![]() ![]() There are two types of MOSFETs: NMOS and PMOS. The MOSFET acts as a switch and turns on or off depending on whether the voltage on it is either high or low. A MOSFET transistor is a voltage-controlled switch. These logic gates are implemented using transistors called MOSFETs. The logic gates are the basic building blocks of all digital circuits and computers. ISRO CS Syllabus for Scientist/Engineer Exam.ISRO CS Original Papers and Official Keys.GATE CS Original Papers and Official Keys.DevOps Engineering - Planning to Production.Python Backend Development with Django(Live).Android App Development with Kotlin(Live).Full Stack Development with React & Node JS(Live).Java Programming - Beginner to Advanced.Data Structure & Algorithm-Self Paced(C++/JAVA).Data Structures & Algorithms in JavaScript.Data Structure & Algorithm Classes (Live).clock) switches ‘n’ times, will be n * C L * Vdd 2, which can gain attention. Thus, amount of energy consumed, if a circuit (e.g. Hence, every time a circuit switches, it consumes C L * Vdd 2 amount of energy. (use the current-voltage relationship to understand the above integration i.e. Let E 0->1 denote energy consumed whenever output switches from ‘0’ to ‘Vdd’.Į 0->1 = ∫ p(t). Now, in order to calculate the amount of energy during a single switching,refer to the following RC equivalent network for CMOS switching from ‘low’ to ‘high’ The power consumed at any instance by a CMOS circuit is given by : The reverse happens whenever C L gets discharged from ‘Vdd’ to ‘0’i.e. part of which is dissipated in Rp, whereas the remaining is stored in C L. Hence, whenever output load C L gets charged from ‘0’ to ‘Vdd’, a finite amount of energy is drawn from the power supply. The charging and discharging current and voltage waveforms could be shown as below : R = Average ‘ON’ resistance of transistorĬonsider the following scenario where output capacitor C L charge from 0 to ‘Vdd’ as input switches from ‘Vdd’ to ‘0’: Hence, a CMOS inverter can be modeled as an RC network, where Substituting ‘Vout’ equal to Vdd/2, and ‘t’ equal to ‘tp’ in above equation, we get the following : Vout = (1-e -t/τ) Vdd, where τ = RC = time constant. First order RC networkĬonsider the following RC network to which we apply a step input. ![]() As we have seen above, the switching behavior of CMOS inverter could be modeled as a resistance R on with a capacitor C L, a simple first order analysis of RC network will help us to model the propagation delay. Now, in order to find the propagation delay, we need a model that matches the delay of inverter. The delay is usually calculated at 50% point of input-output switching, as shown in above figure. The propagation delay high to low (t pHL) is the delay when output switches from high-to-low, after input switches from low-to-high. Many designs could also prefer 30% to 70% for rise time and 70% to 30% for fall time. Fall time (t f) is the time, during transition, when output switches from 90% to 10% of the maximum value. Rise time (t r) is the time, during transition, when output switches from 10% to 90% of the maximum value. In the above figure, there are 4 timing parameters. inverter is the difference in time (calculated at 50% of input-output transition), when output switches, after application of input. The propagation delay of a logic gate e.g. This would be more clear in the next section. The time required for change in ‘Vo’ after application of ‘Vi’, is called as propagation delay of the inverter. But, ‘Vo’ doesn’t changes instantaneously, but after a finite amount of delay after application of ‘Vi’. We understand that when ‘Vi’ switches from low to high (or high to low), ‘Vo’ switches from high to low (or low to high). Hence, during this mode of operation, C L discharges to Vss, through Rn, and ‘Vo’ switches from logic ‘1’ to logic ‘0’, as shown in figure below. This behavior could be modeled as an ‘open switch’ for PMOS and resistance ‘Rn’ for NMOS followed by a capacitor C L. Now, when ‘Vi’ switches from low to high, PMOS turns ‘OFF’, whereas NMOS turns ‘ON’. Hence, during the above mode of operation, C L charges to Vdd, through Rp, and ‘Vo’ switches from logic ‘0’ to logic ‘1’. Also, the wires connected to Vi and Vo of the inverter contribute to the load capacitance C L. These transistors contribute to lot of capacitance, which contribute to C L, load capacitance. In a large circuit, every CMOS is superseded and/or preceded by logic gates, which is again, nothing but a bunch of NMOS and PMOS transistors. ![]() During this operation of CMOS inverter, NMOS is modeled as an ‘open switch’, whereas PMOS is modeled as a resistance ‘Rp’ followed by a capacitor C L. When ‘Vi’ switches from high to low, PMOS turns ‘ON’ whereas NMOS turns ‘OFF’. Now, let us look at the transient response of an inverter. ![]()
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