Editing Resistor–transistor logic (section)

=== One-transistor RTL NOR gate ===
[[File:RTL NPN NOR Gate.svg|right|thumb|200px|Schematic of a one-transistor RTL NOR gate.]]

With two or more base resistors (R<sub>3</sub> and R<sub>4</sub>) instead of one, the inverter becomes a two-input RTL [[NOR gate]] (see the figure on the right). The logical operation [[Logical disjunction|OR]] is performed by applying consecutively the two arithmetic operations [[addition]] and [[Comparison (mathematics)|comparison]] (the input resistor network acts as a parallel ''voltage summer'' with equally weighted inputs and the following common-emitter transistor stage as a ''voltage comparator'' with a threshold about 0.7 V). The equivalent resistance of all the resistors connected to logical "1" and the equivalent resistance of all the resistors connected to logical "0" form the two legs of a composed voltage divider driving the transistor. The base resistances and the number of the inputs are chosen (limited) so that only one logical "1" is sufficient to create base-emitter voltage exceeding the threshold and, as a result, saturating the transistor. If all the input voltages are low (logical "0"), the transistor is cut-off. The [[pull-down resistor]] R<sub>1</sub> biases the transistor to the appropriate on-off threshold.  The output is inverted since the collector-emitter voltage of transistor Q<sub>1</sub> is taken as output, and is high when the inputs are low. Thus, the analog resistive network and the analog transistor stage perform the logic function NOR.<ref>{{cite book |author=IBM |id=Form 223-6889|year=1960 |title=Transistor Component Circuits |publisher=IBM |url=http://ibm-1401.info/Form223-6889-TransistorComponentCircuits.pdf |series=Customer Engineering Manual of Instruction |accessdate=2010-01-04 |quote=The logical function is performed by the input resistor network and the invert function is accomplished by the common emitter transistor configuration... }}</ref>