To understand logic gates, truth tables are often used. A truth  table gives all of the possibilities in binary terms for each  gate containing a characteristic logic function. For example, a  truth table for a NOT gate is illustrated in  Figure 11-61 . Any  input (A) is NOT present at the output (B). This is simple,  but it defines this logic situation. A tiny NOT gate circuit  can be built using transistors that produce these results. In  other words, a circuit can be built such that if voltage arrives  at the gate, no voltage is output or vice-versa. When using transistors to build logic gates, the primary  concern is to operate them within the circuits so the transistors  are either OFF (not conducting) or fully ON (saturated). In  this manner, reliable logic functions can be performed. The  variable voltage and current situations present during the  active mode of the transistor are of less importance.  Figure 11-62  illustrates an electronic circuit diagram that  performs the logic NOT gate function. Any input, either a  no voltage or voltage condition, yields the opposite output.  This gate is built with bipolar junction transistors, resistors,  and a few diodes. Other designs exist that may have different  components. When examining and discussing digital electronic circuits, the  electronic circuit design of a gate is usually not presented. The  symbol for the logic gate is most often used.  [Figure 11-61] The technician can then concentrate on the configuration of  the logic gates in relation to each other. A brief discussion of  the other logic gates, their symbols, and truth tables follow. Another logic gate with only one input and one output is the  buffer. It is a gate with the same output as the input. While  this may seem redundant or useless, an amplifier may be  considered a buffer in a digital circuit because if there is  voltage present at the input, there is an output voltage. If  there is no voltage at the input, there is no output voltage.  When used as an amplifier, the buffer can change the values  of a signal. This is often done to stabilize a weak or varying  signal. All gates are amplifiers subject to output fluctuations.  The buffer steadies the output of the upstream device while  maintaining its basic characteristic. Another application of a  Most common logic gates have two inputs. Three or more  inputs are possible on some gates. When considering the  characteristics of any logic gate, an output of Logic 1 is  sought and a condition for the inputs is stated or examined.  For example,  Figure 11-64  illustrates an AND gate. For an  AND gate to have a Logic 1 output, both inputs have to be  buffer that is two NOT gates, is to use it to isolate a portion  of a circuit.  [Figure 11-63] Buffer Gate AND Gate 11-27 Figure 11-61.  A NOT logic gate symbol and a NOT gate truth table. Figure 11-64.  An AND gate symbol and its truth table.  Figure 11-63.  A buffer or amplifier symbol and the truth table of the  buffer, which is actually two consecutive NOT gates.  Figure 11-62.  An electronic circuit that reliably performs the NOT  logic function.  Input Output D$_{1}$ D$_{2}$ Q$_{1}$ Q$_{2}$ V$_{CC}$ Q$_{3}$ R$_{1}$ R$_{2}$ R$_{3}$ R$_{4}$ Q$_{4}$ Input Output Input 0 1 Output 0 1 The  Buffer  gate A B A 1 0 B 0 1 The  NOT  gate Input Output Input$_{A}$ Input$_{B}$ Output A 0 0 1 1 B 0 1 0 1 0 0 0 1 The  AND  gate