10-08-2012, 10:39 AM
SWITCHING CIRCUITS WITH BJT AND JFET
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PURPOSE:
To investigate the switching properties of BJT and JFET.
THEORY :
BJT : There exists three operating regions of BJT's namely; cut-off, saturation(SAT) and active. In digital applications transistor is operated in cut-off and sat. For this reason the input voltages driving the transistor into cut-off or sat have to be determined.
In order to determine whether the BJT is in cut-off or not, at first the terminals of the transistor is assumed to be open circuit and the voltage VBE is determined. If the condition VBE < VBEon (0.65 V for Si and 0.2 V for Ge transistors, but Ge transistors are no longer in use now) is satisfied, then the BJT is in cut-off.
To investigate the saturation condition, equivalent saturation circuit of BJT is used. In this case currents IB and IC are calculated separately.
If ß IB > IC then BJT is in SAT. If the transistor is neither in cut-off nor in saturation, then it is in active region and used as an amplifier. Circuit models of operation modes are shown in figure 1.
PRELIMINARY WORK :
3.1. Assuming the left hand side of the diode (including the diode) of the
circuit is detached from the inverter to analyze as shown in figure 3.
a ) Find the time constants showing the charge and discharge rates of
the capacitor when the diode is ON or OFF ( The forward resistance of
the diode is 100 and the reverse resistance is 10 M ).
b ) Sketch Vo for Vs = 5 V and 1 KHz. square wave