!
+D=FJAH!
Blocking Oscillators
¤ Blocking oscillators can be employed to generate a pulse of
appreciable peak power and specified shape.
¤ A pulse transformer is used for the purpose of coupling the output of
the blocking oscillator into its input circuit. Pulse transformers are
different from conventional transformers normally used in electrical
engineering.
¤ In a blocking oscillator, the resonance is very sharp and the resonant
frequency of the pulse transformer and its associated stray capacitance is extremely high. In order to flatten the resonance curve of the
blocking oscillator the quality factor Q is kept at a very small value. The
feedback in the blocking oscillator can be regenerative if the relative
winding polarities of the transformer are properly selected.
¤ A blocking oscillator can function as an astable circuit as well as a
monostable circuit depending on the external connections.
¤ The monostable blocking oscillator is generally referred as a triggered
blocking oscillator.
¤ The pulse transformer, employed in a blocking oscillator, is normally
replaced by an ideal transformer in shunt with the magnetizing
inductance L of its primary winding. The current flowing through this
inductance L when the transistor is in saturation is known as the
magnetizing current.
¤ The pulse width tp of a triggered transistor blocking oscillator with base
timing can be expressed as
nLhFE
(13.12)
R
The deficiency in this circuit is its instability of the pulse width due to
variations of hFE with fluctuations in temperature.
tp ª
2
Pulse and Digital Circuits
¤ The pulse width tp of a triggered transistor blocking oscillator with
emitter timing is quite stable, and can be expressed as
tp ª
nL n12L
R
RL
(13.27)
¤ There is a lower limit on the load resistor RL in order to keep the loop
gain of the triggered blocking oscillator with emitter timing less than
unity. This lower limit on RL is expressed by the following inequality.
RL >
FG h
Hh
FE
FE
IJ
K
+ 1 n12R
n
-n
(13.28)
¤ In a blocking oscillator, at the termination of the pulse, the base is
driven abruptly negative, and both base current and collector current
become zero. At this instant an RLC circuit is formed with the primary
inductance L and the inter-turn capacitance C along with the
resistance R of the primary coil with magnetizing current Im flowing
through it. The presence of this RLC circuit causes a sharp current
pulse and the whole phenomenon is known as the blocking oscillator
backswing.
¤ The free-running blocking oscillator is an astable blocking oscillator.
Two astable circuits are popular. The first is a diode-controlled astable
blocking oscillator. The second circuit is an astable blocking oscillator
obtained by adding an R1C1 network in the emitter circuit of a
monostable blocking oscillator.