19.FM - VCO - Varactor I Ham lesson o' de day
So the second main circuit component I've found while researching frequency modulation is the varactor diode (denoted by >||). It is known by many names but is essentially a variable capacitance diode whose capacitance is a function of the voltage impressed on its terminals. These are operated in a reverse-biased fashion. Let's break these parts down a bit.
We'll start with a quick review of diodes. Essentially, it is a circuit component composed of P and N material.
When P-type and N-type semiconductors are first placed together, a passive equilibration process occurs. This is also known as zero bias.
Free electrons from the N-type start migrating towards the P-type as they’re attracted to the holes - bonds without electrons (from the boron atoms not having as many electrons as phosphorus atoms).
Interestingly, those bonds without electrons (holes) also migrate but via diffusion from a high concentration of holes to a lower concentration.
As the free electrons and holes migrate towards each other, they find each other and hook up! <3
This creates a no-man’s-land where there are no charge carriers - ie no holes/free electrons.
Forward Bias has been previously described where the p-type is connected with the positive terminal of the battery and the n-type is connected with the negative terminal of the battery. Essentially electrons flow through the N-type region toward the PN junction. Holes flow through the P-type region in the opposite direction toward the PN junction, and the two species of carriers constantly recombine in the vicinity of the junction. The electrons and holes travel in opposite directions, but they also have opposite charges, so the overall current is in the same direction on both sides of the diode, as required.
Varactor diodes are run in reverse bias where the P-type region is connected to the negative terminal of the power supply and the N-type region to the positive terminal of the power supply. The 'holes' in the P-type material are attracted to the negative terminal and pulled away from the junction. Similarly, because the N-type region is connected to the positive terminal, the electrons will also be pulled away from the junction. Therefore the depletion zone widens, and does so with increasing reverse-bias voltage.
Key Note: The power supply supplies the reverse-bias voltage.
This increases the voltage barrier causing a high resistance to the flow of charge carriers thus allowing minimal electric current to cross the p–n junction. The increase in resistance of the p-n junction results in the junction behaving as an insulator.
The strength of the depletion zone electric field increases as the reverse-bias voltage increases. Once the electric field intensity increases beyond a critical level, the p–n junction depletion zone breaks-down and current begins to flow. Both of these breakdown processes are non-destructive and are reversible, so long as the amount of current flowing does not reach levels that cause the semiconductor material to overheat and cause thermal damage.
So how does this reverse-bias stuff have anything to do with varactor diodes?
http://en.wikipedia.org/wiki/Reverse-biased
