Category: sinusoidal Oscillators

Phase Shift Oscillators One of the important features of an oscillator is that the feedback energy applied should be in correct phase to the tank circuit. The oscillator circuits discussed so far has employed inductor (L) and capacitor (C) combination, in the tank circuit or frequency determining circuit. We have observed that the LC combination in oscillators provide 180o phase shift and transistor in CE configuration provide 180° phase shift to make a total of 360o phase shift so that it would make a zero difference in phase. Drawbacks of LC circuits Though they have few applications, the LC circuits have few drawbacks such as Frequency instability Waveform is poor Cannot be used for low frequencies Inductors are bulky and expensive We have another type of oscillator circuits, which are made by replacing the inductors with resistors. By doing so, the frequency stability is improved and a good quality waveform is obtained. These oscillators can also produce lower frequencies. As well, the circuit becomes neither bulky nor expensive. All the drawbacks of LC oscillator circuits are thus eliminated in RC oscillator circuits. Hence the need for RC oscillator circuits arise. These are also called as Phase–shift Oscillators. Principle of Phase-shift oscillators We know that the output voltage of an RC circuit for a sinewave input leads the input voltage. The phase angle by which it leads is determined by the value of RC components used in the circuit. The following circuit diagram shows a single section of an RC network. The output voltage V1’ across the resistor R leads the input voltage applied input V1 by some phase angle ɸo. If R were reduced to zero, V1’ will lead the V1 by 90o i.e., ɸo = 90o. However, adjusting R to zero would be impracticable, because it would lead to no voltage across R. Therefore, in practice, R is varied to such a value that makes V1’ to lead V1 by 60o. The following circuit diagram shows the three sections of the RC network. Each section produces a phase shift of 60o. Consequently, a total phase shift of 180o is produced, i.e., voltage V2 leads the voltage V1 by 180o. Phase-shift Oscillator Circuit The oscillator circuit that produces a sine wave using a phase-shift network is called as a Phase-shift oscillator circuit. The constructional details and operation of a phase-shift oscillator circuit are as given below. Construction The phase-shift oscillator circuit consists of a single transistor amplifier section and a RC phase-shift network. The phase shift network in this circuit, consists of three RC sections. At the resonant frequency fo, the phase shift in each RC section is 60o so that the total phase shift produced by RC network is 180o. The following circuit diagram shows the arrangement of an RC phase-shift oscillator. The frequency of oscillations is given by $$f_o = frac{1}{2pi RC sqrt{6}}$$ Where $$R_1 = R_2 = R_3 = R$$ $$C_1 = C_2 = C_3 = C$$ Operation The circuit when switched ON oscillates at the resonant frequency fo. The output Eo of the amplifier is fed back to RC feedback network. This network produces a phase shift of 180o and a voltage Ei appears at its output. This voltage is applied to the transistor amplifier. The feedback applied will be $$m = E_i/E_o$$ The feedback is in correct phase, whereas the transistor amplifier, which is in CE configuration, produces a 180o phase shift. The phase shift produced by network and the transistor add to form a phase shift around the entire loop which is 360o. Advantages The advantages of RC phase shift oscillator are as follows − It does not require transformers or inductors. It can be used to produce very low frequencies. The circuit provides good frequency stability. Disadvantages The disadvantages of RC phase shift oscillator are as follows − Starting the oscillations is difficult as the feedback is small. The output produced is small. Learning working make money

Tunnel Diode Oscillator The oscillator circuit that is built using a tunnel diode is called as a Tunnel diode oscillator. If the impurity concentration of a normal PN junction is highly increased, this Tunnel diode is formed. It is also known as Esaki diode, after its inventor. Tunnel Diode When the impurity concentration in a diode increases, the width of depletion region decreases, extending some extra force to the charge carriers to cross the junction. When this concentration is further increased, due to less width of the depletion region and the increased energy of the charge carriers, they penetrate through the potential barrier, instead of climbing over it. This penetration can be understood as Tunneling and hence the name, Tunnel diode. The following image shows how a practical tunnel diode looks like. The symbols of tunnel diode are as shown below. For more details regarding tunnel diodes, please refer our tutorial. Tunnel Diode Oscillator The tunnel diode helps in generating a very high frequency signal of nearly 10GHz. A practical tunnel diode circuit may consist of a switch S, a resistor R and a supply source V, connected to a tank circuit through a tunnel diode D. Working The value of resistor selected should be in such a way that it biases the tunnel diode in the midway of the negative resistance region. The figure below shows the practical tunnel diode oscillator circuit. In this circuit, the resistor R1 sets proper biasing for the diode and the resistor R2 sets proper