Introduction to

Basic Electronics 7

Basic Electronics

• Oscillator

- Introduction

- Resonance

- Hartley Oscillator

- Colpitts Oscillator

- RC Oscillator

Introduction to Oscillator

Oscillators are used in many electronic circuits and systemsproviding the central "clock" signal that controls the sequentialoperation of the entire system.

Oscillators can produce a wide range of different wave shapes andfrequencies that can be complicated or simple depending upon theapplication.

Oscillators are also used in many pieces of test equipment producingeither sinusoidal sine waves, square, sawtooth or triangular shapedwaveforms or just pulses of a variable or constant width.

Introduction to Oscillator Oscillators are basically an Amplifier with "Positive Feedback", (in-

phase) and one of the many problems in electronic circuit design isstopping amplifiers from oscillating while trying to get oscillators tooscillate.

Oscillators works because they overcome the losses of their feedbackresonator circuit either in the form of a Capacitor, Inductor or both byapplying DC energy at the required frequency into this resonatorcircuit.

Then an Oscillator has a gain equal too or slightly greater than 1. Inaddition to these reactive components, an amplifying device such as anOperational Amplifier or Bipolar Transistor is required.

Unlike an amplifier there is no external AC input required to cause theOscillator to work as the DC supply energy is converted by theoscillator into AC energy at the required frequency.

Introduction to OscillatorBasic Feedback Circuit

Introduction to Oscillator

Oscillators are circuits that generate a continuous voltage outputwaveform at a required frequency with the values of the inductors,capacitors or resistors forming a frequency selective LC resonant (ortank) feedback network.

The oscillators frequency is controlled using a tuned or resonantinductive/capacitive (LC) circuit with the resulting output frequencybeing known as the Oscillation Frequency and by making theoscillators feedback a reactive network the phase angle of the feedbackwill vary as a function of frequency and this is called Phase-shift.

Introduction to Oscillator

There are basically types of Oscillators.

1. Sinusoidal Oscillators - These are known as Harmonic Oscillators and are generally a "LC

Tuned-feedback" or "RC tuned-feedback" type Oscillator that generates a sinusoidal waveform which is of constant amplitude and frequency.

2. Non-Sinusoidal Oscillators - These are known as Relaxation Oscillators and generate complex

non- sinusoidal waveforms that changes very quickly from one condition of stability to another such as "Square-wave", "Triangular-wave" or "Sawtoothed-wave" type waveforms.

Resonance

When a constant voltage but of varying frequency is applied to a circuit consisting of an inductor, capacitor and resistor the reactance of both the Capacitor/Resistor and Inductor/Resistor circuits is to change both the amplitude and the phase of the output signal as compared to the input signal due to the reactance of the components used.

At high frequencies the reactance of a capacitor is very low acting as a short circuit while the reactance of the inductor is high acting as an open circuit.

At low frequencies the reverse is true, the reactance of the capacitor acts as an open circuit and the reactance of the inductor acts as a short circuit.

Resonance Between these two extremes the combination of the inductor and

capacitor produces a "Tuned" or "Resonant" circuit that has aResonant Frequency, ( ƒr ) in which the capacitive and inductivereactance's are equaland cancel out each other, leaving only theresistance of the circuit to oppose the flow of current.

This means that there is no phase shift as the current is in phasewith the voltage. Consider the circuit below.

Basic LC Oscillatory Circuit

Damped Oscillation

Damped Oscillation The losses are mainly due to the resistance of the inductors

windings called the "Real Resistance". If this real resistance is lowthe oscillations will die away slowly and if the coil resistance is highthe oscillations die away more rapidly.

The frequency of the oscillatory voltage depends upon the value ofthe inductance and capacitance in the LC circuit.

We know that for Resonance to occur both the capacitive, XC andinductive, XL reactance's must be equal, XL = XC and opposite tocancel out each other out leaving only the resistance in the circuitto oppose the flow of current.

Damped Oscillation Then the frequency at which this will happen is given as:

Damped Oscillation Then by simplifying the equation we get the final equation for

Resonant Frequency in a tuned LC circuit as:

Where:L is the Inductance in HenrysC is the Capacitance in Faradsfr is the Output Frequency in Hertz

This equation shows that if either L or C are decreased, the frequencyincreases.

This output frequency is commonly given the abbreviation of ( fr ) toidentify it as the Resonant Frequency

Oscillator Summary

The basic requirements for an Oscillatory circuit are given as follows.

1. The circuit MUST contain a reactive (frequency-dependant) component either an Inductor, (L) or a Capacitor, (C) and a DC supply voltage.

2. In a simple circuit oscillations become damped due to circuit losses.

3. Voltage amplification is required to overcome these circuit losses.

4. The overall gain of the amplifier must be at least 1, unity.

5. Oscillations can be maintained by feeding back some of the output voltage to the tuned circuit that is of the correct amplitude and in-phase.

