Oscillator

An oscillator is a circuit that produces a repetitive waveform on its output with only the dc supply voltage as an input.A repetitive input signal is not required.The output voltage can be either sinusoidal or nonsinusoidal depending on the type of oscillator.

The basic concept of an oscillator is illustrated in figure(a).Essentially an oscillator converts electrical energy in the form of dc to electrical energy in the form of ac.A basics sinusoidal oscillator consists of an amplifier for gain( either discrete transistor or op-amp ) and a positive feedback circuit that produces phase shift and provides attenuation,as shown in figure(b).

The basic oscillator concept showing three common types of output waveforms sine wave,square wave and sawtooth.

Oscillator Principles:

Positive Feedback:

Positive feedback is characterized by the condition wherein a portion of the output voltage of an amplifier is fed back to the input with no net phase shift,resulting in a reinforcement of the output signal.This basic idea is illustrated in figure.As you can see the in phase feedback voltage Vf is amplified to produce the output voltage which in turn produces the feedback voltage.That is,a loop is created in which the signal sustains itself and a continous sinusoidal output is produced.This phenomenon is called oscillation.

Conditions for Oscillation:

Two conditions are required for a sustained state of oscillation.

1.The phase shift around the feedback loop must be zero degree.

2.The voltage gain Acl,around the closed feedback loop (loop gain) must equal 1 (unity).

The voltage gain around the closed feedback loop(Acl) ia the product of the amplifier gain (Av) and the attenuation (B) of the feedback circuit.

Acl=Av.b

For example,if the amplifier has a gain of 100,the feedback circuit must have a attenuation of 0.01 to make the loop gain equal to 1( that is Av.B=100*0.01=1).These conditions of oscillation are illustrated in figure.

Start-Up Conditions:

So far,you have seen what it takes for an oscillator to produce a continous sine wave output.Now let's examine the requirements for the oscillation to start when the dc supply voltage is turned on.As you know,the unity-gain condition must be met for oscillation to be sustained.For oscillation to begin,the voltage gain around the positive feesback loop must be greater than 1 ,so that the amplitude of the output can be build up to a desired level.The gain must then decrease to 1 so that the output stays at the desired level

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Operational Amplifier

The standard operational amplifier (op-amp) symbol is shown in figure(a).It has two input terminals,the inverting input(-) and the noninverting input(+) and an output terminal.The typical op-amp operates with two dc supply voltages,one positive and the other negative as shown in figure(b).Usually these dcvoltage terminals are left off the schematic symbol for simplicity but are always understood to be there.Some typical op-amp IC packages are shown in figure(c).

The Ideal Op-Amp:

To illustrate what an op-amp is.Let's consider its ideal characteristics.A practical op-amp of course,falls short of these ideal standards,but it is much easier to understand and analyze the device from an ideal point of view.

First,the ideal op-amp has infinite voltage gain and infinite bandwidth.Also it has an infinite input impedence(open),so that it does not load the driving source.Finally,it has a zero output impedence.These characteristics are illustrated in figure.The input voltage Vin appears between the two input terminals and the output voltage is AvVin as indicated by the internal voltage source symbol.The concept of infinite input impedence is a particularly valuable analysis tool for the various op-amp configuration,which will be discussed.

The practical Op-Amp:

Although modern integrated circuit (IC) op-amps approach parameter values that can be treated as ideal in many cases,the ideal device can never be made.

Any device has limitations,peak-to-peak output voltage,for example,is usually limited to slightly lass than the two supply voltages.Output current is also limited by internal restrictions such as power dissipation and component ratings.

Characteristics of a practical op-amp are very high voltage gain,very high input impedence,very low output impedence and wide bandwidth.Three of these are lebelled in figure:

.

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Voltage Multipliers

Voltage Doubler:

Half wave Voltage Doubler:

A voltage doubler is a voltage multiplier with a multiplication factor of two.A half wave voltage doubler is shown in figure below:

During the positive half-cycle of te secondary voltage,diode D1 is forward-biased and D2 is reverse-biased.Capacitor C1 is charged to the peak of the secondry voltage (Vp) less the diode drop with the polarity shown in part (a).During the negative half-cycle,diode D2 is forward-biased and D1 is reverse-biased,as shown in part (b).Since C1 can't discharge,the peak voltage on C1 adds to the secondary voltage to charge C2 to approximately 2Vp.Applying Kirchhoff's law around the loop as shown in part (b),the voltage across C2 is

Vc1-Vc2+Vp=0

Vc2=Vp+Vc1

Neglecting the diode drop of D2,Vc1=Vp,therefore,

Vc2=Vp+Vp=2Vp

Under a no load condition ,C2 remains charged to approximately 2Vp.If a load resistance is connected across the output,C2 discharges slightly through the loadon the next positive half-cycle and is again recharged to 2Vp on the following negative half-cycle.The ersulting output is a half-wave,capacitor filtered voltage.The peak inverse voltage across each diode is 2Vp.

