Diode Varicap
pp 49-53
3.4 Diode choritikotitas variable (Varicap <Varactor)
As mentioned in paragraph 3.1.2, the area of stripping contact PN diode is a capacitor with "reinforcements" the two parts of the region P and N type containing negative and positive charges respectively. The capacitance of the capacitor CT is dependent on the cross section S of the diode, the length L of the region and stripping the dielectric e, according to relation (3.1.3):
3.4.1
It turns out that the capacity varies inversely a CT
naloga the value of the external voltage applied to the ends of the diode when connected in reverse polarity, according to relation (3.4.2) and Figure 3.4.1.
3.4.2
The diode is called a variable capacity diode <Varikap diode (Varicap) <Varactor.
Figure 3.4.1
Change in capacity diode Varicap
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Free reading:
If no external voltage (VR = 0), the relation (3.4.2) is written:
3.4.3
Substituting the value of K the relation (3.4.2) is written:
The relationship (3.4.4) gives the value of capacity with an external (reverse) voltage as a function of the initial value without external voltage and the ratio of source voltages and barrier.
Example 3.4.1
Have the capacity without external voltage of a diode
Varicap, CT (0) = 1,6 pF. To obtain the capacity of the diode when
the applied voltage at the ends is 24 times greater than the potential barrier.
Solution
Applying relation (3.4.4) we get:
The diode Varicap denoted by an arrow on Figure 3.4.2.Symvolismos Varicap diode with the anode and a pykoot <stio K0ithodo showing that the diode is a variable capacitor (Figure 3.4.2 The equivalent circuit of the diode Varicap consists of a resistor RS to is the proper polarization resistance in series with the parallel connection of the variable capacity of CT and impedance inversion time inversion of polarization RR. The value of RS is very small (a few ohms), the value of RR in the order of MO while the price of CT ranges from 1 to 200 pF.
Table 3.4.1 describes
Equivalent circuit of diode Varicap.
features some channels Varicap trade:
Table 3.4.1 Varicap trade
Free reading: The diode Varicap applied to signal modulation in FM and other telecommunication techniques for frequency selection. In a resonant circuit LC, the resonant frequency is given by equation (3.4.5)
3.4.5
When the capacity is changed, is changing the frequency of the tuned circuit. When not in the capacity varies inversely with a changing trend, then the resonant frequency followed a similar manner to the change in voltage. This property is used to transmit audio signal frequency modulated (FM). Similarly, the selected resonant frequency of a radio or television receiver.
The sensitivity of these receivers are great for small change in the capacity of a large change in frequency and vice versa. It turns out that the frequency change during Bi, the capacity will change in DC where:
3.4.6
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Example 3.4.2
A circuit tuned to a frequency f = 100 MHz consists of inductor and capacitor capacity C = 75pF. Calculate the change of capacitance, the circuit to be tuned to another frequency to 80 KHz deviation from the original.
Solution
Equation (3.4.6) becomes:
