Question: I am plotting the voltage expression V(Q2_EMITTER) in my simulation but it does not produce the
emitter voltage for the Q2 transistor that I expect. When I plot the voltage using the node number at the emitter,
the waveform looks correct. Why aren't these two expressions matching?
Answer: There are three voltage expressions that may be available for any BJT transistor in
order to plot the internal collector, emitter, and base nodes of the device. The expressions
take the form of:
V(Part_Emitter)
V(Part_Base)
V(Part_Collector)
where Part is the PART attribute name of the transistor. For example, if the PART attribute of
the transistor has been defined as Q3, then the expressions would be:
V(Q3_Emitter)
V(Q3_Base)
V(Q3_Collector)
These expressions appear when you right click in the Y Expression field to invoke the Variables
list. They are available within the Variables / Node Voltage section.
Note that the key point to these expressions is that they plot the internal nodes of the
transistor. These expressions are only available if the corresponding lead resistance (RB,
RC, or RE) has been defined in the BJT model statement. In your case, the V(Q2_EMITTER) would
produce a value equivalent to the voltage at the emitter node in the schematic minus the voltage
drop across the RE resistor for the Q2 transistor.
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