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Figure 1. Thermal Noise Voltage vs. Resistance sum of the noise in each of the bands N1-Nn.
© 2014 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0 • 6/25/15 3 The tools are now available to calculate the noise of a variety of transducers and see how this unspecified noise affects amplifier (S/N) performance. EXAMPLES Calculations of electret microphone noise with various loads and R IAA equalized phono cartridge noise is done using equations (1)—(7). Center frequencies and frequency bands must be chosen first. Values of the lumped circuit components calculated and noise calculated for each band, then summed for the total noise. Octave bandwidths starting at 25 Hz will be adequate for approximating the noise. In this example, t he microphone capa citance is 10 pF loaded with 5 pF of amplifier and stray capacitance. Two resistive loads will be used to illustrate the effect RL has on the microphone noise. R L1 = 1 GΩ (109), RL2 = 10 GΩ (1010). It is assumed that there is no gain equalization in the amplifiers that follow. The noise calculations are summarized in Table I. The electret or condenser microphone noise (Re(Z)) is reduced when the load resistance is increased. This is one of the cases when a larger resistance means lower noise, not more noise. The second example is the calculation of the RIAA equalized noise of an ADC 27 phono cartridge loaded with CA = 250 pF and R A = 47 k. The cartridge con stants are R s = 1.13 k and L s = 0.75 H (C c may be neglected). The noise calculations are summarized in Table II for this example. The RIAA equalized noise of the ADC 27 phono cartridge and pream p input network was 0.73 µV for the audio band. Typical high quality preamps have noise voltages less than 1 Vv resulting in a 3 dB or more loss in system S/N ra tio when the cartridge noise is added to the preamp noise (in an RMS fashion). CONCLUSIONS Zero noise sources and amplifiers do not exist. Speci fying amplifier noise under ideal conditions will only lead to ideal specifications, not a measure of actual performance. Methods of S/N ratio measurement should be used that reflect the true performance instead of hollow specifications.
© 2014 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0 • 6/25/15 4
© 2014 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0 • 6/25/15 5 REFERENCES 1970) [2] Hallgren, B. I. , "On the Noise Performance of a Magnetic P honograph Pickup". J. Audio Eng Soc iety , Vol. 23, pp. 546 -552. (Sep. 1976) [3] Fristoe, H.T., "The Use o f Q Equations to Solve Complex Electrical Networks". Engineering Research Bulletin, Oklahoma State University, 1964. Author : John Maxwell, February 1977. DISCLAIMER FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE TO ANY PRODUCTS HEREIN TO IMPROVE RELIABILITY, FUNCTION, OR DESIGN. FAIRCHILD DOES NOT ASSUME ANY LIABILITY ARISING OUT OF THE APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN; NEITHER DOES IT CONVEY ANY LICENSE UNDER ITS PATENT RIGHTS, NOR THE RIGHTS OF OTHERS. LIFE SUPPORT POLICY FAIRCHILD’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, or (c) whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in significant injury to the user. 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness.