# 2 The TTL Inverter Full

Document Sample

```					                                     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.
VCC

RB                                       RB

T1

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

1
130   R3
1.6k       R1
4k         RB

T4

Input                                        T2
T1                                         Output

T3

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
2
T2 ON
T3 ON
T4 ONOFF            T1 SATREV ON
T2 SAT
1
T3 SAT
T4 OFF
VOL MAX                                   D                                              E
(0.2V)
Vi
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

2
(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.

T4

T3
VO

Figure 2.4 Use of Diode in Totem-Pole Output

(iv)
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.

RC

VO1           Vi  LO T2 OFF VO1  HI VO 2  LO
T2                        Vi  HI T2 ON VO1  LO VO 2  HI
VO2

RE

Figure 2.5 The Phase Splitting Stage

3
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

4
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
1

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

5
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
have…

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

6

```
DOCUMENT INFO
Shared By:
Categories:
Stats:
 views: 158 posted: 4/21/2011 language: English pages: 6