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Why PMOS Cannot be used as pull down network?
For PDN the output should be pulled to logic low (i.e. GND). If we use NMOS transistor then the voltage level of F is ‘0’ V and if we use PMOS transistor then the voltage level of F is VTp i.e. Threshold voltage of PMOS. Thus NMOS produces strong ‘0’ and PMOS produces weak ‘0’.
Why PMOS transistor is selected as pull up transistor?
Pull up means getting close VDD. So PMOS has VDD as source, naturally when input is zero drain would be pulled up. When output at zero PMOS turns on, it will be pulled high. Pull down means bring output to Zero from One too.
Why PMOS Cannot be used as pull down network and NMOS as pull up network in CMOS logic circuits?
For a NMOS part the Source pin must be connected at the low level, so when turned on the Drain pin can only “pull down” to that same low level. For a PMOS part the Source pin must be connected at the high level, so when turned on the Drain pin can only “pull up” to that same high level.
Why are NMOS transistors known as pull-down devices?
Because of its action of sinking the load current and pulling the output voltage (Vout) down towards the GND, The NMOS transistor in the CMOS inverter circuit is known as a PULL-DOWN device. Similarly, with the input LOW, the PMOS is able to source a large load current. Thus, pulling the output voltage up to Vdd. (PULL-UP).
Why do we always use PMOS as a pull up?
So PMOS has VDD as source, naturally when input is zero drain would be pulled up. When output at zero PMOS turns on, it will be pulled high. Pull down means bring output to Zero from One too. If input is One for an inverter in CMOS, N transistor will be drive the output to Zero as pull down.
Why is PMOS not able to pull down output node?
When Out reaches Vt the PMOS does not conduct anymore, since when Vsg = Vt the device goes in cutoff region. For this reason, the PMOS can’t pull down the output node all the way to gnd, but only to Vt, hence the term weak 0.
When does the PMOS behave like a resistor?
Since In is to gnd, hence 0, the PMOS goes from saturation to linear region when Out = Vt. From now on, the PMOS behaves like a resistor, and keeps charging the capacitor till Out = Vdd. Please note that throughout the whole time, the source to gate voltage of the PMOS, Vsg, is constant and equal to Vdd.