current level for the tank circuit. The parallel combination of resistor Rp inductor L and capacitor C form a tank circuit, which resonates at the selected frequency. When the switch S is closed, the circuit current rises immediately towards the constant value, whose value is determined by the value of resistor R and the diode resistance. However, as the voltage drop across the tunnel diode VD exceeds the peak-point voltage Vp, the tunnel diode is driven into negative resistance region. In this region, the current starts decreasing, till the voltage VD becomes equal to the valleypoint voltage Vv. At this point, a further increase in the voltage VD drives the diode into positive resistance region. As a result of this, the circuit current tends to increase. This increase in circuit will increase the voltage drop across the resistor R which will reduce the voltage VD. V-I characteristic curve The following graph shows the V-I characteristics of a tunnel diode − The curve AB indicates the negative resistance region as the resistance decreases while the voltage increases. It is clear that the Q-point is set at the middle of the curve AB. The Q-point can move between the points A and B during the circuit operation. The point A is called peak point and the point B is called valley point. During the operation, after reaching the point B, the increase in circuit current will increase the voltage drop across the resistor R which will reduce the voltage VD. This brings the diode back into negative resistance region. The decrease in voltage VD is equal to the voltage VP and this completes one cycle of operation. The continuation of these cycles produces continuous oscillations which give a sinusoidal output. Advantages The advantages of a tunnel diode oscillator are as follows − It has high switching speeds. It can handle high frequencies. Disadvantages The disadvantages of a tunnel diode oscillator are as follows − They are low power devices. Tunnel diodes are a bit costly. Applications The applications of a tunnel diode oscillator are as follows − It is used in relaxation oscillators. It is used in microwave oscillators. It is also used as Ultra high speed switching device. It is used as logic memory storage device. After having covered all the major sinusoidal oscillator circuits, it is to be noted that there are many oscillators like the ones mentioned till now. The oscillators which produce sine waveforms are sinusoidal oscillators as discussed. The oscillators which produce non-sinusoidal waveforms (rectangular, sweep, triangular etc.) are non-sinusoidal oscillators which we have discussed in detail in our tutorial. Learning working make money

Clapp Oscillator Another oscillator which is an advanced version of Colpitts oscillator is the Clapp Oscillator. This circuit is designed by making a few changes to the Colpitts oscillator. The circuit differs from the Colpitts oscillator only in one respect; it contains one additional capacitor (C3) connected in series with the inductor. The addition of capacitor (C3) improves the frequency stability and eliminates the effect of transistor parameters and stray capacitances. The following circuit diagram shows the arrangement of a transistor Clapp oscillator. The operation of Clapp oscillator circuit is in the same way as that of Colpitts oscillator. The frequency of oscillator is given by the relation, $$f_o = frac{1}{2 pi sqrt{L.C}}$$ Where $$C = frac{1}{frac{1}{C_1} + frac{1}{C_2} + frac{1}{C_3}}$$ Usually, the value of C3 is much smaller than C1 and C2. As a result of this, C is approximately equal to C3. Therefore, the frequency of oscillation, $$f_o = frac{1}{2 pi sqrt{L.C_3}}$$ It is understood that the Clapp oscillator is similar to the Colpitts oscillator, however they differ in the way the inductances and capacitances are arranged. The frequency stability though is good, can be variable in a Clapp oscillator. A Clapp oscillator is sometimes preferred over a Colpitts oscillator for constructing a variable frequency oscillator. The Clapp oscillators are used in receiver tuning circuits as a frequency oscillator. Learning working make money

Sinusoidal Oscillators – Introduction An oscillator generates output without any ac input signal. An electronic oscillator is a circuit which converts dc energy into ac at a very high frequency. An amplifier with a positive feedback can be understood as an oscillator. Amplifier vs. Oscillator An amplifier increases the signal strength of the input signal applied, whereas an oscillator generates a signal without that input signal, but it requires dc for its operation. This is the main difference between an amplifier and an oscillator. Take a look at the following illustration. It clearly shows how an amplifier takes energy from d.c. power source and converts it into a.c. energy at signal frequency. An oscillator produces an oscillating a.c. signal on its own. The frequency, waveform, and magnitude of a.c. power generated by an amplifier, is controlled by the a.c. signal voltage applied at the input, whereas those for an oscillator are controlled by the components in the circuit itself, which means no external controlling voltage is required. Alternator vs. Oscillator An alternator is a mechanical device that produces sinusoidal waves without any input. This a.c. generating machine is used to generate frequencies up to 1000Hz. The output frequency depends on the number of poles and the