On High frequency applications of Bipolar Junction Transistors (BJT),

it forms inner electrode capacitance across the junctions.

On high frequency such as Radio Frequency, the capacitance may act as short circuit especially on Collector-Base junctions.

circuit diagram bjt

Figure Shows inner electrode capacitance of BJT

A short circuit on Collector-Base junctions form a feed back effect. From the output (collector) to the input(base).
If the feed back is negetive, the gain of the amplifier may get reduce.
And if
the feedback is positive,and provides a phase shift angle which is the multiple of 0,360....... The amplifier starts to oscillate, which make it highly unstable.

To avoid the feedbacking of BJT on high frequencies, we have to connect an additional feedback network externally. Which will neutralise or Unilateralise the internal feedback network.

Circuit diagram of Unilateralised network

The unilateralising network consisting of a resistor Rn and a capacitor Cn, connected in series.
This network will neutralise the possibility of amplifier to oscillates.

Note:- Barkhausen Criteria Provides the necessary condion for an amplifier To oscillate.


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