Definition of Engineering Terms and Phrases
"A" "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M",
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Blocking Oscillator

An Oscillator is a circuit used to produce an alternating voltage or current. When produced by discrete components, an oscillator requires some form of amplification and feedback from the output to the input. With the two circuits shown on this page a transistor is used an the amplifier and a transformer is used to provide feedback.

In this case a Blocking Oscillator is being covered, which is a form of Relaxation Oscillator. This style of oscillator uses either a capacitor or inductor to produce regenerative feedback to generate a non-sinusoidal output. In this specific case an inductor [a transformer] is being used as the reactive component. Both circuits use a transistor as the active component. Basically the circuit is a pulse oscillator that forces the transistor amplifier into cut-off for most of the cycle.

Transformer Coupled Blocking Oscillator

A Blocking Oscillator is a type of waveform generator used to produce a narrow pulse, or trigger. A type of oscillator that blocks the output after completion of a cycle for some predetermined amount of time. In this case a circuit that produces high Duty Cycle pulse train, oscillating between Vcc and ground. The basic collector output is shown in the right side-bar.

Transistor Blocking Oscillator
Blocking Oscillator

Circuit Description

This circuits uses an NPN transistor, and almost any NPN BJT should work as long as it's rated for the frequency of operation. Resistor R1 provides forward bias for the transistor Base circuit. The frequency of oscillation is determined by components C1 and T1, although the values are not provided. Note the polarity change between L1 and L2 windings of transformer T1. Collector current flowing through the primary [L1] of the transformer is induced into the secondary [L2] 180 degrees out of phase. This feedback is regenerative and pushes the transistor into saturation.

The DC bias provided by R1 starts transistor Q1 conducting as soon as power is applied. Current begins flowing through Q1 and the primary of T1, which induces a voltage in the secondary winding L2. The phasing dots on the transformer indicate a 180-degree phase shift between the primary and secondary windings. The negative going voltage on the primary translates a positive voltage to the Base of the transistor [through C1], forcing the transistor to increase current flow [t0]. As the collector current flow increases, the voltage coupled through the transformer saturates the Base.

As the collector turns full on, the transistor output drops to zero [t1]. The current has been increasing through the transformer. When the collector current reaches maximum and the coil becomes saturated, magnetic flux between the windings, L1 to L2, stops changing. The capacitor [C1] now discharges through the Base resistor and cuts off the bias voltage to the Base circuit, and than shutting off the Collector current.

Circuit Time Constant

The Time Constant for any circuit is determined by the components and their values. The pulse width of the output waveform is determined by the length of time it takes the transformer to saturate [TC= L/R] after the transistor has turned on and begins conducting. The greater the inductance of the transformer the longer the output pulse width [t0 to t1].

Once saturation is reached and the capacitor is allowed to discharge, it most do so through the resistor. So the discharge time [TC= RC] determines how long the transistor will be turned off [holding the output at Vcc]. Once the capacitor has discharged, the Base is again forward biased and the transistor is turned on and the cycle repeats.

Tertiary Coupled Blocking Oscillator

This is basically the same circuit as described previously. However this circuit uses a transformer with a Tertiary Winding, or additional secondary winding. In addition the output is taken across the transformer and not the Collector of the transistor.

Transistor Blocking Oscillator
Blocking Oscillator

In this case the circuit uses an PNP transistor. Notice that the polarity of the transformer windings have changed to compensate for the reverse polarity of the transistor. A Tertiary winding is a third winding on a transformer, having the same turns ratio as the second winding. The third winding [secondary winding] L3 is being used to produce the output.

The circuit also uses an external input trigger.

These blocking oscillators describe here require a transformer to operate. Other oscillator styles which use a capacitor would be much smaller, although having the same complexity. Refer to this link for another example of a Relaxation Oscillator, which uses a UJT and capacitor to produce a sawtooth waveform.

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