2 2.1
The TTL Inverter
Circuit Structure
The circuit diagram of the Transistor Transistor Logic inverter is shown in Fig. 2.1. This circuit overcomes the limitations of the single transistor inverter circuit. Some of the notable features of the circuit structure of the TTL Logic family are as follows: (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
Fig. 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 shown in Fig. 2.3 and referred to as a totem-pole output , can actively source or sink current to or from capacitive loads and allows the output voltage to be defined independently of the load connected to the gate. Resistor, R 3 , serves to limit current. Under steady-state conditions, only one transistor is ON at a time.
T4
OFF
T3
RL
T4
T3
ON
OFF
ON
RL
Fig. 2.3 Output Current Driving Transistors
1
R3
130Ω
R1
1.6k Ω
RB
4k Ω
T4
Input
T2
T1
Output T3
Vi
VO
A
T1 SAT T2 OFF T3 OFF T4 ON
B
4
3
VOH MIN
VO
R2
1k Ω
C
(2.86V)
T1 SAT T2 ON T3 OFF T4 ON T1 SAT T2 ON T3 ON T4 ON→OFF
2
1
T1 SAT→REV ON T2 SAT T3 SAT T4 OFF
D
VOL MAX
E
(0.2V)
0
0.5V
1
2
VIL MAX
VIH MIN
(1.2V)
(1.4V)
3
4
Vi
5
Fig. 2.1 Circuit Diagram & Transfer Characteristic of a 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 Fig. 2.4.
T4
D
T3
Fig. 2.4
VO
Use of Diode in Totem-Pole Output
(iii) 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 Fig. 2.5.
RC Vi
=
LO T2 OFF VO 1
=
HI VO 2
=
LO
Vi
=
HI T2 ON
=
LO VO 2
=
HI
VO1
T2
VO 1
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, T 1, may conduct in either forward or reverse mode. Drawing up a table of conduction states accordingly with reference to Fig. 2.1 gives: INPUT
T1
T2
T3
T4
D
OUPUT
LO
ONFOR
OFF
OFF
ONCUT-IN
ONCUT-IN
HI
HI
ONREV
ON
ON
OFF
OFF
LO
LO in action
2.3
HI out
and
HI in
-
LO out
⇒
This is logic inverter
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 Fig. 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 the point of cut-in.
VO
=
VCC
VO
=
5 − 0.6 − 0.4
−
VBE 4 CUT IN
Point A : Vi
−
=
4
−
=
VD CUT IN −
4V
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 reach the point of conduction. Then:
Vi
=
VBE 2 CUT IN −
Point B : Vi
−
=
VCE 1 SAT
=
0.5V
0.6 − 0.1
VO
=
=
0.5V
4V
Point C As the input voltage is further increased, T 2 becomes more conducting, turning fully ON. Base current to T 2 is supplied by the forward biased base-collector junction of T1 which is still in saturation (i.e both junctions of T 1 are forward biased). Eventually, T3 reaches the point of conduction. This happens when:
Vi
=
Vi
=
VBE 2 ON 0.7
+
VBE 3 CUT
+
0.6
−
− − IN
0.1
=
VCE 1 SAT
1.2V
Note that with transistor T3 at cut-in, VBE 3 = 0.6V which means that the current through R 2 is 0.6V/1kΩ = 0.6mA. With operation in the linear active region, the collector current in T2 is αF IE2 ≈ 0.97 0.58mA. The voltage drop across R 1 is then VR1 = 0.58mA 0.94V. Under this condition the voltage drop across T2 is:
VCE 2
=
VCC
VCE 2
=
5 − 0.94 − 0.6 = 3.46V
−
VR 1
−
× 0.6
× 1.6
=
kΩ =
VR 2
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 VO
= =
VCC
−
VR 1
−
VBE 4 ON
5 − 0.94 − 0.7 − 0.5
Point C :
Vi
=
5
1.2V
VO
VD ON
−
=
=
2.86V
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. As T2 becomes more conducting, its collector current increases. This in turn increases the voltage drop across R 1 which in turn means that the voltage across T 2, i.e. VCE2, decreases. This falls below the requirement for conduction in T 4 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
Vi
=
0.8
VO
+
VBE 3 ON
−
VCE 1SAT
0.7 − 0.1
=
1.4V
+
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 Fig. 2.1 to identify the critical points, we have:
ViL MAX
=
1.2V
VOL MAX
=
0.2V
NML
=
1.0V
ViHMIN
=
1.4V
VOHMIN
=
2.8V
NMH
=
1.4V
The manufacturer’s specification guarantees:
ViL MAX
=
0.8V
VOL MAX
=
0.4V
NML
=
0.4V
ViHMIN
=
2.0V
VOHMIN
=
2.4V
NMH
=
0.4V
6
