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https://designers-guide.org/forum/YaBB.pl Design >> Analog Design >> Fully Differential Amplifier closed loop gain (superposition assumption) https://designers-guide.org/forum/YaBB.pl?num=1510329623 Message started by monglebest on Nov 10th, 2017, 8:00am |
Title: Fully Differential Amplifier closed loop gain (superposition assumption) Post by monglebest on Nov 10th, 2017, 8:00am I don't understand why output is considered ground when calculating Vin's superposition at amplifier input pin. http://www.ti.com/lit/an/slyt157/slyt157.pdf From the above TI notes, I learnt that I should apply superposition Vn=(Vin-)*R2/(R1+R2) + (Vout+)*R1/(R1+R2) (1) Vp=(Vin+)*R4/(R3+R4) + (Vout-)*R3/(R3+R4) (2) Then based on the differential amplifier equation (Vp-Vn)*Adiff=(Vout+)-(Vout-), I can calculate the closed loop gain. My question is about the superposition equations (1). Why can I assume Vout+ node is grounded when considering (Vin-)'s superposition, which give me Vn=(Vin-)*R2/(R1+R2). If the amplifier is OTA, the output node is high impedance. Can anyone explain to me why in this superposition, I can obtain Vn=(Vin-)*R2/(R1+R2)? I can understand the other way when calculating Vn=(Vout+)*R1/(R1+R2) because Vin- voltage source should be grounded in Vout+'s superposition. What is missing in my understanding? |
Title: Re: Fully Differential Amplifier closed loop gain (superposition assumption) Post by buddypoor on Nov 11th, 2017, 2:45am The superposition principle requires that the influence of the various signal sources is considered successively - all sources grounded (set to zero) except one! |
Title: Re: Fully Differential Amplifier closed loop gain (superposition assumption) Post by monglebest on Nov 13th, 2017, 12:56pm buddypoor wrote on Nov 11th, 2017, 2:45am:
Hi, buddypoor I understand what you say when I execute the calculation of superposition. But why can I do that? In normal operation, there is only voltage source at input, the output should be connected with cap load. Or maybe I am asking the wrong question when trying to analyze the gain calculation of fully differential pair? Do you know other way to calculate fully differential amplifier differential gain? Regards Gaofeng |
Title: Re: Fully Differential Amplifier closed loop gain (superposition assumption) Post by Frank_Heart on Nov 17th, 2017, 10:50am monglebest wrote on Nov 10th, 2017, 8:00am:
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Title: Re: Fully Differential Amplifier closed loop gain (superposition assumption) Post by monglebest on Nov 29th, 2017, 10:08am Frank_Heart wrote on Nov 17th, 2017, 10:50am:
Hi, Frank: Can you explain it more clearly what's the black magic that closed loop does here? Thank you Gaofeng |
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