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https://designers-guide.org/forum/YaBB.pl Design >> Analog Design >> Why while testing an AMPLIFIER (test chip) in feedback configuration both R-C fe https://designers-guide.org/forum/YaBB.pl?num=1266231079 Message started by Manas on Feb 15th, 2010, 2:51am |
Title: Why while testing an AMPLIFIER (test chip) in feedback configuration both R-C fe Post by Manas on Feb 15th, 2010, 2:51am Why while testing an AMPLIFIER (test chip) in feedback configuration both R-C feed back circuit is used. Please see the attached file for the Diagram. |
Title: Re: Why while testing an AMPLIFIER (test chip) in feedback configuration both R-C fe Post by raja.cedt on Feb 15th, 2010, 4:05am hi mans, due to op amp input parasitic capacitance, if you use only resistive divider you don't see proper step response due to additional pole at the op amp input. So probably they are ding pole zero compensation with the help of this passive network. Thanks, rajasekhar. |
Title: Re: Why while testing an AMPLIFIER (test chip) in feedback configuration both R-C fe Post by Manas on Feb 15th, 2010, 11:24pm Hi Rajsekhar, So ur telling if we use Resistive feedback the loop-gain Phase is getting degraded due to the i/p Cap of OPAMP and hence the stability and BW. If this is the problem then one can use ONLY Capacitor feed-back and get out of the jail. This can also give the DC -ve Feed-back.Why bother about both RESISTOR and CAPACITOR |
Title: Re: Why while testing an AMPLIFIER (test chip) in feedback configuration both R-C fe Post by Berti on Feb 16th, 2010, 12:31am Rajasekhar, can you please elaborate on your answer. Why should the input capacitance of the amplifier degrade settling? Manas, for which frequencies is this testing configuration used? I think that the capacitors provide the actual (low) impedance feedback while the resistors are only used to define the DC operating point. Would you use only resistors, the amplifier would be resistively loaded, while using only capacitors, the DC operating point at the input of the amplifier will not be defined. Regards |
Title: Re: Why while testing an AMPLIFIER (test chip) in feedback configuration both R-C fe Post by Manas on Feb 16th, 2010, 12:59am Berti wrote on Feb 16th, 2010, 12:31am:
Berti, The testing configuration is used for to Characterize an OPAMP (to find UGB,Slew rate,IM3 and so on). Also these type of configuration is very common. Any paper you open you will find this type of set-up for testing the Chip(OPAMP). I have some doubts on your Comments--> 1)Why one needs low impedance feed-back.What does it actually mean? 2)Why you are saying Cap divider can't give DC biasing point. Two capacitors can divide the DC voltage like Resistive Divider. Isn't It :) |
Title: Re: Why while testing an AMPLIFIER (test chip) in feedback configuration both R-C fe Post by pancho_hideboo on Feb 16th, 2010, 2:16am Manas wrote on Feb 16th, 2010, 12:59am:
What voltage do you think it results in ? If feedback networks are composed of only capacitor, - It is difficult to fix DC bias points properly. It could results in extreme bias points, e.g. all devices are saturated or cut off. This is very true for CMOS OP Amp. - No feedback effect at DC and low frequency. It could cause unstability. If feedback networks are composed of only resistor, - It could be unstable for high frequency. |
Title: Re: Why while testing an AMPLIFIER (test chip) in feedback configuration both R-C fe Post by Manas on Feb 16th, 2010, 3:27am pancho_hideboo wrote on Feb 16th, 2010, 2:16am:
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Title: Re: Why while testing an AMPLIFIER (test chip) in feedback configuration both R-C fe Post by pancho_hideboo on Feb 16th, 2010, 3:31am Manas wrote on Feb 16th, 2010, 3:27am:
Both charge(Qx) and capacitance(Cx) of inverted node are unknown. And Qx might be negative. Manas wrote on Feb 16th, 2010, 3:27am:
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Title: Re: Why while testing an AMPLIFIER (test chip) in feedback configuration both R-C fe Post by raja.cedt on Feb 16th, 2010, 6:57am hi barthi, i feel due to op amp input cap there will be one extra pole which may impact Ts...what's the issue in this thanks Rajasekhar. |
