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The circuit arrangement of a phase-shift oscilla­tor using N-P-N transistor in CE configuration is shown in figure. As usual, the voltage divider R₁-R₂ provides dc emitter base bias, R_E and C_E combination provides temperature stability and prevent ac signal degeneration and collector resistor R_C controls the collector voltage. The oscillator output voltage is capacitively coupled to the load by C_c.

In case of a transistor phase shift oscillator, the output of the feedback network is loaded appreciably by the rela­tively small input resistance (h_ie) of the transistor. Hence, instead of employing voltage series feedback (as used in case of FET phase shift oscillator), voltage shunt feedback is used for a transistor phase shift oscillator, as shown in figure.In this circuit, the feedback signal is cou­pled through the feedback resistor R’ in series with the amplifier stage input resistance h^. The value of R’ should be such that when added with amplifier stage input re­sistance h_ie, it is equal to R i.e., R’ + h_ie = R.

Operation of Circuit

The circuit is set into oscillations by any random or variation caused in the base current, that may be either due to noise inherent in the transistor or minor variation in voltage of dc power supply. This variation in base current is amplified in collector circuit. The output of the amplifier is supplied to an R-C feedback network. The R-C network produces a phase shift of 180° between output and input voltages. Since CE amplifier produces a phase reversal of the input signal, total phase shift becomes 360° or 0° which is essential for regeneration or for sus­tained oscillations] The output of this network is thus in the same phase as the originally assumed input to the amplifier and is applied to the base terminal of the tran­sistor. Thus sustained variation in collector current between saturation and cut-off values are obtained. R-C phase shift network is the frequency determining network, as already explained in case of FET phase-shift oscillator.

Applications

The phase shift oscillator is well suited to the range of frequencies from several hertz to several hundred kilohertz (20Hz to 200 kHz), and so includes the audio frequency range (upto 20 kHz). For generating different audio-frequencies, variable air capacitors are employed as circuit elements in the phase-shift network. It is possible to vary the frequency in the range of about 1 : 10 because the range of capacitors can be varied in the ratio of 10 : 1 (typically from 40 p F to 450 p F). For variations of frequency over a large range, the three capacitors are usually ganged so as to vary the capacitance of the three capacitors si­multaneously. Such a variation keeps the input impedance to the phase-shift network con­stant and also keeps constant the magnitudes of β and αβ. Thus the amplitude of oscillations will remain unaffected as the frequency is adjusted. The phase-shift oscillator is operated in class A so as to keep distortion to the minimum. Frequency range from 20 Hz to 200 Hz, 200 Hz to 2 kHz, 2 kHz to 20 kHz and 20 kHz to 200 kHz can be obtained by using different set of resistors.

Phase-shift oscillators are not suitable for higher frequency operation because at higher frequency, the internal phase shift of the transistor and reduction in h_fe cause difficulties in designing the circuit. The frequency of the oscillator cannot be changed easily.

Phase Shift Oscillator

We select the so called phase shift oscillator  as a first example as it exemplifies very simply the principles set forth in the previous blog post. The circuit is drawn to show clearly the amplifier and feedback network. The circuit consists of a common source FET amplifier followed by a three section R-C phase shift network. The amplifier stage is self-biased with a capacitor bypassed source resistor R_s and a drain bias resistance R_D. The output of the last section is supplied back to the gate. If the loading of the phase-shift network on the amplifier can be assumed to be negligible, a phase shift of 180° between the amplified output voltage V_out and the input voltage V_in at the gate is produced by the amplifier itself. The three-section R-C phase shift network produces an additional phase shift, which is a function of frequency and equals 180° at some frequency of operation. At this frequency the total phase shift from the gate around the circuit and back to gate will be exactly zero. This particular frequency will be the one at which the circuit will oscillate provided that the magnitude of the amplification is sufficiently large. In a FET phase-shift oscillator voltage series feedback [that is, feedback voltage proportional to the output voltage V_out and supplied in series with the input signal at the gate is used.

