Oscillators. Hartley, Colpitts, UJT relaxation. ECE/MEA Engg College S.R.K. 9/13/2007 Authored by: Ramesh.K

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1 Oscillators Hartley, Colpitts, UJT relaxation. S.R.K 9//007 Authored by: Ramesh.K This documents contains a brief note about the principle of sinusoidal oscillator and some general oscillator circuits

2 Contents. OSCIATORS.... A GENERA FORM OF C OSCIATORS.... COPITTS OSCIATOR HARTEY OSCIATOR UJT -BASICS UJT REAATION OSCIATOR... 9 REFERENCES... 0 Oscillators 9//007

3 . OSCIATORS An Electronic device, that generate oscillations (Signals), is called an oscillator. Simply says an oscillator receives DC energy and converts it into AC energy of desired frequency. The frequency of oscillations depends up on the constants of the device. Oscillators are extensively used in electronic equipments. Oscillators can produce sinusoidal or non sinusoidal signals. Sinusoidal Oscillators. An electronic device that generates, sinusoidal oscillations of desired frequency is known as a sinusoidal oscillator. Basic Principles Of Sinusoidal Oscillator. The basic structure of a sinusoidal oscillator consists of an amplifier and a frequency selective network connected in a positive feedback loop as shown in fig. i Amplifier (gain A) o =A i o Frequency selective network A feedback amplifier is one that produce a feed back voltage V f which is in phase with the input signal. A phase shift of 80 o is produced by the amplifier and a further phase shift of 80 o is introduced by the feedback network. Thus the signal is shifted by 60 o and fed to the input. That is feedback voltage is in phase with the input signal. But, oscillator is a circuit which produces oscillations without any external signal source.a signal V in is first supplied to the network and removed.then a feedback signal V f is still applied to the input signal. The oscillator will respond to this signal and V f will be amplified and send to the o/p. the feed back n/w will send a portion of the o/p again back to the i/p. Hence the amplifier receives another i/p cycle and another o/p cycle is produced. This process continues and amplifier will produce oscillations without any ext input.that is oscillations. Where A is the loop gain., at this condition the circuit will produce In the above circuit i = - f + s, when s =0, i =- f =-A I or A. Barkhausen Criterion: The frequency for which a sinusoidal oscillator wil operate is the frequency for which the total phase shift introduced,- as a signal proceeds from the input terminals through the amplifier and feedback network, and back to the input,- is precisely zero ( or of course an integral multiple of π). That is the frequency of a sinusoidal oscillator is determined by the condition that the loop gain phase shift is zero. Another condition is that the magnitude of the feed back signals (o/p of the mixing circuit i.e. f =- f. Oscillations will not be sustained if, at the oscillator frequency, the magnitude of the product of the transfer gain of the amplifier A and the feedback factor of the feedback network is less than unity. Oscillators 9//007

4 The condition of unity loop gain, -A =, is called Barkhausen criterion. This condition implies that, of course, both that, and phase of A is zero. i.e. f, for,. means that, there exists o/p signals even in the absence of an externally applied signal voltage.. A GENERA FORM OF C OSCIATORS Many resonant-circuit oscillators can be expressed by the general structure as shown below (fig ).The active element may be an operational amplifier, a BJT amplifier or an FET. In the analysis that follows we assume an active device with infinite input resistance.fig shows a linear equivalent circuit of fig, using an amplifier with negative gain Av and output resistance Ro. Clearly the topology is that voltage series feedback. R o A v V Fig oop Gain: The value of A will be obtained by considering the circuit of Fig to be a feedback amplifier with o/p is taken from the terminals and and with i/p terminals and. The gain without feedback is given by, - v R o () Oscillators 9//007 The Feedback factor is given by, - The load impedance consists of a series combination of and parallel to. i.e. ( + )// ( ) () ()

