Online Digital Electronics Test: Combinational logic with transistors

A	B	C	QA	QB	QC	Y
L	L	L
L	L	H
L	H	L
L	H	H
H	L	L
H	L	H
H	H	L
H	H	H

Online Digital Electronics Test: Combinational logic with transistors

Digital Electronics conceives transistors as switches. Briefly, we consider only two states of transistors: fully-ON and fully-OFF. So, levels of currents and voltages are considered sufficient large or low to firmly maintain a state or another.

NPN transistors are ON when I_B>0 (V_input= V_cc) and is OFF in other cases;
PNP transistors are ON when I_B<0 (V_input= V_GND) and is OFF in other cases;
N channel MOS transistors are ON when V_G=V_cc and is OFF in other cases;
P channel MOS transistors are ON when V_G=V_GND and is OFF in other cases;

Possible output states are: L - Low, H - High, Z -High Impedance, and "—" prohibited. The last situation usually occurs when the logic state of the input produce a short-circuit at the power source terminals.

IC current sign convention: A current is considered positive if it flows into the device terminal.

Last updated: March 9, 2015