LM120K15883 National Semiconductor, LM120K15883 Datasheet

LM120K15883

Manufacturer Part Number

LM120K15883

Description

TO-3

Manufacturer

National Semiconductor

Datasheet

1.LM120K15883.pdf (8 pages)

Specifications of LM120K15883

Date_code

05+

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Noise Specs Confusing?

It’s really all very simple — once you understand it. Then,

here’s the inside story on noise for those of us who haven’t

been designing low noise amplifiers for ten years.

You hear all sorts of terms like signal-to-noise ratio, noise

figure, noise factor, noise voltage, noise current, noise

power, noise spectral density, noise per root Hertz, broad-

band noise, spot noise, shot noise, flicker noise, excess

noise, I/F noise, fluctuation noise, thermal noise, white

noise, pink noise, popcorn noise, bipolar spike noise, low

noise, no noise, and loud noise. No wonder not everyone

understands noise specifications.

In a case like noise, it is probably best to sort it all out from

the beginning. So, in the beginning, there was noise; and

then there was signal. The whole idea is to have the noise

very small compared to the signal; or, conversely, we desire

a high signal-to-noise ratio S/N. Now it happens that S/N is

related to noise figure NF, noise factor F, noise power, noise

voltage e

happens that any noisy channel or amplifier can be com-

pletely specified for noise in terms of two noise generators e

and i

All we really need to understand are NF, e

is a rundown on these three.

NOISE VOLTAGE, e

SHORT-CIRCUIT INPUT RMS NOISE VOLTAGE is simply

that noise voltage which would appear to originate at the

input of the noiseless amplifier if the input terminals were

shorted. It is expressed in nanovolts per root Hertz

at a specified frequency, or in microvolts in a given frequency

band. It is determined or measured by shorting the input

terminals, measuring the output rms noise, dividing by am-

plifier gain, and referencing to the input. Hence the term,

equivalent noise voltage. An output bandpass filter of known

characteristic is used in measurements, and the measured

value is divided by the square root of the bandwidth

data is to be expressed per unit bandwidth or per root Hertz.

FIGURE 1. Noise Characterization of Amplifier

as shown in Figure 1.

, and noise current i

, or more properly, EQUIVALENT

. To simplify matters, it also

AN007414

, and i

. So here

00741401

National Semiconductor

Application Note 104

May 1974

The level of e

typically it increases at lower frequencies as shown in Figure

2. This increase is 1/f NOISE.

NOISE CURRENT, i

OPEN-CIRCUIT RMS NOISE CURRENT is that noise which

occurs apparently at the input of the noiseless amplifier due

only to noise currents. It is expressed in picoamps per root

Hertz

given frequency band. It is measured by shunting a capacitor

or resistor across the input terminals such that the noise

current will give rise to an additional noise voltage which is i

x R

gain, referenced to input, and that contribution known to be

due to e

the total measured noise. If a capacitor is used at the input,

there is only e

bandpass filter and converted to

if appropriate; typically it increases at lower frequencies for

op amps and bipolar transistors, but increases at higher

frequencies for field-effect transistors.

NOISE FIGURE, NF is the logarithm of the ratio of input

signal-to-noise and output signal-to-noise.

where: S and N are power or (voltage)

FIGURE 2. Noise Voltage and Current for an Op Amp

(or X

and resistor noise is appropriately subtracted from

cin

). The output is measured, divided by amplifier

at a specified frequency or in nanoamps in a

is not constant over the frequency band;

and i

, or more properly, EQUIVALENT

cin

. The i

is measured with a

levels

00741402

www.national.com

(1)

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