signal magnitude into account. This is the ratio of signal power to noise power, according to the following formula: / 20 * log Sig Tot V S N V =

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where V Tot is the total input-referred noise voltage and V Sig represents the expected signal amplitude. V Sig is a user-defined value that you enter in the Noise Analysis dialog. The default value is 1mV. To get a better feeling for these noise concepts, let's study the following circuit, an equivalent circuit of a basic amplifier (noise.tsc). Select Analysis.Noise Analysis. The following dialog appears The dialog allows you to set the Start frequency , End frequency , the Number of points for which the noise is to be computed, and the Signal amplitude . You may also select the required analysis results–Output Noise, Total Noise, Input Noise, and Signal to Noise.
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