Diode Limiters or Clipper Circuits Circuits which are used to clip off portions of signal voltages above or below certain levels are called limiters or clippers.
Types of Clippers Positive Clipper Negative Clipper
Positive Clipper The circuit that limits or clips the positive part of the input voltage, is called a positive limiter or positive clipper.
Working of Positive Clipper As the input voltage goes positive, the diode becomes forward-biased and conducts current. Since the cathode is at ground potential (0V), the anode cannot exceed 0.7V (for Si). So point A is limited to +0.7V when the input voltage exceeds this value. When the input voltage goes back below 0.7V, the diode is reverse-biased and appears as an open. The output voltage looks like the negative part of the input voltage, but with magnitude determined by the voltage divider formed by R 1 and load resistor R L ;
Negative Clipper The circuit that limits or clips the negative part of the input voltage, is called a negative limiter or negative clipper.
Working of Negative Clipper If the diode is turned around, the negative part of the input voltage is clipped off. When the diode is forward-biased during the negative part of the input voltage, point A is held at -0.7V by the diode drop. When the input voltage goes above 0.7V, the diode is no longer forward- biased; and a voltage appears across R L proportional to input voltage.
Practical Problem What will be the output waveform across R L in the limiter circuit shown in figure
Clamper Circuits Clamping is the process of introducing a dc level into an ac signal. Clampers are also known as dc restorers.
Types of Clampers Positive Clampers Negative Clampers
Positive Clampers The circuit inserts a positive dc level in the output waveform.
Working of Positive Clamper When the input voltage initially goes negative, the diode is forward biased, allowing the capacitor to charge to near the input (Vp(in) 0.7V). Just after the negative peak, the diode is reverse-biased. This is because the cathode is held near Vp(in) 0.7V by the charge on the capacitor. The capacitor can only discharge through high resistance of RL. So, from the peak of one negative half-cycle to the next, the capacitor discharges very little. For good clamping action, the RC time constant should be at least ten times the period of the input frequency.
Working of Positive Clamper The net effect of the clamping action is that the capacitor retains a charge approximately equal to the peak value of the input less the diode drop. The capacitor voltage acts essentially as a battery in series with the input voltage. The dc voltage of the capacitor adds to the input voltage by superposition.
Negative Clampers The circuit inserts a negative dc level in the output waveform.
Working of Negative Clamper If the diode is turned around, a dc voltage is added to the input voltage to produce the output voltage.
Zener Diode Reverse-biased heavily doped pn-junction diode which operated in the breakdown region.
Junction Breakdown By increasing reverse bias voltage, a point is reaches where the junction breakdown and reverse current rises sharply to a value limited only by the external resistance connected in series with the junction. This breakdown voltage depends on the width of the depletion region, which depends upon doping level.
Types of Breakdown in Zener Diode There are two types of reverse breakdown in zener diode; Avalanche Breakdown It occurs in both rectifier diode and zener diode at a sufficiently high reverse voltage. Zener Breakdown It occurs in a zener diode at low reverse voltages.
Avalanche Breakdown It occurs in junctions which have wide depletion layers due to light doping level. Minority carriers (accelerated by this field) collide with semiconductor atoms in depletion region. On collision with valance electrons, covalent bonds are broken and electron-hole pairs are generated. These newly-generated charge carriers also accelerated by electric field resulting in more collisions and hence further production of charge carriers. This leads to an avalanche (flood) of charge carriers. Note:- Junction have very low resistance in breakdown region.
Zener Breakdown It occurs in junctions which have narrow depletion layers due to heavily doped. Breakdown voltage setup a very strong electric field across this narrow layer. This field rupture or break the covalent bonds; thereby creating electron-hole pairs. A slight further increase in reverse voltage is capable of producing large number of current carriers. Note:- Junction have very low resistance in breakdown region.
Zener Biasing Diode should be; Reverse Biased Have voltage across it greater than V z Be in a circuit where current is less than I zmax Note:- If a zener diode is forward-biased, it operates in the same way as a rectifier diode does.
V / I Characteristics V z = Zener breakdown voltage I zmin = Minimum current to sustain zener breakdown I zmax = Maximum zener current limited by maximum power dissipation For an ideal zener diode, the voltage does not change once it goes into breakdown. It means that V z remains constant even when I z increases considerably.
V / I Characteristics As the reverse voltage (V R ) is increased, the reverse current (I R ) remains extremely small upto the knee of the curve. The reverse current is also called the zener current, I Z. At this point, the breakdown effect begins; the internal zener resistance, also known as zener impedance (Z Z ), begins to decrease as the reverse current increase rapidly. Note that from the bottom of the knee, the zener breakdown voltage (V Z ) remains nearly constant.
Zener Impedance A change in zener current (ΔI Z ) produces a small change in zener voltage (ΔV Z ). This is represented as zener impedance (Z Z ), as Z Z = ΔV Z ΔI Z
Voltage Regulation A voltage regulation is a measure of a circuit s ability to maintain a constant output voltage even when either input voltage or load current varies.
Operating Principle Zener voltage (V z ) remains constant even zener current (I z )increases considerably.
As the input voltage increases, the output voltage remains constant
As the input voltage decreases, the output voltage remains constant
Voltage Regulation V IN = Input d.c voltage whose variations are to be regulated R L = Load resistance across which a constant voltage V out is required, is connected in parallel with the diode
Regulation Condition Applying Kirchhoff s Current Law; I = I z + I L Under all conditions V out = V z V in = IR + V out V in = IR + V z
Case 1 Let R is kept constant but supply voltage V in is increased slightly. It will increase I. This increase in I will be absorbed by the zener diode without affecting I L. The increase in V in will be dropped across R thereby keeping V out (or V z ) constant.
Case 2 V in is fixed but I L varies. When I L increases, diode current I z decreases thereby keeping I and hence IR drop constant. In this way V out remains constant. When I L decreases, I z increases in order to keep I and hence IR drop constant. Again V out remains unaffected. V in - IR = V out V out = Vin (I z + I L ) R R = V in V out I z + I L When the diode current reaches its maximum value I L becomes zero ; R = V in - V out I z (max)
LED Operating Principle When the device is forward-biased, electrons cross the pn-junction from the n-type material and recombine with the holes in the p-type material. When the recombination takes place, the recombining electrons release energy in the form of heat and light. This process is known as electroluminescence.
LED
LED Symbol
Various impurities are added during the doping process to establish the wavelength of the emitted light (i.e the color of light). LED Semiconductor Material Gallium Arsenide (GaAs) (emit infrared radiation) Gallium Arsenide Phosphate (GaAsP) (red or yellow light) Gallium Phosphate (GaP) (red or green light)
LED Biasing The forward voltage across an LED is considerably greater than for a silicon diode (typically the maximum V f for LED is between 1.2V and 3.2V). Reverse breakdown for an LED is much less than for a silicon rectifier diode (3V to 10V).
Light Output The LED emits light in response to a sufficient forward current. The amount of power output translated into light is directly proportional to the forward current.
Seven-Segment Display Application Combinations of the segments form the ten decimal digits. Each segment in the display is an LED. By forward-biasing selected combinations of segments any decimal digit and decimal point can be formed.