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Dec 7th, 2019, 9:00am
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Symmetric zeros, does it matter? (Read 250 times)
Jacki
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Symmetric zeros, does it matter?
Mar 13th, 2019, 7:55am
 
Hello,

   If we look at the two-stage differential-to-single-ended opamp, the first stage to the second stage is connected by the diode-connected current mirror, and in this way, we will get two symmetric zeros at the differential output of the first stage. I know this zero is at high frequency, but I wonder if the two zeros can be considered as a "common-mode zero", and for the differential signal the zero will not be harmful, and their influence cannot compensated?
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kumar.g
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Re: Symmetric zeros, does it matter?
Reply #1 - Aug 12th, 2019, 7:56am
 
The two zeros appear because of Cgd of input MOSFETs. Typically they occur after the unity gain frequency of the amplifier and so can be safely ignored.
Additionally, this zero is for both the common mode and differential mode signal paths.
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Jacki
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Re: Symmetric zeros, does it matter?
Reply #2 - Aug 13th, 2019, 1:09am
 
Thank you for your reply. But if we derive the expression, in fact we cannot see these two zeros, i think maybe it is internally compensated each other.
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Horror Vacui
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Re: Symmetric zeros, does it matter?
Reply #3 - Aug 13th, 2019, 2:06am
 
The differential amplifier relies on two identical amplifying devices (=transistors). If both device have the same imperfection, they will cancel out in the differential signal path. On the other hand, common mode current will flow through these capacitors, and it will alter the common mode at the output, but this is at a very high frequency and therefore it can be neglected.
The transit frequency which is often used as a metric for transistor speed is approximated as gm/Cg, where Cg is the effective capacitance at the gate. The zero you are talking about lies at gm/Cgd, which frequency is higher than f_T by more than a factor of 2! Therefore there is no signal amplification here. If we take into account that diff amps without reactive matching, inductive peaking or reactive resonance operate up to lets say f_T/20 at best, the zero due to Cgd of the diff pair is not important at all.
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