speed of rotation of the armature. The following points highlight the differences between an alternator and an oscillator − An alternator converts mechanical energy to a.c. energy, whereas the oscillator converts d.c. energy into a.c. energy. An oscillator can produce higher frequencies of several MHz whereas an alternator cannot. An alternator has rotating parts, whereas an electronic oscillator doesn’t. It is easy to change the frequency of oscillations in an oscillator than in an alternator. Oscillators can also be considered as opposite to rectifiers that convert a.c. to d.c. as these convert d.c. to a.c. You can get a detailed description on rectifiers in our tutorial. Classification of Oscillators Electronic oscillators are classified mainly into the following two categories − Sinusoidal Oscillators − The oscillators that produce an output having a sine waveform are called sinusoidal or harmonic oscillators. Such oscillators can provide output at frequencies ranging from 20 Hz to 1 GHz. Non-sinusoidal Oscillators − The oscillators that produce an output having a square, rectangular or saw-tooth waveform are called non-sinusoidal or relaxation oscillators. Such oscillators can provide output at frequencies ranging from 0 Hz to 20 MHz. We will discuss only about Sinusoidal Oscillators in this tutorial. You can learn the functions of non-sinusoidal oscillators from our tutorial. Sinusoidal Oscillators Sinusoidal oscillators can be classified in the following categories − Tuned Circuit Oscillators − These oscillators use a tuned-circuit consisting of inductors (L) and capacitors (C) and are used to generate high-frequency signals. Thus they are also known as radio frequency R.F. oscillators. Such oscillators are Hartley, Colpitts, Clapp-oscillators etc. RC Oscillators − There oscillators use resistors and capacitors and are used to generate low or audio-frequency signals. Thus they are also known as audio-frequency (A.F.) oscillators. Such oscillators are Phase –shift and Wein-bridge oscillators. Crystal Oscillators − These oscillators use quartz crystals and are used to generate highly stabilized output signal with frequencies up to 10 MHz. The Piezo oscillator is an example of a crystal oscillator. Negative-resistance Oscillator − These oscillators use negative-resistance characteristic of the devices such as tunnel devices. A tuned diode oscillator is an example of a negative-resistance oscillator. Nature of Sinusoidal Oscillations The nature of oscillations in a sinusoidal wave are generally of two types. They are damped and undamped oscillations. Damped Oscillations The electrical oscillations whose amplitude goes on decreasing with time are called as Damped Oscillations. The frequency of the damped oscillations may remain constant depending upon the circuit parameters. Damped oscillations are generally produced by the oscillatory circuits that produce power losses and doesn’t compensate if required. Undamped Oscillations The electrical oscillations whose amplitude remains constant with time are called as Undamped Oscillations. The frequency of the Undamped oscillations remains constant. Undamped oscillations are generally produced by the oscillatory circuits that produce no power losses and follow compensation techniques if any power losses occur. Learning working make money

Hartley Oscillator A very popular local oscillator circuit that is mostly used in radio receivers is the Hartley Oscillator circuit. The constructional details and operation of a Hartley oscillator are as discussed below. Construction In the circuit diagram of a Hartley oscillator shown below, the resistors R1, R2 and Re provide necessary bias condition for the circuit. The capacitor Ce provides a.c. ground thereby providing any signal degeneration. This also provides temperature stabilization. The capacitors Cc and Cb are employed to block d.c. and to provide an a.c. path. The radio frequency choke (R.F.C) offers very high impedance to high frequency currents which means it shorts for d.c. and opens for a.c. Hence it provides d.c. load for collector and keeps a.c. currents out of d.c. supply source Tank Circuit The frequency determining network is a parallel resonant circuit which consists of the inductors L1 and L2 along with a variable capacitor C. The junction of L1 and L2 are earthed. The coil L1 has its one end connected to base via Cc and the other to emitter via Ce. So, L2 is in the output circuit. Both the coils L1 and L2 are inductively coupled and together form an Auto-transformer. The following circuit diagram shows the arrangement of a Hartley oscillator. The tank circuit is shunt fed in this circuit. It can also be a series-fed. Operation When the collector supply is given, a transient current is produced in the oscillatory or tank circuit. The oscillatory current in the tank circuit produces a.c. voltage across L1. The auto-transformer made by the inductive coupling of L1 and L2 helps in determining the frequency and establishes the feedback. As the CE configured transistor provides 180o phase shift, another 180o phase shift is provided by the transformer, which makes 360o phase shift between the input and output voltages. This makes the feedback positive which is essential for the condition of oscillations. When the loop gain |βA| of the amplifier is greater than one, oscillations are sustained in the circuit. Frequency The equation for frequency of Hartley oscillator