Hello guys ...
please help me on this...
According to theory in order to increase the input impedance bootstrap capacitor is employed…which couples the same output voltage to point B ..and as the circuit is a voltage follower Vin = Vout ..thus the voltage difference across R1 is very small ..which means that no or very little current will flow through R1 ..which can be also visualised as a very high resistance present..
Analysis:
Without bootstrap capacitor Cb:
Zin = R1; figure 1..
With bootstrap capacitor:
Zin = (1+Aol)R1 ....where Aol = open loop gain of opamp.
As its clear from above equation that input impedance increase by a factor of (1+Aol)..
since from figure 1. we have
i1 = V1/R1 = 500mv / 68K = 7.35 mA.
from figure 2 ..we have
Zin = (999.996  999.998)mV / 7.35mA (forget Zin = (1+Aol) R1 for the time being)
= 2.72* 10^4 k
But the value of Zin should be very 2 large ..but is not agreeing with the theory.. why?? please justify..
please help me on this...
According to theory in order to increase the input impedance bootstrap capacitor is employed…which couples the same output voltage to point B ..and as the circuit is a voltage follower Vin = Vout ..thus the voltage difference across R1 is very small ..which means that no or very little current will flow through R1 ..which can be also visualised as a very high resistance present..
Analysis:
Without bootstrap capacitor Cb:
Zin = R1; figure 1..
With bootstrap capacitor:
Zin = (1+Aol)R1 ....where Aol = open loop gain of opamp.
As its clear from above equation that input impedance increase by a factor of (1+Aol)..
since from figure 1. we have
i1 = V1/R1 = 500mv / 68K = 7.35 mA.
from figure 2 ..we have
Zin = (999.996  999.998)mV / 7.35mA (forget Zin = (1+Aol) R1 for the time being)
= 2.72* 10^4 k
But the value of Zin should be very 2 large ..but is not agreeing with the theory.. why?? please justify..
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