AN-6605

APPLICATION NOTE

© 2014 Fairchild Semiconductor Corporation

www.fairchildsemi.com

Rev. 1.0 • 6/25/15

2

1

2

n

(4)

Where VEQ = equalized source noise (V) and An =

magnitude of the equalized gain at the center of each

noise band (V/V).

SOURCE NOISE

Models are needed for capacitive and inductive systems

such that noise calculations can be made. Namely, the real

part of the impedance needs to be determined.

A lumped model of a capacitive source, such as condenser

or electret microphone, consists of the microphone and

stray capacitance shunted by a load resistance.

Figure 2.

Lumped Model of a Capacitive

Microphone

It should be noted that for any particular microphone, the

5).

The inductive source (phono cartridges and tape heads) is

more complex to analyze because it has a much more

complex model. The simplified lumped model of a phono

cartridge or tape head consists of a series inductance and

resistance shunted by a small capacitor.

Each phono

cartridge or tape head has a recommended load consisting

of a specified shunt resistance and capacitance. A model

for the inductive source and preamp input network is

shown in Figure 3.

Figure 3.

Phono Cartridge or Tape Head and

Preamp Input Network

This circuit is quite formidable to analyze and needs,

further simplification. Through the use of Q equations, a

series L-R is transformed to a parallel L-R.

Simplifying the input network to:

Figure 4.

Simplified Inductive Source Network.