Title: Re: Why while testing an AMPLIFIER (test chip) in feedback configuration both R-C fe Post by Berti on Feb 16th, 2010, 11:44pm Rajasekhar, Quote:
This 'feeling' can be resolved by simple hand-calculations. However, since the input cap of the opamp is basically in parallel to the input capacitance I don't see how an additional pole should be created. Manas, I was wondering about the frequencies of interest since already for reasonably high frequencies (>1MHz) the impedance will be dominated by the capacitor (for the values given in your figure). The resistors therefore only serves to define the DC operating point. - as Pancho explained quite well. Cheers |
Title: Re: Why while testing an AMPLIFIER (test chip) in feedback configuration both R-C fe Post by Manas on Feb 17th, 2010, 9:52pm link=1266231079/0#6 date=1266319631]Suppose the cap (Cin) from inverting node to Gnd(no ac signal) has +ve Charge, then the OPAMP o/p voltage will fall this in turn sucks charge from the inverting node through the F/B CAP(Cf) and the -node will be reverted back to voltage at the + node of the OPAMP as -ve DC feed-back still exists in case of CAP divider.[/quote]I can't understand what you want to claim. IN RESPONSE TO ABOVE MENTIONED Q--> I MEAN TO SAY THAT ANY SPURIOUS CHARGE CAN BE REMOVED BECAUSE OF OPAMP ACTION AND IN THE STEADY STATE IT WILL BE SAME AS IF YOU DON'T HAVE ANY CHARGE IN THE CAP. Both charge(Qx) and capacitance(Cx) of inverted node are unknown. And Qx might be negative. IN RESPONSE TO ABOVE Qn--> THE VOLTAGE AT THE ASKED NODE IS --> = Vdd * [C2/(C1+CX+C2)]. AS CHARGE REDISTRIBUTION HAPPENS. PLEASE NOTE THAT IF C1>>CX (AS MENTIONED IN MY DIAGRAM 2.5pf), THE EFFECT OF Cx CAN BE NEGLECTED AND ONE CAN GET THE DESIRED VOLTAGE DIVISION. |
Title: Re: Why while testing an AMPLIFIER (test chip) in feedback configuration both R-C fe Post by Manas on Feb 17th, 2010, 9:58pm raja.cedt wrote on Feb 16th, 2010, 6:57am:
Rajasekhar Why you r telling there will be pole ? if there is no resistor feed-back ?? Don't u think one can give DC -ve feed-back with CAP only ?? |
Title: Re: Why while testing an AMPLIFIER (test chip) in feedback configuration both R-C fe Post by Manas on Feb 17th, 2010, 9:58pm raja.cedt wrote on Feb 16th, 2010, 6:57am:
Rajasekhar Why you r telling there will be pole ? if there is no resistor feed-back ?? Don't u think one can give DC -ve feed-back with CAP only ?? |
Title: Re: Why while testing an AMPLIFIER (test chip) in feedback configuration both R-C fe Post by pancho_hideboo on Feb 18th, 2010, 12:48am Manas wrote on Feb 17th, 2010, 9:52pm:
First, Vx is a floating node. So Qx can not be removed. Second, it could result in stagnated bias points. So OP Amp can't work correctly. Manas wrote on Feb 17th, 2010, 9:52pm:
C1, C2 and Cx share Qx in initial state. Qx is not charged in only small Cx. So after restribution of charge, I can't expect Vx correctly. If feedback networks are composed of only capacitor, - It is difficult to fix DC bias points properly. It could results in extreme bias points, e.g. all devices are saturated or cut off. This is very true for CMOS OP Amp. - No feedback effect at DC and low frequency. It could cause unstability. If feedback networks are composed of only resistor, - It could be unstable for high frequency. Manas wrote on Feb 17th, 2010, 9:58pm:
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Title: Re: Why while testing an AMPLIFIER (test chip) in feedback configuration both R-C fe Post by pancho_hideboo on Feb 18th, 2010, 12:53am The following is duplicated. Please delete this post. http://www.designers-guide.org/Forum/YaBB.pl?num=1266231079/12#12 |
Title: Re: Why while testing an AMPLIFIER (test chip) in feedback configuration both R-C fe Post by Manas on Feb 18th, 2010, 4:06am pancho_hideboo wrote on Feb 18th, 2010, 12:48am:
C1, C2 and Cx share Qx in initial state. Qx is not charged in only small Cx. So after restribution of charge, I can't expect Vx correctly. If feedback networks are composed of only capacitor, - It is difficult to fix DC bias points properly. It could results in extreme bias points, e.g. all devices are saturated or cut off. This is very true for CMOS OP Amp. - No feedback effect at DC and low frequency. It could cause unstability. If feedback networks are composed of only resistor, - It could be unstable for high frequency. SORRY, I AM WRONG. AS IT IS A FLOATING NODE, THE Qx CHARGE CAN'T BE REMOVED. HENCE THAT POINT CAN'T HAVE DESIRED VOLTAGE DUE TO OPAMP ACTION. SORRY AGAIN FOR THE WRONG INTERPRETATION. AGAIN THANKS A LOT !! |
Title: Re: Why while testing an AMPLIFIER (test chip) in feedback configuration both R-C fe Post by pancho_hideboo on Feb 18th, 2010, 4:09am Manas wrote on Feb 18th, 2010, 4:06am:
But unstability issue never can be resolved even if you implement startup circuit with feedback networks compopsed of only capacitor. |
Title: Re: Why while testing an AMPLIFIER (test chip) in feedback configuration both R-C fe Post by Manas on Feb 18th, 2010, 11:06pm But unstability issue never can be resolved even if you implement startup circuit with feedback networks compopsed of only capacitor. [/quote] Thanks A Lot !! Can u provide any example or cases where the above mentioned issue has happened or the designers have faced the problems?? You may point to any reference paper or material.. |
Title: Re: Why while testing an AMPLIFIER (test chip) in feedback configuration both R-C fe Post by pancho_hideboo on Feb 19th, 2010, 1:37am Manas wrote on Feb 18th, 2010, 11:06pm:
Here there is no feedback effect around DC and low frequency. This is almost open loop high gain OP Amp alone. So it could cause unstability around low frequency due to contiunous time operation. On the other hand, capacitor only feedback networks are used in switched capacitor amplifier. The reason why it could not be unstable is due to its discrete time operation. |
Title: Re: Why while testing an AMPLIFIER (test chip) in feedback configuration both R-C fe Post by Manas on Feb 22nd, 2010, 1:51am This is Just for Completion of the Discussion of the POST. I have attached the desired files |
Title: Re: Why while testing an AMPLIFIER (test chip) in feedback configuration both R-C fe Post by Manas on Feb 22nd, 2010, 1:52am Manas wrote on Feb 22nd, 2010, 1:51am:
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Title: Re: Why while testing an AMPLIFIER (test chip) in feedback configuration both R-C fe Post by Manas on Feb 22nd, 2010, 1:52am Manas wrote on Feb 22nd, 2010, 1:52am:
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Title: Re: Why while testing an AMPLIFIER (test chip) in feedback configuration both R-C fe Post by pancho_hideboo on Feb 22nd, 2010, 2:03am For only resistor feedback networks, you should consider instability due to load capacitance rather than input capacitance. |
Title: Re: Why while testing an AMPLIFIER (test chip) in feedback configuration both R-C fe Post by Manas on Feb 22nd, 2010, 2:39am pancho_hideboo wrote on Feb 22nd, 2010, 2:03am:
Of-Course due to Load Capacitance the Phase of the Stand-Alone OPAMP phase degrades even if there is resistive feed-back or not ( is a lose statement. It may improves the stability). If I break the loop at the o/p node of the OPAMP and apply a Vtest at the break point then the Resistors and the i/p cap forms a Low-pass filter and adds a effective pole in the LOOP-gain as the Feed-back ressistors are kept quite high for not reducing (loading) OPMAP o/p resistance. I wonder how due to the resistive feed-back and the load capacitance the pole is added in the Loop-gain. Putting the my doubt i other way--> How resistive feed-back degrades the pole caused by the load Capacitor |
Title: Re: Why while testing an AMPLIFIER (test chip) in feedback configuration both R-C fe Post by pancho_hideboo on Feb 22nd, 2010, 2:44am Manas wrote on Feb 22nd, 2010, 2:39am:
Then consider resistance only feedback with it and load capacitance. It could result in positive feedback. |
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