5 oopgain A - A v R o (4) A - Av( ) ( ) - Av {{ ( ) ( )} Ro ( ) Ro ( ) Reactive elements: et =j, =j, =j 4, be the three impedances (either capacitive or inductive). For capacitor,. - (j ( - j Av ) jr j o( ) (- ( Av ) jr o( ) (5) For loop gain to be real (zero phase shift). The imaginary term in the above equation should be equal to zero, i.e A v - A v A v oopgain A ( ) ( ) (6) (7) is positive and at least unity in magnitude, then and must have the same sign (A v is positive).in other words, they must be the same kind of reactance, either both inductive or capacitive. Then from the equation =-( + ) must be inductive if and are capacitive, or vice versa. For Colpitts oscillator and are capacitors and is an inductor. For Hartley oscillator and are inductors and is a capacitor.. COPITTS OSCIATOR The Colpitts oscillator is shown below. In this two capacitors are placed across a common inductor as shown below so that C, C and forms the tank (tuned) circuit. Working: When the power supply is turned on, C and C get charged.these capacitors then discharge through the coil, setting up oscillations whose frequency depends on the values of, C,and C.The oscillations across C are applied to the Base-Emitter junction and appears in the amplified form in the collector circuit. The amount of feedback depends on the values of C and C. Smaller the C the greater will be the feedback (from Eqn (5)). The capacitors C and C act as a simple voltage divider. Therefore the points and are 80 o out of phase. A further 80 o phase shift is introduced by the amplifier. Hence a proper positive fed back is obtained (for sustained un-damped oscillation). Oscillators 9//007 4

6 oop Gain: The above circuit can be redrawn by its equivalent circuit by replacing and by its equivalent capacitive impedances -j (=-j/ωc ) and j (=-j/ωc respectively and by inductive impedance j (=jω) and proceeding the similar steps as given under the section I.e. and from eqn (6 ) + = - Or fo ( C ) Where = C and f o is the frequency of oscillation. C C Fig 4. HARTEY OSCIATOR It is also an C tuned oscillator and has the following advantages Oscillators 9//007 Adaptability to wide range of frequencies. Easy tuning. Working: A Hartley oscillator using BJT is shown below. When the power supply is turned on due to some transient disturbances in the circuit the collector current starts increasing and charges the capacitor C. C then discharges through and setting up oscillations. These oscillations across are applied to the base-emitter junction and appear in the amplified form at the collector. The coil couples collector circuit energy back by means of mutual induction b/w and. In this way, energy is continuously supplied back to the tank ( - -C ) ckt to overcome the losses in it. Consequently continues un-damped oscillations will obtain. The loop gain and the frequency of oscillations can be found by preceding the steps as explained under the section. 5

7 The above circuit can be replaced by an equivalent circuit as shown in fig 4 with The loop gain is given by ( Where f o is the frequency of oscillation of the ckt. f (( o ) j C ) C Oscillators 9//007 6

8 5. UJT -BASICS A Uni junction transistor (UJT) is a three terminal semiconductor switching device. It has a unique characteristic that when it is triggered, the emitter current increases regeneratively until it is limited by emitter power supply. Construction: The fig below shows the basic structure of a UJT. Symbol It consists of a n-type silicon bar with electrical connection on each end, named by base leads B and B. There is a p-n junction near to B (than B). The lead to this connection is called Emitter ) Since the device has only one junction and three leads,it is commonly called a unijunction transistor ) It is also called double base diode. ) The emitter is a heavily doped p-type and the n region is Operation: The main operational difference b/w the FET and UJT is, the former is operated normally with the gate junction reverse-biased, whereas the useful behavior of the later occurs when the emitter is forward-biased. When a voltage V BB is applied,with emitter open as shown in fig in such a way that the B is always positive w.r.t B, a voltage gradient is establishing along the n-type bar. Since emitter is away from B and near to B, more than half of V BB appears b/w emitter and B. The voltage (The voltage drop across E and B ) b/w E and B establishes a rev bias on the on the p-n junction and emitter current is cut-off. Of course a small leakage current will flows from B to E due to minority carriers. If a positive voltage is applied at the emitter, the p-n junction will remain rev biased so long as the i/p voltage is less than V. If the i/p exceeds V the p-n junction becomes forward biased. Under this condition, the holes from E are repelled towards base B by the negative potential at B.This increases the accumulation of holes in the E to B region. Fig. V E =0V n Basic structure Oscillators 9//007 Hence the internal resistance b/w the E and B decreases, resulting in an increased emitter current I E. As more holes are injected, a condition: of saturation will eventually be reached. At this point, the emitter current is limited by the emitter power supply only. The device is now in the ON state. If a negative pulse is applied to the emitter, the p-n junction become rev biased and the emitter current is cut-off. The device now said to be in the OFF state Fig 7

