THE PRINCIPLES OF OSCILLATION BASIC INFORMATION AND TUTORIALS

A small signal voltage amplifier is shown in Fig. 1.3. In Fig. 1.3(a) the operational amplifier has no external components connected to it and the signal is fed in as shown.

The operational amplifier has an extremely high gain under these circumstances and this leads to saturation within the amplifier. As saturation implies working in the non-linear section of the characteristics, harmonics are produced and a ringing pattern may appear inside the chip.

As a result of this, a square wave output is produced for a sinusoidal input. The amplifier has ceased to amplify and we say it has become unstable. There are many reasons why an amplifier may become unstable, such as temperature changes or power supply variations, but in this case the problem is the

very high gain of the operational amplifier.

Figure 1.3(b) shows how this may be overcome by introducing a feedback network between the output and the input. When feedback is applied to an amplifier the overall gain can be reduced and controlled so that the operational amplifier can function as a linear amplifier.

Note also that the signal fedback has a phase angle, due to the inverting input, which is in opposition to the input signal (Vi).

Negative feedback can therefore be defined as the process whereby a part of the output voltage of an amplifier is fed to the input with a phase angle that opposes the input signal. Negative feedback is used in amplifier circuits in order to give stability and reduced gain.

Bandwidth is generally increased, noise reduced and input and output resistances altered. These are all desirable parameters for an amplifier, but if the feedback is overdone then the amplifier becomes unstable and will produce a ringing effect.

In order to understand stability, instability and its causes must be considered. From the above discussion, as long as the feedback is negative the amplifier is stable, but when the signal feedback is in phase with the input signal then positive feedback exists.

Hence positive feedback occurs when the total phase shift through the operational amplifier (opamp) and the feedback network is 360° (0°). The feedback signal is now in phase with the input signal (Vi) and oscillations take place.

PARTS OF OSCILLOSCOPE DEFINITION AND BASICS TUTORIAL

The most important part of an oscilloscope is a large, cone-shaped glass tube with a vacuum inside. It operates the way the old "vacuum tube" radios worked, before the invention of transistors. That is, there is a tungsten wire "filament" at the small end, which is heated by a low voltage but high current source, which could be a battery.

Electrons tend to fly off this hot wire, almost like vapor from a boiling liquid. The battery and filament assembly is the left-hand part of Fig. 8.

Since the electrons are negatively charged, they get strongly attracted to a wire hoop which has a high positive charge on it, from another power source, shown here as thousand-volt battery. (This would work, but in a regular oscilloscope, 120 volt ac from a wall socket is changed into thousand-volt dc by methods that will be explained later in the course.)

Some of the electrons get collected by the "anode" hoop and returned to the battery, but many are going so fast that their momentum tends to keep them going to the right, rather than changing direction quickly enough to get caught by the hoop, so they fly through the large hole in the middle. Since the beam of electrons is coming from the negatively charged filament (the "cathode" of the 1,000V circuit), this beam is called the "cathode ray." The whole tube is sometimes called a "cathode ray tube," or "CRT."

It can be an important part of a TV receiver, in which case it is called the "picture tube." The electrons continue until they eventually hit the "screen," which has a thin coating of "phosphor" material, such as zinc silicate. This contains impurities that convert the kinetic energy of the electrons into a visible spot of light, and that can be seen outside, through the glass tube.

The inside of the tube also has a thin conductive coating on it, to carry the electrons back to the power supply. A grid of thin wires is attached to a "terminal" outside the tube (the small white circle), and if a strong negative voltage is applied to the grid, it can stop or at least decrease the intensity of the visible light spot.

This is not always used in an oscilloscope, but it is an important part of a TV picture tube. Four metal plates are on the top, bottom, and sides of the tube, and they are all connected to outside terminals (only two of which are shown in this diagram, for simplicity).

If the top one is slightly charged + and the bottom one –, via the "vertical" terminals, the electron beam (cathode ray) is attracted upward, as shown in the diagram, and the spot of light thus appears up high on the screen. Similarly, if the "horizontal" plates are charged by some outside voltage, the spot will move to the side (not shown here).

PRINCIPLES OF OSCILLATION BASIC AND TUTORIALS

HOW PRINCIPLES OF OSCILLATION WORKS?


A small signal voltage amplifier is shown in Fig. 1.3. In Fig. 1.3(a) the operational amplifier has no external components connected to it and the signal is fed in as shown. The operational amplifier has an extremely high gain under these circumstances and this leads to saturation within the amplifier.

As saturation implies working in the non-linear section of the characteristics, harmonics are produced and a ringing pattern may appear inside the chip. As a result of this, a square wave output is produced for a sinusoidal input. The amplifier has ceased to amplify and we say it has become unstable.

