AN2867 datasheet(9/41 Pages) STMICROELECTRONICS

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AN2867

Oscillator theory

The part of the I-V curve represented in purple shows a negative transresistance:

while the parts of the curve in blue shows a positive transresistance:

Figure 3. I-V curve of a dipole showing a negative transresistance area (in purple)

2.2

Transconductance

Like the conductance which is defined as the inverse of the resistance, the

transconductance is also defined as the inverse of the transresistance. Transconductance

can also be defined as the differential conductance which expressed by the formula:

2.3

Negative-resistance oscillator principles

An oscillation loop is made of two branches (see Figure 4):

•

The active branch of the oscillation loop which is composed of the oscillator itself. This

branch is responsible for providing enough energy at startup to make the oscillation

start and build up until it reaches the stable oscillation phase. When a stable oscillation

is reached, the oscillator branch provides enough energy to compensate for the

oscillation loop passive branch losses.

•

The passive branch is mainly composed of the resonator, the two load capacitors and

all the parasitic capacitances.

ΔV

ΔI

--------

() VC

()

–

() IC

()

–

---------------------------------

ΔV

ΔI

--------

() VA

()

–

() IA

()

–

--------------------------------

ΔV

ΔI

--------

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