BJT at high frequencies

Under reverse bias condition the capacitance at the junction is called transition or space charge capacitance.

Under forward bias condition the capacitance is called diffusion or storage capacitance.

At high frequencies, BJT cannot be analysed by h-parameters.

Giacolleto model - hybrid π equivalent ckt

Desirable fractures of hybrid π equivalent circuit (ckt)

(1)        The value of components in the equivalent ckt. are independent of frequencies.

(2)        The values of all the resistive components in the equivalent ckt. can be determined from the known or

specified values of h-parameters at low frequencies.

(3)        The results obtained by using this equivalent ckt. agrees with the experimental result.

The components of the equivalent ckt. exist in the form of π hence the name.

For small signal behaviour the transistor at its input port behaves as a resistor.

The output port is a dependent current source.

Because the base (B) is lightly doped all the depletion region lies entirely in the Base region. So, when the collector

voltage is increased the depletion region in the base increases.

rce --> This resistance is added to compensate for the change in IC due to change in VCE.


 

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