5.1 Introduction – The transistor at Radio Frequency
203
Figure 5.1-5 Section of the BJT electrical model which affects the input impedance of the device
The primary contributors to the input impedance of a BJT are and , neither of which
the designer has any control over. The resistance
on the other hand is a very small resistance
and hence has little influence.
and can vary in size depending on circuit layout and the
designer may try and minimise them. However they may still have a significant effect when
operating in high frequency bands.
( )
5.1.3 Output Impedance
Let us now look into the transistor terminals from the output side and see how we can
simplify the model shown in Figure 5.1-3. Firstly, since
is very large (>100K) compared to other
elements, it may be ignored. The same reasoning applies to
. The simplified circuit is shown in
Figure 5.1-6 where we have inserted an explicit source resistance
and we will assume that an
input impedance
has been connected between the base and emitter terminals.
Figure 5.1-6 Section of the BJT electrical model which affects the output impedance of the device
The output impedance of a transistor typically decreases with frequency. One would assume
that
and are the determining factors in any output impedance calculation and that they alone
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