What Is Virtual Ground In Op Amp-Operational Amplif

What Is Virtual Ground In Op Amp

The invert­ing input con­fig­u­ra­tion of the oper­a­tional  ampli­fi­er and we will see the con­cept of Vir­tu­al Ground In Op Amp

Virtual Ground In Op Amp
Vir­tu­al Ground In Op Amp

Working Principle Of Virtual Ground Op-Amp

  • We have seen the basics of this oper­a­tional ampli­fi­er and we had seen  that this op-amp is a very high gain dif­fer­en­tial amplifier. 
  • And the gain of the op-amp used to be in the range of 10 to the pow­er 5 to the 10 to the  pow­er 6. 
  • And we had seen that even if apply a very small amount of dif­fer­en­tial input volt­age  between the input ter­mi­nals of the op-amp, then also the out­put will be get sat­u­rat­ed  towards the bias­ing points. 
  • And then we had seen the volt­age trans­fer curve of this op-amp. 
  • And then we had seen that even if we apply the small input volt­age to this op-amp then  also the out­put will be get sat­u­rat­ed either towards the pos­i­tive or the neg­a­tive sat­u­ra­tion  voltages. 
  • Now, let’s say the sat­u­ra­tion volt­age for the op-amp is 10V, and the open loop gain  of this opamp is let’s say 10 to the pow­er 6. 
  • So, this op-amp will get sat­u­rat­ed at 10 micro­volts of a dif­fer­en­tial input voltage. 
  • So, now when­ev­er we want to use this op-amp as an ampli­fi­er, we need to use it in the  lin­ear region.  That means in this region. 
  • So, that the input and out­put have a lin­ear relationship. 
  • But when­ev­er we are using this op-amp in open loop con­fig­u­ra­tion, then this lin­ear range  is very small. 
  • So, if we want to use this op-amp as an ampli­fi­er, then we need to some­how con­trol the gain of  this amplifier. 
  • And we can do so, by apply­ing the feed­back from out­put to the input side.  So, there are two ways, by which we can apply this feedback 
  • One is pro­vid­ing feed­back from out­put to this pos­i­tive input ter­mi­nal.  Or let’s say non-invert­ing input terminal. 
  • Now, when­ev­er we are pro­vid­ing the feed­back from this out­put to this non-invert­ing input  then that kind of feed­back is known as the pos­i­tive feedback. 
  • Because here the frac­tion of an out­put volt­age is get­ting added to this non-invert­ing input. 
  • Now, when­ev­er in any sys­tem we are using the pos­i­tive feed­back, then this pos­i­tive feed­back  leads that sys­tem to the insta­bil­i­ty.  So, we can not use this pos­i­tive feed­back alone.
  • So now here to con­trol the gain of the op-amp, we need to go for this neg­a­tive feedback. 
  • That means we need to feed the frac­tion of the out­put volt­age to this invert­ing input  terminal. 
  • Now, here three ways by which we can apply the input to this op-amp. 
  • The first is apply­ing the input to this non-invert­ing input ter­mi­nal and ground­ing this neg­a­tive  input terminal. 
  • Sec­ond is pro­vid­ing the input to this invert­ing input ter­mi­nal and ground­ing this pos­i­tive  terminal. 
  • And third is pro­vid­ing the input to both non-invert­ing as well as invert­ing input terminals 
  • So, first, we will see the case when we are apply­ing the input to this invert­ing input  terminal. 
  • So, let’s say we have applied input to this invert­ing input ter­mi­nal through one resis­tor R1. 
  • And we are pro­vid­ing neg­a­tive feed­back from out­put to this invert­ing input ter­mi­nal via  this feed­back resis­tor Rf. 
  • So, now when­ev­er op-amp is used in this con­fig­u­ra­tion then this con­fig­u­ra­tion is known as the invert­ing  op-amp configuration. 
  • So, now in this con­fig­u­ra­tion let’s find out the rela­tion­ship between this out­put and the  input voltages. 
  • And let’s see how we can con­trol the gain of op-amp by using this feed­back resis­tor  Rf and this resis­tor R1. 
  • So, let’s find the rela­tion­ship between this out­put and input in terms of this feed­back  resis­tor Rf and R1. 
  • So, now before we derive this expres­sion, let’s under­stand the con­cept of vir­tu­al ground  in the op-amp. 
  • And this con­cept of vir­tu­al ground is applic­a­ble when we are pro­vid­ing the neg­a­tive feed­back  to this op-amp. 
  • Now, let’s say for the giv­en op-amp the open-loop gain of this op-amp is 10 to the pow­er 6. 
  • And we know that the out­put volt­age Vout of op-amp can be giv­en as A times the dif­fer­en­tial  input voltage. 
  • That is the input volt­age between these invert­ing and the non-invert­ing input terminals. 
