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In CMOS designs, why size of PMOS is kept larger than size of NMOS?
- a)To get higher drive strength
- b)To reduce power dissipation
- c)To get balanced rise/fall time
- d)All of above
Correct answer is option 'C'. Can you explain this answer?
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Most Upvoted Answer
In CMOS designs, why size of PMOS is kept larger than size of NMOS?a)T...
To maximize the switching speed of a logic gate, for example, an inverter, it is best if the rise and fall time of the logic gate’s output signal is the same.
For this to occur, the top side transistors of the logic gate must switch current into the output of the logic gate at the same magnitude as the low side transistors.
Since PMOS transistors (high side) have approximately half the mobility of NMOS transistors (low side), it is necessary to add two parallel PMOS devices to the high side to achieve the equivalent magnitude currents.
In saturation
NMOS:
Cox and voltage are the same for both side, when
Considering the same length as it is a fixed constraint for the circuit.
that’s why we take PMOS size greater than N-MOS
NMOS:
Cox and voltage are the same for both side, when
Considering the same length as it is a fixed constraint for the circuit.
that’s why we take PMOS size greater than N-MOS
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In CMOS designs, why size of PMOS is kept larger than size of NMOS?a)T...
Introduction:
In CMOS designs, the size of PMOS (p-channel metal-oxide-semiconductor) transistors is typically kept larger than the size of NMOS (n-channel metal-oxide-semiconductor) transistors. This is done to achieve balanced rise/fall times, as well as to optimize drive strength and power dissipation.
Explanation:
The size of a MOS transistor determines its width and length, which directly affects its electrical characteristics. Here's why the size of PMOS is kept larger than NMOS:
1. Balancing Rise/Fall Times:
In a CMOS circuit, both PMOS and NMOS transistors are used to implement complementary logic functions. When the input signal transitions from low to high (0 to 1), the PMOS transistor should turn off quickly, and the NMOS transistor should turn on quickly. Similarly, when the input signal transitions from high to low (1 to 0), the PMOS transistor should turn on quickly, and the NMOS transistor should turn off quickly.
By making the PMOS transistor larger, its drive strength increases. This means it can provide more current during switching, leading to faster turn-on and turn-off times. On the other hand, the NMOS transistor can be made smaller to achieve a balanced rise/fall time because it has inherently higher mobility and can switch faster compared to PMOS.
2. Drive Strength:
The drive strength of a transistor determines its ability to drive capacitive loads and overcome parasitic resistances. By making the PMOS transistor larger, its drive strength increases, allowing it to drive larger loads efficiently. This is especially important for complementary logic designs where the PMOS transistor is responsible for driving the pull-up network.
3. Power Dissipation:
Power dissipation in CMOS circuits occurs during switching events when both PMOS and NMOS transistors are conducting simultaneously. By making the PMOS transistor larger, its on-resistance (Rds(on)) decreases, resulting in lower power dissipation during the pull-up phase. This helps in reducing overall power consumption in CMOS designs.
Conclusion:
In summary, the size of the PMOS transistor is kept larger than the NMOS transistor in CMOS designs to achieve balanced rise/fall times, improve drive strength, and reduce power dissipation. This helps in optimizing the performance, power consumption, and reliability of CMOS circuits.
In CMOS designs, the size of PMOS (p-channel metal-oxide-semiconductor) transistors is typically kept larger than the size of NMOS (n-channel metal-oxide-semiconductor) transistors. This is done to achieve balanced rise/fall times, as well as to optimize drive strength and power dissipation.
Explanation:
The size of a MOS transistor determines its width and length, which directly affects its electrical characteristics. Here's why the size of PMOS is kept larger than NMOS:
1. Balancing Rise/Fall Times:
In a CMOS circuit, both PMOS and NMOS transistors are used to implement complementary logic functions. When the input signal transitions from low to high (0 to 1), the PMOS transistor should turn off quickly, and the NMOS transistor should turn on quickly. Similarly, when the input signal transitions from high to low (1 to 0), the PMOS transistor should turn on quickly, and the NMOS transistor should turn off quickly.
By making the PMOS transistor larger, its drive strength increases. This means it can provide more current during switching, leading to faster turn-on and turn-off times. On the other hand, the NMOS transistor can be made smaller to achieve a balanced rise/fall time because it has inherently higher mobility and can switch faster compared to PMOS.
2. Drive Strength:
The drive strength of a transistor determines its ability to drive capacitive loads and overcome parasitic resistances. By making the PMOS transistor larger, its drive strength increases, allowing it to drive larger loads efficiently. This is especially important for complementary logic designs where the PMOS transistor is responsible for driving the pull-up network.
3. Power Dissipation:
Power dissipation in CMOS circuits occurs during switching events when both PMOS and NMOS transistors are conducting simultaneously. By making the PMOS transistor larger, its on-resistance (Rds(on)) decreases, resulting in lower power dissipation during the pull-up phase. This helps in reducing overall power consumption in CMOS designs.
Conclusion:
In summary, the size of the PMOS transistor is kept larger than the NMOS transistor in CMOS designs to achieve balanced rise/fall times, improve drive strength, and reduce power dissipation. This helps in optimizing the performance, power consumption, and reliability of CMOS circuits.
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In CMOS designs, why size of PMOS is kept larger than size of NMOS?a)To get higher drive strengthb)To reduce power dissipationc)To get balanced rise/fall timed)All of aboveCorrect answer is option 'C'. Can you explain this answer?
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