2 The TTL Inverter Full

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					                                     2     The TTL Inverter

2.1     Circuit Structure
The circuit diagram of the Transistor Transistor Logic inverter is shown in Figure 2.1. This
circuit overcomes the limitations of the single transistor inverter circuit.
(i)    An input transistor, T1, which performs a current steering function, can be thought of as a
       back-to-back diode arrangement.

                               RB                                       RB

            Figure 2.2 Equivalent of Input Current-Steering Transistor

       The transistor can operate in either forward or reverse mode to steer current to or from T2 .
       Since it has a forward current gain, it provides a higher discharge current to discharge the
       base of T2 when turning it off.

(ii)   The output transistor pair, T3 and T4 referred to as a totem-pole output, provides the ability
       to actively source or sink current and is useful for driving capacitive loads. Resistor, R 3 ,
       serves to limit current. Under steady-state conditions, only one transistor is ON at a time.

                  T4 OFF        RL
                                                                          T4   ON

                          T3   ON                                 T3 OFF       RL

                           Figure 2.3 Output Current Driving Transistors

                                                                       130   R3
                                                      1.6k       R1
                                     4k         RB


                 Input                                        T2
                                            T1                                         Output


                         Vi                                                             VO
                                                      1k         R2

          VO                                         T1 SAT
                     A        B
          4                                          T2 OFF
                                                     T3 OFF
                                                     T4 ON
                                                                   T1 SAT
                                                                   T2 ON
          3                                                        T3 OFF
      VOH MIN                               C
       (2.86V)                                                     T4 ON

                                                              T1 SAT
                                                              T2 ON
                                                              T3 ON
                                                              T4 ONOFF            T1 SATREV ON
                                                                                   T2 SAT
                                                                                   T3 SAT
                                                                                   T4 OFF
       VOL MAX                                   D                                              E
                 0        0.5V       1                    2              3         4            5

                                  VIL MAX       VIH MIN
                                   (1.2V)        (1.4V)

Figure 2.1 Schematic Diagram and Transfer Characteristic of a Standard TTL Inverter

(iii)   The diode, D, serves to increase the effective VBE of T4 which allows T4 to be turned

        OFF before T3 turns ON fully. This prevents large surge currents from flowing when both
        transistors conduct during transitions between logic states. The disadvantage is that the
        high logic voltage is reduced by an amount of the diode drop as shown in Figure 2.4.



                          Figure 2.4 Use of Diode in Totem-Pole Output

        Finally, T2 is a “phase splitter” driving transistor to drive the output stage. It allows the
        logic condition to be phase-splitted in opposite directions so that the output transistors can
        be driven in anti-phase. This allows T3 to be ON when T4 is OFF and vice versa as
        shown in Figure 2.5.


                             VO1           Vi  LO T2 OFF VO1  HI VO 2  LO
                 T2                        Vi  HI T2 ON VO1  LO VO 2  HI


                               Figure 2.5 The Phase Splitting Stage

2.2 Logical Operation
The logical functioning of the circuit can be established by determining the state of conduction of
each transistor in turn from input to output for all possible combinations of input states.
Transistors can be taken as either ON or OFF. Note that the input transistor, T1, may conduct in
either forward or reverse mode. Drawing up a table of conduction states accordingly with
reference to Figure 2.1 gives:

               INPUT             T1        T2      T3       T4       D              OUPUT

                 LO          ONfor        OFF      OFF     ON        ON              HI

                 HI          ONrev         ON      ON      OFF      OFF              LO

LO in -    HI out and     HI in       -   LO out     This is inverter action

2.3 Transfer Characteristic
       The transfer characteristic can be deduced by applying a slowly increasing input voltage
and determining the sequence of events which takes place with regard to changes in the states of
conduction of each transistor and the critical points at which the onset of these changes occur.
Consider the circuit and transfer characteristic of Figure 2.1.

Point A
With the input LO and the base current supplied to T1, this transistor can conduct in the forward
mode. Since the only source of collector current is the leakage of T2 then T1 is driven into
saturation. This ensures that T2 is OFF which, in turn, means that T3 is OFF. While there is no
load present, there are leakage currents flowing in the output stage which allow the transistor T 4
and the diode D to be barely conducting at cut-in.

                            VO  VCC  VBE 4             CUT IN
                                                                    VD   CUT IN

                            VO  5  0.6  0.4  4V
                            Point A : Vi  0V, VO  4V

Point B
As the input voltage is slowly increased, the above condition prevails until, with T1 ON in
saturation, the voltage at the base of T2 rises to the point of conduction. Then

                Vi  VBE 2        CUTIN
                                            VCE 1   SAT
                                                                0.6  0.1  0.5V
                 Point B : Vi  0.5V VO  4V

Point C
As the input voltage is further increased, T2 becomes more conducting, turning fully ON. Base
current to T2 is supplied by the forward biased base-collector junction of T1 which is still in
saturation. Eventually, T3 reaches the point of conduction. This happens when

                         Vi  VBE 2         ON
                                                  VBE 3       CUT IN
                                                                          VCE 1   SAT

                         Vi  0.7  0.6  0.1  1.2V

Note that with transistor T3 at cut-in, VBE 3 = 0.6V which means that the current through R2 is
0.6V/1k = 0.6mA. With operation in the linear active region, the collector current in T2 is
α F I E2  0.97  0.6 = 0.58mA.

The voltage drop across R1 is then VR1 = 0.58mA  1.6 k = 0.94V.
Under this condition the voltage drop across T2 is:

                           VCE 2  VCC  VR  VR         1               2

                           VCE 2  5  0.94  0.6  3.46V
This confirms that T2 is still operating in the forward active mode.

With T3 beginning to conduct there is a conduction path for current through T4 and the diode, D,
which then turns fully ON. In this case:

                              VO  VCC  VR  VBE 4 ON  VD ON

                             VO  5  0.94  0.7  0.5  2.86V
                           Point C :             Vi  1.2V VO  2.86V

Point D
As the input voltage is further increased, T2 conducts more heavily, eventually saturating. T3 also
conducts more heavily and eventually reaches the point of saturation also. As T2 becomes more
conducting, its collector current increases. This in turn increases the voltage drop across R1 which
in turn means that the voltage across T2 i.e. VCE2 drops. This falls below the requirement for
conduction in T4 and the diode, D, so that both of these turn OFF prior to the saturation of T3.
When T3 reaches the edge of saturation:

                        Vi  VBE 2     SAT
                                               VBE 3  VCE 1
                                                      ON          SAT

                        Vi  0.8  0.7  0.1  1.4V
                        VO  VCE 3      SAT
                                               0.2V
                        Point D : Vi  1.4V,                VO  0.2V

2.4     Noise Margins
Using points C and D on the transfer characteristic in Figure 3.1 to identify the critical points, we

               ViL MAX  1.2V           VOL MAX  0.2V              NM L  1.0V
               ViH MIN  1.4V           VOH MIN  2.8V              NM H  1.4V

The manufacturer’s specification guarantees

              ViL MAX  0.8V            VOL MAX  0.4V                  NML  0.4V
              ViH MIN  2.0V           VOH MIN  2.4V               NMH  0.4V


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