Astable Blocking Oscillator; With small variations of the Monostable Blocking Oscillator circuit, it can be provided to work into astable mode. The astable blocking. A blocking oscillator is a simple configuration of discrete electronic components which can .. ff and “The astable blocking oscillator” p. ff. Figure shows the diagram of an astable blocking oscillator with an. RC circuit in series with the base of the transistor. This circuit includes, in addition to the.

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An oscillator is a circuit that provides an alternating voltage or current by its ownwithout any input applied. An Oscillator needs an amplifier and also a feedback from the output. The feedback provided should be regenerative feedback which along with the portion of the output signal, contains a component in the output signal, which is in phase with the input signal.

An oscillator that uses a regenerative feedback to generate a nonsinusoidal output is called as Relaxation Oscillator. We have already seen UJT relaxation oscillator. Another type of relaxation oscillator is the Blocking oscillator.

Pulse Circuits – Blocking Oscillators

A blocking oscillator is a waveform generator that is used to produce narrow pulses or trigger pulses. While having the feedback from the output signal, it blocks the feedback, after a cycle, for certain predetermined time. This feature of blocking the output while being an oscillatorgets the name blocking oscillator to it.

In the construction of a blocking oscillator, the transistor is used as an amplifier and the transformer is used for astqble. The transformer used here is a Pulse transformer. The symbol of a pulse transformer is as shown below.

A Pulse transformer is one which couples a source of rectangular pulses of electrical energy to the load. Keeping the shape and other properties of pulses unchanged. They are wide band transformers with minimum attenuation and zero or minimum phase change. The regenerative feedback is made easy by using pulse transformer. The output can be fed back to the input in the same phase by properly choosing the winding polarities of the pulse transformer.

Blocking oscillator is such a free-running oscillator made using a capacitor and a pulse transformer along with a oacillator transistor which is cut off for most of blockihg duty cycle producing periodic pulses. Using the blocking oscillator, Astable and Monostable operations are possible. But Bistable operation is not possible.

Let us go through them. If the blocking oscillator needs a single pulse, to change its state, it is called as a Monostable blocking oscillator circuit. These Monostable blocking oscillators can be of two types. In both of these, a timing resistor R controls the gate width, which when placed in the base of transistor becomes base timing circuit and when osfillator in the emitter of transistor becomes emitter timing circuit.

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To have a clear understanding, let us discuss the working of base timing Monostable Multivibrator. A transistor, a pulse transformer for feedback and a resistor in the base of the transistor constitute the circuit of a transistor triggered Monostable blocking oscillator with base timing.

Blocking oscillator

The pulse transformer used here has a turns ratio of n: A resistance R is connected in series to the wstable of the transistor which controls the pulse duration. Initially the transistor is in OFF condition. As shown in the following figure, VBB is considered zero or too low, which is negligible. But when a negative trigger is applied at the collector, the voltage gets reduced.

Because of the winding polarities of the transformer, the collector voltage goes osci,lator, while the oscillatpr voltage rises. Then, a small base current is observed. This raises the collector current which decreases the collector voltage. This action cumulates further, which increases the collector current and decreases the collector voltage further. With the regenerative feedback action, if the loop gain increases, the transistor gets into saturation quickly.

But this is not a stable state. When the transistor gets into saturation, the collector current increases and the base current is constant.

Now, the collector current slowly starts charging the capacitor and the voltage at the transformer reduces. Due to the transformer winding polarities, the base voltage gets increased. This in turn decreases the base current.

Navy Electricity and Electronics Training Series (NEETS), Module – RF Cafe

This cumulative action, throws the transistor into cut off condition, which is the stable state of the circuit. The main disadvantage of this circuit is that the output Pulse width cannot be maintained stable. We know that the collector current is. As the h FE is temperature dependent and the pulse width varies linearly with this, the output pulse width cannot be stable.

Also h FE varies with the transistor used. Anyways, this disadvantage can be eliminated if the resistor is placed in emitter, which means the solution is the emitter timing circuit. When the above condition occurs, the transistor turns OFF in the emitter timing circuit and so a stable output is obtained.

If the blocking oscillator can change its state automatically, it is called as an Astable blocking oscillator circuit. These Astable blocking oscillators can be of two types.

In diode controlled Astable blocking oscillator, a diode placed in the blockinv changes the state of the blocking oscillator. While in the RC controlled Bpocking blocking oscillator, a timing resistor R and capacitor C form a network in the emitter section to control the pulse timings.

To have a clear understanding, let us discuss the working of Diode controlled Astable blocking oscillator. The diode controlled Astable blocking oscillator contains a pulse transformer in the collector circuit.


A capacitor is connected in between transformer secondary and the base of the transistor. The transformer primary and the diode are connected in the collector. An initial pulse is given at the collector of the transistor to initiate the process and from there no pulses ashable required and the circuit behaves as an Astable Multivibrator. The figure below shows the circuit of a diode controlled Astable blocking oscillator.

Pulse Circuits Blocking Oscillators

Initially the transistor is in OFF state. To initiate the circuit, a negative trigger pulse is applied at the collector. The diode whose anode is connected to the collector, will be in reverse biased condition and will be OFF by the application of this negative trigger pulse. This pulse is applied to the pulse transformer and due to the winding polarities as indicated in the figuresame amount of voltage gets induced without any phase inversion.

This voltage flows through the capacitor towards the base, contributing some base current. This base current, develops some base to emitter voltage, which when crosses the cut-in voltage, pushes the transistor Q 1 to ON. Now, the collector current of the transistor Q 1 raises and it gets applied to both the diode and the transformer. The voltage that gets induced into the transformer primary windings induces some voltage into the transformer secondary winding, using which the capacitor starts charging.

As the capacitor will not deliver any current while it is getting charged, the base current i B stops flowing. This turns the transistor Q 1 OFF. Hence the state is changed. Now, the diode which was ON, has some voltage across it, which gets applied to the transformer primary, which is induced into the secondary.

Now, the current flows through the capacitor which lets the capacitor discharge. Hence the base current i B flows turning the transistor ON again. The output waveforms are as shown below. As the diode helps the transistor to change its state, this circuit is diode controlled.

Also, as the trigger astabe is applied only at the time of initiation, whereas the circuit keeps on changing its state all by its own, this circuit asgable an Astable oscillator. Hence the name diode controlled Astable blocking oscillator is given. Another type of circuit uses R and C combination in the emitter portion of the transistor and it is called as RC controlled Astable blocking oscillator circuit.

Pulse Circuits – Blocking Oscillators Advertisements.