is given as $$f = frac{1}{2 pi sqrt{L_T C}}$$ $$L_T = L_1 + L_2 + 2M$$ Here, LT is the total cumulatively coupled inductance; L1 and L2 represent inductances of 1st and 2nd coils; and M represents mutual inductance. Mutual inductance is calculated when two windings are considered. Advantages The advantages of Hartley oscillator are Instead of using a large transformer, a single coil can be used as an auto-transformer. Frequency can be varied by employing either a variable capacitor or a variable inductor. Less number of components are sufficient. The amplitude of the output remains constant over a fixed frequency range. Disadvantages The disadvantages of Hartley oscillator are It cannot be a low frequency oscillator. Harmonic distortions are present. Applications The applications of Hartley oscillator are It is used to produce a sinewave of desired frequency. Mostly used as a local oscillator in radio receivers. It is also used as R.F. Oscillator. Learning working make money

Oscillator Circuit An Oscillator circuit is a complete set of all the parts of circuit which helps to produce the oscillations. These oscillations should sustain and should be Undamped as just discussed before. Let us try to analyze a practical Oscillator circuit to have a better understanding on how an Oscillator circuit works. Practical Oscillator Circuit A Practical Oscillator circuit consists of a tank circuit, a transistor amplifier, and a feedback circuit. The following circuit diagram shows the arrangement of a practical oscillator. Let us now discuss the parts of this practical oscillator circuit. Tank Circuit − The tank circuit consists of an inductance L connected in parallel with capacitor C. The values of these two components determine the frequency of the oscillator circuit and hence this is called as Frequency determining circuit. Transistor Amplifier − The output of the tank circuit is connected to the amplifier circuit so that the oscillations produced by the tank circuit are amplified here. Hence the output of these oscillations are increased by the amplifier. Feedback Circuit − The function of feedback circuit is to transfer a part of the output energy to LC circuit in proper phase. This feedback is positive in oscillators while negative in amplifiers. Frequency Stability of an Oscillator The frequency stability of an oscillator is a measure of its ability to maintain a constant frequency, over a long time interval. When operated over a longer period of time, the oscillator frequency may have a drift from the previously set value either by increasing or by decreasing. The change in oscillator frequency may arise due to the following factors − Operating point of the active device such as BJT or FET used should lie in the linear region of the amplifier. Its deviation will affect the oscillator frequency. The temperature dependency of the performance of circuit components affect the oscillator frequency. The changes in d.c. supply voltage applied to the active device, shift the oscillator frequency. This can be avoided if a regulated power supply is used. A change in output load may cause a change in the Q-factor of the tank circuit, thereby causing a change in oscillator output frequency. The presence of inter element capacitances and stray capacitances affect the oscillator output frequency and thus frequency stability. The Barkhausen Criterion With the knowledge we have till now, we understood that a practical oscillator circuit consists of a tank circuit, a transistor amplifier circuit and a feedback circuit. so, let us now try to brush up the concept of feedback amplifiers, to derive the gain of the feedback amplifiers. Principle of Feedback Amplifier A feedback amplifier generally consists of two parts. They are the amplifier and the feedback circuit. The feedback circuit usually consists of resistors. The concept of feedback amplifier can be understood from the following figure below. From the above figure, the gain of the amplifier is represented as A. The gain of the amplifier is the ratio of output voltage Vo to the input voltage Vi. The feedback network extracts a voltage Vf = β Vo from the output Vo of the amplifier. This voltage is added for positive feedback and subtracted for negative feedback, from the signal voltage Vs. So, for a positive feedback, Vi = Vs + Vf = Vs + β Vo The quantity β = Vf/Vo is called as feedback ratio or feedback fraction. The output Vo must be equal to the input voltage (Vs + βVo) multiplied by the gain A of the amplifier. Hence, $$(V_s + beta V_o)A = V_o$$ Or $$AV_s + Abeta V_o = V_o$$ Or $$AV_s = V_o(1 – Abeta)$$ Therefore $$frac{V_o}{V_s} = frac{A}{1 – Abeta}$$ Let Af be the overall gain (gain with the feedback) of the amplifier. This is defined as the ratio of output voltage Vo to the applied signal voltage Vs, i.e., $$A_f = frac{Output : Voltage}{Input : Signal : Voltage} = frac{V_o}{V_s}$$ Rrom the above two equations, we can understand that, the equation of gain of the feedback amplifier with positive feedback is given by $$A_f = frac{A}{1 – Abeta}$$ Where Aβ is the feedback factor or the loop gain. If Aβ = 1, Af = ∞. Thus the gain becomes infinity, i.e., there is output without any input. In another words, the amplifier works as an Oscillator. The condition Aβ = 1 is called as Barkhausen Criterion of oscillations. This is a very important factor to be always kept in mind, in the concept of Oscillators. Learning working make money