Oscillator Summary

6. Oscillations can only occur when the feedback is "Positive" (self-regeneration).

7. The overall phase shift of the circuit must be zero or 360o so that the output signal from the feedback network will be "In-phase" with the input signal.

Hartley Oscillator

When the circuit is oscillating, the voltage atpoint X (collector), relative to point Y(emitter), is 180o out-of-phase with thevoltage at point Z (base) relative to point Y.

At the frequency of oscillation, theimpedance of the Collector load is resistiveand an increase in Base voltage causes adecrease in the Collector voltage.

Then there is a 180o phase change in thevoltage between the Base and Collector andthis along with the original 180o phase shiftin the feedback loop provides the correctphase relationship of positive feedback foroscillations to be maintained.

Hartley Oscillator The amount of feedback depends upon the

position of the "tapping point" of theinductor.

If this is moved nearer to the collector theamount of feedback is increased, but theoutput taken between the Collector and earthis reduced and vice versa.

Resistors, R1 and R2 provide the usualstabilizing DC bias for the transistor in thenormal manner while the capacitors act asDC-blocking capacitors.

Hartley Oscillator In this Hartley Oscillator circuit, the DC Collector current flows

through part of the coil and for this reason the circuit is said to be "Series-fed" with the frequency of oscillation of the Hartley Oscillator being given as.

Note: LT is the total inductance if two separate coils are used.

The frequency of oscillations can be adjusted by varying the "tuning" capacitor, C or by varying the position of the iron-dust core inside the coil (inductive tuning) giving an output over a wide range of frequencies making it very easy to tune. Also the Hartley Oscillator produces an output amplitude which is

constant over the entire frequency range.

Colpitts Oscillator

In some ways the Colpitts Oscillator is the opposite to the HartleyOscillator.

Like the Hartley oscillator, the tuned tank circuit consists of an LCresonance circuit connected between the Collector and Base of thetransistor amplifier.

The basic configuration of the Colpitts Oscillator resembles that oftheHartley Oscillator but the difference being is that the centre tappingis now made from a "Capacitive Voltage Divider" network instead of atapped autotransformer type inductor as shown below.

Colpitts Oscillator

The transistor amplifiers Emitter isconnected to the junction ofcapacitors, C1 and C2 which areconnected in series with the requiredexternal phase shift is obtainedin a similar manner to that in theHartley Oscillator.

The amount of feedback isdetermined by the ratio of C1 and C2which are generally "ganged"together to provide a constantamount of feedback.

Colpitts Oscillator

The frequency of oscillations for aColpitts Oscillator is determined bythe resonant frequency of the LCtank circuit and is given as:

RC Oscillator

A single stage amplifier will produce 180o of phase shift between itsoutput and input signals when connected in a class-A configuration.

For an oscillator to oscillate sufficient feedback of the correct phase, ie"Positive Feedback" must be provided with the amplifier being used asan inverting stage to achieve this.

In a RC Oscillator the input is shifted 180o through the amplifier stageand 180o again through a second inverting stage giving us"180o + 180o = 360o" of phase shift which is the same as 0o thereby givingus the required positive feedback.

In a Resistance-Capacitance Oscillator or simply an RC Oscillator,we make use of the fact that a phase shift occurs between the input to a RCnetwork and the output from the same network.

RC Oscillator

RC Phase-Shift Network

RC Oscillator The circuit on the left shows a single Resistor-Capacitor network and

whose output voltage "leads" the input voltage by some angle less than90o.

An ideal RC circuit would produce a phase shift of exactly 90o.

The amount of actual phase shift in the circuit depends upon the values ofthe Resistor, Capacitor and the chosen frequency of oscillations with thephase angle ( Φ ) being given as;

RC OscillatorBasic RC Oscillator Circuit

RC Oscillator The RC Oscillator which is sometimes called a Phase Shift

Oscillator, produces a sine wave output signal using regenerative feedback from the Resistor/Capacitor combination.

This regenerative feedback from the RC network is due to the ability of the capacitor to store an electric charge, (similar to the LC tank circuit).

This Resistor/Capacitor feedback network can be connected as shown above to produce a leading phase shift (Phase Advance Network) or interchanged to produce a lagging phase shift (Phase Retard Network) the outcome is still the same as the sine wave oscillations only occur at the frequency at which the overall phase-shift is 360o.

RC Oscillator By varying one or more of the resistors or capacitors in the phase-

shift network, the frequency can be varied and generally this is done using a 3-ganged variable capacitor.

If all the resistors, R and the capacitors, C are equal in value, then the frequency of oscillations produced by the oscillator is given as:

Where:f is the Output Frequency in HertzR is the Resistance in OhmsC is the Capacitance in FaradsN is the number of RC stages and in our example N = 3