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BJT suitability of transistor types simple circuit

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Determine the most suitable transistor from TABLE B for use in the circuit of FIGURE 3. You may assume in all cases that the transistor is biased with VBE = 0.6 V. Justify your choice in detail.

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Solution Summary

Choosing a transistor type based on biasing of circuit, power requirements and power handling, collector current are examined.

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Determine Quiescence or Q point voltages and currents at set points in the circuit.

The Q point biasing points are determined by
Considering DC conditions so ignore (remove for analysis) any capacitors in the circuit
Considering that no input is present and biasing set by the voltage divider at the input
The circuit is thus redrawn to represent this scenario

The Q point base voltage V_BQ is thus obtained as the voltage appearing across
the voltage divider formed by 22 kΩ and 2.7 kΩ resistors across the supply, thus


Assuming a base emitter voltage V_BE=0.6V we can then determine the Q point
of the emitter voltage, V_BQ from


We can now determine the Q point currents, initially the emitter current, i_EQ
that flows from Ohm's Law

i_EQ=V_EQ/R_E =2/100=20mA

We assume the BJT transistor draws very little base current (a good
simplification as it has a very high input resistance when connected in
common emitter configuration which is the case). Then we can determine the
Q point collector current, ...

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