14.1 Introduction – Pulse and Digital Circuits


A transformer is a device that transfers electrical energy from one winding to the other through inductive coupling. A changing current in the primary winding creates a changing magnetic field. This changing magnetic field induces a changing voltage in the secondary winding by mutual induction. Transformers work on Faraday’s law of induction which says that the induced electromotive force or EMF in any closed circuit is equal to the time rate of change of the magnetic flux through the circuit. A pulse transformer is one which is optimized for transmitting rectangular electrical pulses, that is, pulses with negligible rise and fall times and a relatively constant amplitude. To minimize distortion of the pulse shape, a pulse transformer needs to have low values of leakage inductance and distributed capacitance, a high open-circuit inductance and a duty cycle of less than one. The energy stored in the coil during the pulse must be cleaned out before the next pulse. Pulse transformers are wide-band transformers, i.e., they pass a wide band of frequencies with minimum attenuation and no or minimum phase change. These pulse transformers can be used for many applications:

  • The desired output pulse amplitude can be obtained by appropriately choosing the turns ratio of the windings in the transformer.
  • A centre-tapped transformer can be used to derive positive and negative pulses.
  • The pulse transformer can be used as an inter-stage coupling element in applications where there is a need to block the dc component.
  • The pulse transformer winding has associated inductance. This, in conjunction with resistance R, can be used as a high-pass RL configuration that serves as a differentiator circuit to derive sharp pulses that can be used to trigger other circuits.

Blocking oscillators are pulse-shaping or pulse-generation circuits. Unlike multivibrators, these circuits use a single transistor to achieve regeneration, which is made possible by the use of a 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. A blocking oscillator is a single-transistor electronic circuit which can produce a free-running signal, requiring only a capacitor and broad-band (pulse) transformer. The circuit is called a blocking oscillator because the transistor is cut-off or blocked for most of the duty cycle, producing periodic pulses. If the device employed in the circuit is initially OFF, on the application of a trigger, the regenerative circuit quickly turns it ON. However, after a finite time interval, the device switches into the OFF state, ending the quasi-stable state. This circuit is called a monostable blocking oscillator. This monostable circuit can be modified to generate oscillations and the resultant circuit is called an astable blocking oscillator. However, bistable operation is not possible in a blocking oscillator as a single transistor is used with a transformer as a coupling element for regeneration. In this chapter, we consider astable and monostable blocking oscillators. The influence of core saturation on the pulse width of a monostable circuit is also considered.