The frequency of the oscillator output depends upon the values of capacitors C and re­sistors R used in the phase shift network. Using basic RC circuit analysis technique, it can be shown that the network phase shift is 180° when

X_c = √6 R    or     1 / 2∏fc = √6 R  or f = 1 / / 2∏ R c √6

The frequency can be adjusted over a wide range if variable capacitors are used.As well as phase shifting, the R-C network attenuates the amplifier output. Network analysis shows that when the necessary phase shift of 180° is obtained, this network attenuates the output voltage by a factor of 1/29. This means that the amplifier must have a voltage gain of 29 or more. When the amplifier voltage gain is 29 and feedback factor of R-C network, β= 1/29 then the loop gain is β A = 1, the amplifier phase shift of – 180° combined with the network phase shift of + 180° gives a loop phase shift of zero. Both of these conditions are necessary to satisfy the Barkhausen criteria. If the amplifier gain is much greater than 29, the oscillator output waveform is likely to be distorted. When the gain is slightly greater than 29, the output is usually a reasonably pure sinusoidal.

The advantages and disadvantages of phase shift oscillators are given below :

Advantages.

• It is cheap and simple circuit as it contains resistors and capacitors (not bulky and expensive high-value inductors).
• It provides good frequency stability.
• The phase shift oscillator circuit is much simpler than the Wien bridge oscillator circuit because it does not need negative feedback and the stabilization arrangements.
• The output is sinusoidal that is quite distortion free.
• They have a wide frequency range (from a few Hz to several hundred kHz).
• They are particularly suitable for low frequencies, say of the order of 1 Hz, as these frequencies can be easily obtained by using R and C of large values.

Disadvantages.

• The output is small. It is due to smaller feedback.
• It is difficult for the circuit to start oscillations as the feedback is usually small.
• The frequency stability is not as good as that of Wien bridge oscillator.
• It needs high voltage (12 V) battery so as to develop sufficiently large feedback volt­age.

Applications.

FET phase-shift oscillator is used for generating signals over a wide frequency range. The frequency may be varied from a few Hz to 200 Hz by employing one set of re­sistors with three capacitors ganged together to vary over a capacitance range in the 1 : 10 ratio. Similarly the frequency ranges of 200 Hz to 2 kHz, 2 kHz to 20 kHz and 20 kHz to 200 kHz can be obtained by using other sets of resistors.

An introduction to Audio Oscillators also known as RC Oscillators

So far, we have considered the oscillators which use L-C tuned circuit that causes a phase shift of 180° due to inductive or capacitive coupling in addition to a 180° phase shift produced by the transistor itself. The oscillators employing L-C elements, called the L-C oscillators, are very popular for generating high frequency oscillations but they cannot be employed for generation of low frequency oscillations as they become too bulky and expensive. R- C oscil­lators are commonly used for generating audio-frequencies as they provide good frequency stability and waveform. Also, with the advent of IC technology, R-C network is the only feasible solution, as it is very difficult to make a too high value inductance in an integrated circuit. Two com­monly used R-C oscillators are (i) R-C phase shift oscillator and (ii) Wien bridge oscillator. Another commonly used audio-oscillator is the beat frequency oscillator (BFO). So these three audio-oscillators will be discussed here.

Basic Principles of R-C Oscillators.

For producing oscillations in an oscillator circuit we need positive feedback which means that the voltage signal feedback should be in phase with the input signal. For providing a positive feedback at one particular frequency, an inverting amplifier may be used with a feedback network that causes a phase shift of 180° at the desired frequency of oscillation, as shown in fig. (a). The 180° phase shift in the feedback signal can be obtained by a suitable network consisting of three R-C sections, as shown in fig. (b).

When a phase-shift network such as that indicated in fig. is used in a phase shift oscillator, the R’s and C’s must be selected so as to produce a phase shift of 180° at the desired frequency of oscillation. The output of the voltage amplifier is fed to the input to the phase-shift network. Thus V₁ = V_out. The output resistance of the amplifier is designed to be very small in comparison to the input impedance of the phase-shift network. The output voltage of the phase-shift network, V₂ is fed into the input of the amplifier i.e., V₂ = V_in. The amplifier’s input impedance must be much larger than the output impedance of the phase-shift network.

Alternatively, a positive feedback can be obtained by using two stages of amplifiers each giving a phase shift of 180°. A part of this output is fedback to the input through a feedback network without causing any further phase shift. Wien bridge oscillator operates on this principle.