9 Equivalent Circuit: IB ) R B depends upon the bias voltage and it is variable ) R B is the resistance offered by the n type material b/w emitter and base B ) When V E =0V and V BB =0V; R BB =R B +R B (4k-0k) 4) When a voltage is applied, The voltage across R B, VE IE V D RB nvbb RB VBB But Fig 4 Equivalent circuit. Where is called the intrinsic stand off ratio i.e. This voltage V rev biases the junctn, therefore emitter current is zero. If now a progressively rising voltage is applied at the emitter, the diode will become forward biased when i/p voltage exceeds nvbb by V D (drop across the junctn diode). i.e. Where Vp is the peak- point voltage. When the diode starts conducting, due to the accumulation of holes, the resistance R B decreases (indicated by variable resistance). When the i/p positive voltage is less than the peak-point voltage Vp, the p-n junction remain reverse biased and the emitter current is practically zero. However, when the i/p voltage exceeds Vp, RB falls from a several thousands of ohms to a small value The diode is now forward biased and the emitter current quickly reaches to a saturation value limited by R B (~0ohms) and fwd resistance of p-n junction (~00 ohms). Characteristics (brief) ) Peak point voltage: The voltage above which the p-n junction becomes properly fwd biased. ) Negative resistance region: In this region the emitter current gradually increases with a corresponding decrease in emitter voltage ) Valley point: The point from which the emitter voltage increases and emitter current goes to saturation 4) Cut-off region: In this region the emitter p-n junction is reverse biased and there will be only a reverse current (There will be a negative potential on the emitter, w. r. t the base, which reverse biases the emitter -base (B) junction). Cut-off region VE Vp Peak point Ip Negative resistance region Saturation region Valley point IE Oscillators 9//007 8

10 6. UJT REAATION OSCIATOR The fig below shows a UJT relaxation oscillator, where the discharging of a capacitor through UJT can develop a saw-tooth wave form o/p. when the power supply is turned on, the capacitor C Charges through R. During the charging period, the voltage across the capacitor rise in an exponential manner until it reaches the peak point voltage. At this time, the UJT switches to its low resistance conducting mode and capacitor is discharged b/w Emitter E and Base B. As the capacitor voltage flies back to zero, the emitter ceases to conduct and the UJT is switched-off. The next cycle then begins, allowing the capacitor to charge again. Due to this alternate charging and discharging of the capacitor and the action of UJT (ON and OFF) continues and thus oscillations are produced. The frequency of the o/p saw-tooth wave can be varied by changing the value of R, since this controls the time constant, τ =CR of the capacitor charging circuit. Assume that the capacitor is initially uncharged, the voltage V c across the capacitor prior to breakdown is, V c =V BB (-e -t/ CR ) The discharge of the capacitor occurs when V c =η V BB i.e., η V BB = V c =V BB (-e -t/ CR ) Or, η = (-e -t/ CR ) Or, t = CR log e (/-η) sec. i.e. the frequency of oscillation, f = /t = / (CR log e (/-η)) Hz Vc t Oscillators 9//007 9

11 A practical Circuit with o/p wave forms References. Micro Electronic Circuits, Sedra/Smith, 4th edition. Integrated Electronics: Analog and Digital Circuits And Systems, Millman & Halkias. Principles of Electronics, V.K Mehta BY Ramesh.K Electronics and Communication MEA Engg College Oscillators 9//007 0

LECTURE NOTES ELECTRONIC CIRCUITS II SYLLABUS

LECTURE NOTES ELECTRONIC CIRCUITS II SYLLABUS FATIMA MICHAEL COLLEGE OF ENGINEERING & TECHNOLOGY Madurai Sivagangai Main Road Madurai - 625 020. [An ISO 9001:2008 Certified Institution] LECTURE NOTES EC6401 ELECTRONIC CIRCUITS - II SEMESTER: IV /