07-08-2012, 11:41 AM
DIGITAL INTEGRATED CIRCUITS
Introduction
Using advanced photographic technique a manufacturer can produce miniature circuit on the surface of a small peice of semiconductor material called a chip. Such a circuit is called an integrated circuit (IC) because the components - transistors, diodes, resistors are an integral part of the chip. Depending upon no. of gates integrated on a single chip the various Ics are classified as follows
Bipolar Families
In tha bipolar category there are three basic families
• Diode Transistor Logic (DTL)
• Transistor Transistor Logic (TTL)
• Emitter Coupled Logic (ECL)
DTL uses diode and Transistor: this design was once popular now obsolete.
TTL uses transistor almost exclusively; it has become most popular family of SSI & MSI chips.
MOS Families
The following are the MOS families
• PMOS ( p- Channel MOSFETs)
• NMOS (n- Channel MOSFETs)
• CMOS (Complementry MOSFETs)
PMOS , the oldest and the slowest type is becoming obsolete.
NMOS dominates the LSI field in its use for Microprocessors and memories.
CMOS, a push pull arrangement of n- and p- channel MOSFETs is extensively used where low power consumption is needed , as in pocket calculators.
Output Transistor OFF
Fig .2 shows the circuit operation with one input 0V.
The transistor Q1 is driven ON providing the Low collector emitter saturation voltage of Q1 as input to the base of Q2.
With the base of Q2 held at 0.05 V, Q2 remains OFF with the output going upto +5 V
Open Collector Output
Instead of totem pole output some TTL devices have an open collector output. This means they use only the lower transistor of a totem pole pair. Fig .5 shows the two input NAND gate with an open collector output. Because the collector of Q3 is open , a GATE like this will not work properly until an external pull up resistor is connected
Tri-State Output
A very popular output connection that incorporates the benefits of Totem pole and open collector in a single circuit is a Tri state output connection.
Using a common pull up resistor with open collector devices is called a passive pull up because the supply voltage pulls the output voltage up to a high level when all the transistors are cutt off. There is another approach known as active pull up. It uses a modified totem pole output to speed up the charging of stray output capacitance. The effect is to dramatically lower down the time constant. The circuit with tri-state is shown in Fig 7. With control input C open or HIGH (+5 V) any input A or B LOW will result in Q1 going ON with Q2 and Q3 both cut off . Transistor Q4 is then driven ON as is Q5 the output then goes to a HIGH level (near 3.6 V). With control C input open or HIGH Q1 operates in reverse ON driving Q2 and Q3 both ON. The output is then LOW (Q4 and Q5 both held off). Thus with control input C open the circuit operates a totem pole output circuit. With control input LOW, the Diode D1 keeps b and Q5 both off. Transistor 1 is driven ON holding Q2
TTL Circuits
A series of TTL circuits are available under manufacturers numbering starting with a designation 74 This family of TTL citcuits provides various logical and functional characteristics with well specified speed , power dissipation features. Any device in 74 series works over a temperature range of 0 to 70C and a over a supply range of 4.75 to 5.25 V. This is adequate for commercial applications.
Introduction
Using advanced photographic technique a manufacturer can produce miniature circuit on the surface of a small peice of semiconductor material called a chip. Such a circuit is called an integrated circuit (IC) because the components - transistors, diodes, resistors are an integral part of the chip. Depending upon no. of gates integrated on a single chip the various Ics are classified as follows
Bipolar Families
In tha bipolar category there are three basic families
• Diode Transistor Logic (DTL)
• Transistor Transistor Logic (TTL)
• Emitter Coupled Logic (ECL)
DTL uses diode and Transistor: this design was once popular now obsolete.
TTL uses transistor almost exclusively; it has become most popular family of SSI & MSI chips.
MOS Families
The following are the MOS families
• PMOS ( p- Channel MOSFETs)
• NMOS (n- Channel MOSFETs)
• CMOS (Complementry MOSFETs)
PMOS , the oldest and the slowest type is becoming obsolete.
NMOS dominates the LSI field in its use for Microprocessors and memories.
CMOS, a push pull arrangement of n- and p- channel MOSFETs is extensively used where low power consumption is needed , as in pocket calculators.
Output Transistor OFF
Fig .2 shows the circuit operation with one input 0V.
The transistor Q1 is driven ON providing the Low collector emitter saturation voltage of Q1 as input to the base of Q2.
With the base of Q2 held at 0.05 V, Q2 remains OFF with the output going upto +5 V
Open Collector Output
Instead of totem pole output some TTL devices have an open collector output. This means they use only the lower transistor of a totem pole pair. Fig .5 shows the two input NAND gate with an open collector output. Because the collector of Q3 is open , a GATE like this will not work properly until an external pull up resistor is connected
Tri-State Output
A very popular output connection that incorporates the benefits of Totem pole and open collector in a single circuit is a Tri state output connection.
Using a common pull up resistor with open collector devices is called a passive pull up because the supply voltage pulls the output voltage up to a high level when all the transistors are cutt off. There is another approach known as active pull up. It uses a modified totem pole output to speed up the charging of stray output capacitance. The effect is to dramatically lower down the time constant. The circuit with tri-state is shown in Fig 7. With control input C open or HIGH (+5 V) any input A or B LOW will result in Q1 going ON with Q2 and Q3 both cut off . Transistor Q4 is then driven ON as is Q5 the output then goes to a HIGH level (near 3.6 V). With control C input open or HIGH Q1 operates in reverse ON driving Q2 and Q3 both ON. The output is then LOW (Q4 and Q5 both held off). Thus with control input C open the circuit operates a totem pole output circuit. With control input LOW, the Diode D1 keeps b and Q5 both off. Transistor 1 is driven ON holding Q2
TTL Circuits
A series of TTL circuits are available under manufacturers numbering starting with a designation 74 This family of TTL citcuits provides various logical and functional characteristics with well specified speed , power dissipation features. Any device in 74 series works over a temperature range of 0 to 70C and a over a supply range of 4.75 to 5.25 V. This is adequate for commercial applications.