There are many reasons why an amplifier may become unstable, such as temperature changes or power supply variations, but in this case the problem is the very high gain of the operational amplifier.

Figure 1.3(b) shows how this may be overcome by introducing a feedback network between the output and the input. When feedback is applied to an amplifier the overall gain can be reduced and controlled so that the operational amplifier can function as a linear amplifier.

Note also that the signal fedback has a phase angle, due to the inverting input, which is in opposition to the input signal (Vi).

Negative feedback can therefore be defined as the process whereby a part of the output voltage of an amplifier is fed to the input with a phase angle that opposes the input signal. Negative feedback is used in amplifier circuits in order to give stability and reduced gain.

Bandwidth is generally increased, noise reduced and input and output resistances altered. These are all desirable parameters for an amplifier, but if the feedback is overdone then the amplifier becomes unstable and will produce a ringing effect.

In order to understand stability, instability and its causes must be considered. From the above discussion, as long as the feedback is negative the amplifier is stable, but when the signal feedback is in phase with the input signal then positive feedback exists.

Hence positive feedback occurs when the total phase shift through the operational amplifier (opamp) and the feedback network is 360° (0°). The feedback signal is now in phase with the input signal (Vi) and oscillations take place.

COLPITTS OSCILLATORS BASIC CIRCUIT TUTORIALS

WHAT ARE COLPITTS OSCILLATORS?

Colpitts oscillators are similar to the shunt fed Hartley oscillator circuit except the Colpitts oscillator, instead of having a tapped inductor, utilizes two series capacitors in its LC circuit.

With the Colpitts oscillator the connection between these two capacitors is used as the center tap for the circuit. A Colpitts oscillator circuit is shown at Figure 2-5, and you will see some similarities with the Hartley oscillator.

Colpitts Oscillators

The simplest Colpitts oscillator to construct and get running is the “series tuned” version, more often referred to as the “Clapp Oscillator.” Because there is no load on the inductor, a high “Q” circuit results with a high L/C ratio and of course much less circulating current.

This aids drift reduction. Because larger inductances are required, stray inductances do not have as much impact as perhaps in other circuits.

The total capacitive reactance of the parallel combination of capacitors depicted as series tuning below the inductor in a series tuned Colpitts oscillator or “Clapp oscillator” should have a total reactance of around 200 ohms.

Not all capacitors may be required in your particular application. Effectively all the capacitors are in series in a Colpitts oscillator, i.e. they appear as parallel connected but their actual values are in fact in series.

Ideally, your frequency determining components L1 and the parallel capacitors should be in a grounded metal shield. The FET used in the Colpitts oscillator is the readily available 2N4416A.

Note, the metal FET case is connected to the circuit ground. The output from the Colpitts oscillator is through output capacitor 47 pF; this should be the smallest of values possible, consistent with continued reliable operation into the next buffer amplifier stage.

HARTLEY OSCILLATOR CIRCUIT TUTORIALS

HARTLEY OSCILLATOR DIAGRAM AND INFORMATION

This oscillator is very similar to the Colpitts except that it has a split inductance. It is represented in a similar way to the Colpitts, as seen in Fig. 1.14. It may be designed using a similar approach to the Colpitts but it has the disadvantages of mutual inductance between the coils, which causes unpredictable frequencies, and also the inductance is more difficult to vary. 

When two coils are placed in close proximity to one another the flux due to the magnetic field of one interacts with the other. Hence an induced voltage is applied to the second coil due to the rate of change of flux. Similarly, flux due to the magnetic field of the second coil may cut the first coil, also inducing a voltage in it. 

This is referred to as mutual induction, in contrast to self-inductance which is caused by lines of magnetic force cutting a single coil. Hence the rate of change of flux in one coil affects the other.

Splitting a single coil causes similar effects and mutual inductance exists between the two parts. As can be seen from equations (1.14), (1.15) and (1.16), the gain and frequency are dependent on the mutual inductance, and these parameters may be difficult to achieve as the tapping point has to be precise.

Two practical circuits are shown in Fig. 1.15. In both circuits the frequency is given by

where LT = L1 + L2 + 2M as both coils are virtually in series; note that M is the mutual inductance. The β factor and gain are

The remarks made earlier concerning loading and Q factors also apply here. While the Hartley and Colpitts oscillators have a similar design, the Hartley is easier to tune while the Colpitts requires two ganged capacitors. 

An advantage of using a Colpitts oscillator is the reduction in low-capacitance paths which can cause spurious oscillations at high frequencies. This is mainly due to the inter-electrode capacitance of the semiconductors. 

The Hartley oscillator, on the other hand, can produce several LC combinations due to the capacitance between the turns of the coil and thus cause spurious oscillations. It is for this reason that the Colpitts oscillator is often used as the local oscillator in receivers.

Hartley Oscillator Diagram