3i.1 Recall
that a diode will conduct electrons only in one direction and that a
small potential difference (forward bias) is needed to turn the diode
on.
A
diode is a semiconductor device that will conduct electrons in only one
direction. They are usually made of Germanium or silicon.
To turn the diode on a small potential difference (forward bias) is required. The actual voltage depends on the type of diode. In a forward biased silicon diode it takes a voltage across the diode of 0.65V before it starts to conduct.
In the drawing, the positive of
the power supply is connected to the anode of the diode. The negative
is connected to the cathode. The electrons are repelled towards and
into the depletion zone and current flows.
If the power source is
reversed, electrons are attracted to the positive of the power supply,
the negative pole of the power source attracts the positive "holes".
The area in the middle of the diode , the depletions zone increase in
size and current stops flowing.
When an alternating voltage is applied to a diode it will only conduct for half of the cycle.
Understand the use of a diode to produce direct current from the alternating current leaving a transformer (rectification). Identify the AC and rectified (pulsed DC) waveforms.
When
an alternating voltage is applied to a single diode it will only
conduct for half of the cycle. This produces a simple DC output
consisting of pulses.
By using 4 diodes, full rectification can be achieved where the DC is
smoother, but not totally. Capacitors can be used to smooth out the
pulses
3i.2 Understand
that in a rectifier circuit a capacitor can store a charge during the
conducting part of the cycle and release it during the non-conducting
part, providing a smoothing effect and a smoother DC output. Identify
the resulting waveforms. In a rectifier circuit a capacitor can store a charge during the
conducting part of the cycle and release it during the non-conducting
part, providing a smoothing effect and a smoother DC output.
The capacitor is often called a smoothing capacitor.
Note the shape of the output DC voltage
Capacitor used in a rectifier Circuit to provide smoother DC
3i.3 Recall
that a light emitting diode (LED) is made from a material that will
produce light when passing a suitable direct current.
These
are designed so that when they are forward-biased, the electrons enter
the "holes" and emit light. They are used on the front panels of some
rigs to show that the set is turned on. The advantage over the old
filament bulb is that they do not produce a lot of heat and are
therefore more efficient, using less power. They are also used to
form alphanumeric displays in LED displays.
Light Emitting Diodes (LED)
This is the symbol for an LED:
3i.4 Recall
that a variable capacitance diode behaves like a capacitor when reverse
biased, and that the capacitance can be varied by changing the applied
potential difference.
A
variable capacitance diode changes its capacitance depending on the
potential difference (voltage) applied. Thus a diode and a variable
resistor can be used in place of a variable capacitor. As variable
capacitors are quite expensive this can be a viable alternative. In
addition an RF stage with several bandpass filters can be made tunable
over a range of frequencies.
When a diode is reverse biased there is a large depletion zone in the
centre. This acts like the dialectric of a capacitor. The P and N
regions of the diode act like the plates of a capacitor. As the reverse
voltage is changed the depletion zone changes altering the capacitance.
All diodes can act like capacitors, but varicap diodes are designed specially for particular ranges of capacitance.
