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Question about the minimum allowable input voltage in Unity Gain Op-Amp

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firsttimedesigning

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From the book it says that the minimum allowable input voltage is Vcss + Vgs1. Vcss is the voltage across the current source. The maximum input voltage is Vdd- Vgs3 +Vth1. I can see how the authors gets these two voltage. What I dont understand is that what would happen if the input voltage drops below the minimum allowable input voltage? and the input voltage gets above the max allowable input voltage? Does one of the transistors go into triode region? if so, how?

I am guessing that when it is below the minimum allowable input voltage, some of the transistors will not conduct the current. But I cannot see how does that happen...
 

Re: Unity Gain Op-Amp

When the input voltage goes below Vcss+Vgs1 then the current source Iss will not have enough voltage across it. So when you implement it using a MOS its Vds will decrease below its Vgs-Vth point and it will go into triode, and thus decrease its current.
When the input voltage goes above the maximum the input transistor M1 will go into triode since the Vgs of M3 is constant since the current in it is set by the tail current so that Vgs will eat into the Vds of M1 causing it to go into triode. Note when the input voltage is increased the voltage at the top of the current source node will also increase since the Vgs if M1 is fixed by the current source Iss.
I think this should make the working clear.
 
Re: Unity Gain Op-Amp

This question can be very nicely answered using an oscilloscope. What happens is that when the voltage drops below a certain level, the transistors get switched off while if the voltage goes above Vmax, the transistors enter into the triode region thus introducing harmonics into the output waveform.
Thus on the screen you see a straight line at the bottom while a curved line can be seen at the top of the waveform.
Cheers
 

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