  • Now, here let’s assume that through this neg­a­tive feed­back we are con­trol­ling the out­put volt­age  of this op-amp in as such way that the out­put volt­age is always less than the sat­u­ra­tion  voltage. 
  • Or we can say that we are oper­at­ing this op-amp in a lin­ear region. 
  • Now, here this dif­fer­en­tial input volt­age is noth­ing but the dif­fer­ence between this  invert­ing and the non-invert­ing input terminals. 
  • So, we can write this dif­fer­en­tial input volt­age as (Vplus ) — (Vmi­nus)  that is equal to 10 microvolts. 
  • Now, here this 10 micro­volts is very small sig­nal and we can almost neglect it.  So, we can write this (Vplus) — (Vmi­nus) as approx­i­mate­ly equal to zero volts. 
  • Or we can say that Vplus that is equal to Vminus. 
  • It means that the invert­ing and the non-invert­ing input ter­mi­nals are at the same potential. 
  • Or we can say that there is vir­tu­al short between this invert­ing and the non-invert­ing  input terminals. 
  • Now, here the term vir­tu­al means that these two ter­mi­nals are not actu­al­ly short-cir­cuit­ed  but they are vir­tu­al­ly short-circuited. 
  • But they are vir­tu­al­ly short-cir­cuit­ed.  So, what­ev­er volt­age that appears at one ter­mi­nal, the exact same volt­age will appear at anoth­er  terminal. 
  • So, now in this con­fig­u­ra­tion, this non-invert­ing input ter­mi­nal is ground­ed.  So, we can say that Vplus that is equal to zero.  So, accord­ing to this con­clu­sion, Vmi­nus should be equal to zero. 
  • It means that this ter­mi­nal is not actu­al­ly ground­ed but it will act as a vir­tu­al ground. 
  • So, this neg­a­tive feed­back will ensure that the dif­fer­ence between this invert­ing and  the non-invert­ing input is very small or we can say that it is almost negligible. 
  • And because of that, we can con­sid­er these both input ter­mi­nals at the same potential. 
  • So, this con­cept is known as the vir­tu­al ground concept. 
  • So, now let’s use this con­cept if vir­tu­al ground and let’s derive the expres­sion between  this Vout and Vin. 
  • Now, here let’s say the cur­rent I1 is the cur­rent that is flow­ing through this resis­tor  R1 and let’s say this node is node X. 
  • And let’s say the cur­rent that is flow­ing through this resis­tor Rf is If. 
  • Now, in the last video, we had seen that the op-amp has very high input imped­ance or if we con­sid­er the ide­al op-amp then the input imped­ance of the op-amp is infinite. 
  • It means that I1 is equal to If.  So, now we can write this I1 as, Vin minus Vx, divide by R1. 
  • Where Vx is the volt­age at this par­tic­u­lar node.  Like­wise, we can write this cur­rent If as Vx minus Vout, divide by Rf. 
  • Now, if we apply the con­cept of vir­tu­al ground, then this node x should have zero potential. 
  • Because this non-invert­ing input ter­mi­nal is ground­ed.  So, the val­ue of Vx should be equal to zero. 
  • So, we can write this expres­sion as Vin divide by R1 that is equal to minus Vout divide by  Rf. and if we rearrange it then we can write it  as Vout by Vin, that is equal to minus Rf divide by R1. 
  • And this expres­sion is known as the closed loop gain for this invert­ing op-amp configuration. 
  • So, as you can see here, just by chang­ing the val­ue of this Rf and R1 we can con­trol  the gain of this op-amp. 
  • And we can use this op-amp as an ampli­fi­er.  Now, here the neg­a­tive sign indi­cates that the out­put volt­age is 180 degree out of phase  with respect to input voltage. 
  • So, let’s say, if we have applied the sinu­soidal sig­nal at the input, then at the out­put we  will get the ampli­fied sinu­soidal sig­nal which is hav­ing a 180-degree phase with respect  to the input signal. 
  • And that is why this con­fig­u­ra­tion of the op-amp is known as the invert­ing configuration. 
  • Because the out­put will be get invert­ed with respect to input voltage. 
  • So, let’s say in this case if Rf is equal to 2 kilo-ohms and R1 is equal to 1 kilo-ohm, 
  • then the gain of the op-amp will be equal to 2. and sup­pose if we apply the 1 volt of  a sig­nal, then at the out­put we will get 2V of a sig­nal which is 180 degree out of phase  with respect to 1V signal. 
  • So, in this way, by con­trol­ling the val­ue of Rf and R1, we can con­trol the gain of this  op-amp and we can use this op-amp as an amplifier. 
  • So, I hope in this arti­cle, you under­stood the invert­ing op-amp con­fig­u­ra­tion and the con­cept  of vir­tu­al ground. 

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