Full Wave Voltage Doubler:

A full-wave doubler is shown in figure.When the secondary voltage is positive,D1 is forward-biased and C1 charges to approximately Vp,as shown in part (a).During the negative half-cycle,D2 is forward-biased and C2 charges to approximately Vp,as shown in part (b).The output voltage,2Vp is taken across the two capacitors in series.

Voltage Tripler:

The ddition of another diode-capacitor section to the half-wave voltage doubler creats a voltage tripler,as shown in figure.The operationis as follows:On the positive half-cycle of the secondary voltage,C1 charges to Vp through D1.During the negative half-cycle,C2 charges to 2Vp through D2,as described for the doubler.During the next positive half-cycle,C3 charges to 2Vp through D3.The tripler output is taken across C1 and C3,as shown in figure.

Voltage Quadrupler:

The addition of still another diode-capacitor section,as shown in figure,produces an output four times the peak secondary voltage,C4 charges to 2Vp through D4 on a negative half-cycle.The 4Vp output is taken across C2 and C4,as shown.In both the tripler and qudrupler circuits,the PIV of each diode is 2Vp.

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Bipolar Junction Transistor ( BJT )

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The bipolar junction transistor is constructed with three doped semiconductor regions separated by two pn junctions.The three regions are emitter,base and collector.There are two types of bipolar junction transistors.

One type consists of two n regions separated by p region called npn.
Second type consists of two p regions separated by n region called pnp.


fig.1 ( npn )
fig.2 ( pnp )

The pn junction joining the base and emitter regions called base-emitter junction.Also the pn junction joining the base collector regions called base-collector junction.The three leads are connected to these regions and named as E, B and C for emitter ,base and collector respectively.
The base region is lightly doped and thin,emitter is heavily doped and collector is moderaftely doped.
The bipolar term is for both holes and electrons as carriers in transistor.

Transistor Operation:

To operate the transistor properly as an amplifier, The two pn juctions must be correctly biased with proper dc voltage.For npn and pnp both the BE base-emitter junction is forward biased and BC base-collector junction is reverse biased known as forward-reverse bias.
Let's examine what happen when BE and BC junctions in npn transistor are forward-reverse biased.The forward bias for BE junction narrows the BE depletion region and reverse bias for BC junction widens the BC depletion region. Read More!

Diode Limiter

The diode limiter also called Clipper as it is used to limit the input voltage.when the positive cycle of the input wave comes, the diode becomes forward biased (fig. 2). As the cathode now at zero volts 0 v,the anode cannot exceed 0.07 volts assuming diode is a silicon diode.

So it limited to 0.7 volts as positive cycles comes from input voltage.When negative cycle comes the diode become reverse biased and at the output load we only get the negative cycle with clipped positive cycle.

And if we put the diode in reverse we will get the positive cycle with the clipped negative cycle
as shown below in the figure.





Figure 1: Shows the input
Figure 2: positive cycle clipped.
Figure 3: Negative cycle clipped.
Figure 4: Both the positive and negatice cycles are clipped. Read More!

P Type Semiconductors

When the trivalent impurity atoms like aluminium (Al), boron (B), indium (In) and gallium (Ga) having three valence electrons are added to silicon,after forming covalent bond one hole remains in silicon which need fourth electrons.As much as impurities atoms added more holes are produced called majority carriers.
The number of holes can be controled by impurites atoms added to silicon.The majority of current carriers is P-type material.There are also some free electrons produced when electron holes are thermally treated.These electrons are the minority carriers in p-type material. Read More!

N Type Semiconductors

Doping:

The conductivity of silicon and germenium can be drastically increased by the controlled addition of impurities to the intrinsic (pure) semiconductor material. This process, called doping, increases the number of current carriers (electrons or holes).

N Type Semiconductor:

To increase the number of conduction band electrons , pentavalent impurity atoms like arsenic (As), phosphorus (P), bismith(Bi) and antimony (Sb) are added.
Each pentavalent atom leaves an extra free conduction electron after combining with the silicon atom forming covalent bond.As pentavalent atom gives an electron so called donar atom.
The number of conduction electrons can be controled by the impurities ato added to silicon.

Most of current carriers are electron,silicon doped with pentavalent atoms is an n-type semiconductor material.The electrons are called majority carriers.There are also some holes when electrons are paired when thermally processed.These holes are called minority carriers. Read More!