Biomedical Engineering Reference
In-Depth Information
0.8
0.6
0.4
0.2
0
Φ= 0, y =- I - cosθ
θ
θ
(a)
1.0
0.8
0.6
0.4
0.2
0
-0.2
Φ=π/2, y =- sinθ
θ
θ opt
θ
(b)
-0.4
θ opt
0
-0.2
-0.4
-0.6
-0.8
-1.0 0
θ
I - cosθ
θ
Φ=π, y =-
(c)
π
FIGURE 2.11
AC resistance versus transit angle for three different injection phase delays, (a) Φ = 0, (b) Φ = n 2 ,
(c) Φ = n .
is no injection phase delay (Φ = π), the AC resistance never becomes negative.
Therefore, no oscillation can occur when there is no injection phase delay.
The maximum negative AC resistance then occurs when the AC voltage just
starts into its negative half cycle. The injected carriers drift across the drift
region of length W at their saturation velocity, v s , during the negative half
cycle of the AC voltage. Therefore, the frequency of oscillation is f = v s /2 W .
Several important characteristics of the microwave packages used for
IMPATT oscillators are the cavity and the heat sink material at the base of
the package. The cavity envelops the IMPATT diode, and its dimensions are
such that oscillations at the correct frequency are obtained. The packaging
material must have a relatively low coefficient of expansion relative to tem-
perature in order to ensure proper oscillation over the specified temperature
range. The heat sink material must be a good thermal conductor because of
the heat buildup from the high electric fields present in the avalanche region
that cause impact ionization current.
 
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