Blocking Oscillators

Inductor saturation plays a crucial role in the operation of blocking oscillators. It is the phenomenon where the inductor's core material reaches its maximum magnetization capability, causing the inductance value to drop significantly. This sudden change in inductance affects the energy storage and coupling characteristics of the transformer, thereby influencing the oscillation frequency and pulse characteristics of the blocking oscillator.

In a blocking oscillator, the inductor is typically connected in series with the collector of a transistor. As the the inductor current increases, the magnetic flux in the inductor core builds up. When the flux reaches a certain threshold, the core material saturates, causing the inductance value to plummet.

When power is applied to the circuit above, R1 provides forward bias and transistor Q1 conducts.

 Current flow through Q1 and the primary of T1 induces a voltage in L2. 

The phasing dots on the transformer indicate a 180-degree phase shift. 

As the bottom side of L1 is going negative, the bottom side of L2 is going positive. 

The positive voltage of L2 is coupled to the base of the transistor through C1, and Q1 conducts more.

 This provides more collector current and more current through L1. This action is regenerative feedback. Very rapidly, sufficient voltage is applied to saturate the base of Q1. 

Once the base becomes saturated, it loses control over collector current. The circuit now can be compared to a small resistor (Q1) in series with a relatively large inductor (L1), or a series RL circuit.

The operation of the circuit to this point has generated a very steep leading edge for the output pulse. The waveform diagram shows the idealized collector and base waveforms. 

Once the base of Q1  becomes saturated, the current increase in L1 is determined by the time constant of L1 and the total series resistance. From T0 to T1 the current increase (not shown) is approximately linear. The voltage across L1 will be a constant value as long as the current increase through L1 is linear.

At time T1, L1 saturates. At this time, there is no further change in magnetic flux and no coupling from L1 to L2. 

C1, which has charged during time TO to T1, will now discharge through R1 and cut off Q1. This causes collector current to stop, and the voltage across L1 returns to 0.

The length of time between T0 and T1 (and T2 to T3 in the next cycle) is the pulse width, which depends mainly on the characteristics of the transformer and the point at which the transformer saturates. 

A transformer is chosen that will saturate at about 10 percent of the total circuit current. This ensures that the current increase is nearly linear. 

In practical circuits it is necessary to protect the base (and collector) of the transistor from excessive voltages.  In particular a diode is often needed to protect the base emitter junction from negative voltage breakdown (-3 to -12V).