Analog Electronics 3-58 Multivibrators and Blocking Oscillators
Vee = Rey (ler +h) + (Ry + Ry) (10)
Using equation (8) in equation (10),
Voce -TifR, # Roy + Ry]
a Rg = ecm tl +Rei + Rel w(t)
cn
= Rt Rey) = Rex) | (Rey) ++ (12)
‘This completes the design.
3.10 Introduction to Blocking Oscillators.
A special type of wave generator which is used to produce a single narrow pulse or
train of pulses is called a blocking oscillator.
The two important elements of a blocking oscillator are, 1. An active device like
transistor 2. A pulse transformer.
A pulse transformer is used to couple output of the transistor back to the input. The
nature of such feedback through pulse transformer is controlled by relative winding
polarities of a pulse transformer.
Key Point ; The properly controlled winding polarities produce a regenerative feedback
The circuit with such a regenerative feedback produces a single pulse or pulse train
and calied a biucking oscillator. If it is used to produce a single pulse, then it is called
monostable operation of a blocking oscillator and if used to produce pulse train then it is
called astable operation of a blocking oscillator.
Such a transformer coupled configuration of a blocking oscillator is important in many
practical applications. These are concerned with the timing of some other circuits. These
circuits are used in frequency dividers, counter circuits and for switching the other circuits
on and off at the specific times.
Before discussing the blocking oscillator circuits let us study briefly the characteristics
of a pulse transformer.
3.11 Pulse Transformer
‘A pulse transformer is basically a transformer which couples a source of pulses of
electrical energy to the load; keeping the shape and other properties of pulses unchanged.
Key Point : The voltage level of the pulse can be raised or lowered by designing the proper
turns ratio for the pulse transformer.Analog Electronics 3-59 Multivibrators and Blocking Oscillators
The characteristics of pulse transformers are,
1, Generally iron cored and small in size.
2. The leakage inductance is minimum.
3. The interwinding capacitance is low.
4, The cores have high permeability.
5. They have high magnetizing inductance
The pulse transformers are designed to handle fast waveforms like train of pulses and
hence iron cored pulse transformers can be approximated to be ideal transformers, from
analysis point of view.
‘The ideal pulse transformer model is shown in the Fig. 3.44.
Fig. 3.44 Ideal pulse transformer model
‘As transformer is assumed to be an ideal, the primary, secondary and source
resistances are neglected. All capacitances are also neglected. The core losses and
nonlinearities in the magnetic circuit are also neglected.
Let = Primary inductance
= Secondary inductance
= Mutual inductance
= Source and V, = Output response
= Load resistance
= Primary tums
= Secondary tums
= Primary current
= Secondary current
The coefficient of coupling between primary and secondary is K. Its relation with
transformer inductances is given by,
evr ZF PZ BELT
ve fl)Analog Electronics 3-60 Multivibrators and Blocking Oscillators.
Key Point : For an idea! transformer K = 1.
Let n= Ns 2 Transformation ratio
°
Then for an ideal transformer, various ratios can be abtained as,
++ (2)
Key Point : The voltage and current ratios are inversely proportional to each other.
3.11.1 Practical Equivalent Circuit
Adding the effects of various resistances and capacitances to an ideal transformer, a
practical equivalent circuit of a pulse transformer can be obtained.
In the equivalent circuit, transformer windings are removed and various values and
parameters on secondary side are reflected to the primary side.
So reflected output voltage on primary side is V,/n where n is the transformation
ratio, as given by equation (2). If Ry is the load resistance and R} is secondary winding
resistance then the total resistance R,, reflected to primary side can be written as,
The total effective shunt capacitance reflected on primary side is denoted as C. This
capacitor C includes the transformer capacitance along with the reflected external shunt
loading capacitance, which is dominant on the output side. The total leakage inductance is
denoted as series inductance 6 while the magnetizing inductance is denoted as shunt
inductance L.
The resistance R, is the combined effect of primary winding resistance and the source
impedance which is assumed to be resistive. Such an equivalent circuit of a pulse
transformer, including resistances, inductances and total shunt capacitance is shown in the
Fig. 3.45. All values are reflected on the primary side.
Fig. 3.45 Equivalent circuit of pulse transformer