if you just consider Vref, at equilibrium, there is no current going through R2. so node A's potential is Vref * R3/(R3+R4) = Vo.

so the gain there is R3/(R3+R4) with regards to Vref.

When you say "...just consider Vref, at equilibrium...", apparently you mean to set Vin to zero so we're only considering the effect of Vref.

If Vin is zero, then node A is also at zero volts and the voltage "gain" from Vref to node A is zero. Even if Vin is at some non-zero voltage, applying a voltage to Vref causes no change in the voltage at node A (the - input of the opamp), so the voltage gain from Vref to node A is still zero.

If Vin is at zero volts, the left end of R2 is at zero volts, and if Vref is not zero, then the right end of R2 is at some non-zero voltage, so there is current in R2 when Vin is zero and Vref is not zero.

If Vin is at zero volts (effectively grounded), then node A is also at zero volts and R3 is connected between the + and - inputs of the opamp, and does nothing with respect to the voltage gain from Vref.

The gain from Vref to the opamp output is:

\(- \frac{R2 R5+R1(R2+R5)}{R1 R4}\)

and doesn't involve R3.

If R5 is replaced with a short, then the voltage gain from Vref to the opamp output would be:

\(- \frac{R2}{R4}\)

again not involving R3.

With the full circuit in operation, the output impedance which drives R5 is:

\(\frac{R1R3R4(1+Av)+R2(R3+R4)}{R3R4+(R1+R2)(R3+R4)}\)

where Av is the opamp open loop gain. A real opamp's Av will vary with frequency, of course.

 

When you say "...just consider Vref, at equilibrium...", apparently you mean to set Vin to zero so we're only considering the effect of Vref.

If Vin is zero, then node A is also at zero volts and the voltage "gain" from Vref to node A is zero. Even if Vin is at some non-zero voltage, applying a voltage to Vref causes no change in the voltage at node A (the - input of the opamp), so the voltage gain from Vref to node A is still zero.

If Vin is at zero volts, the left end of R2 is at zero volts, and if Vref is not zero, then the right end of R2 is at some non-zero voltage, so there is current in R2 when Vin is zero and Vref is not zero.

If Vin is at zero volts (effectively grounded), then node A is also at zero volts and R3 is connected between the + and - inputs of the opamp, and does nothing with respect to the voltage gain from Vref.

The gain from Vref to the opamp output is:

\(- \frac{R2 R5+R1(R2+R5)}{R1 R4}\)

and doesn't involve R3.

If R5 is replaced with a short, then the voltage gain from Vref to the opamp output would be:

\(- \frac{R2}{R4}\)

again not involving R3.

With the full circuit in operation, the output impedance which drives R5 is:

\(\frac{R1R3R4(1+Av)+R2(R3+R4)}{R3R4+(R1+R2)(R3+R4)}\)

where Av is the opamp open loop gain. A real opamp's Av will vary with frequency, of course.

Indeed, as the gain increases, the frequency decreases and the error rate decreases.

 

Indeed, as the gain increases, the frequency decreases and the error rate increases.

I'm not sure what error rate you mean in this particular circuit, but generally in opamp circuits, as the gain increases the error decreases rather than increasing.

 

I'm not sure what error rate you mean in this particular circuit, but generally in opamp circuits, as the gain increases the error decreases rather than increasing.

I'm afraid not; that is what I read in "Op-Amps for Everyone" by Ron Mancini. It seems sensible to me.

 

I'm afraid not; that is what I read in "Op-Amps for Everyone" by Ron Mancini. It seems sensible to me.

Could you scan or take a picture of the page where he says that and post it here?

 

Could you scan or take a picture of the page where he says that and post it here?

if you don't have THAT book, you should seriously consider buying one NOW because it is FREE from TI.

 

I'm afraid not; that is what I read in "Op-Amps for Everyone" by Ron Mancini. It seems sensible to me.

Would you tell me the page in the pdf version:
focus.ti.com/lit/an/slod006b/slod006b.pdf

where he says that?

 

Would you tell me the page in the pdf version:
focus.ti.com/lit/an/slod006b/slod006b.pdf

where he says that?

now you have it, you should read it thoroughly and you will benefit a lot from it.

 

Would you tell me the page in the pdf version:
focus.ti.com/lit/an/slod006b/slod006b.pdf

where he says that?

Go to this link: http://focus.ti.com/lit/an/slod006b/slod006b.pdf Fourth Paragraph, beginning with "As shown in prior chapters..."

 

I got it.

In post #42, you said:

"Indeed, as the gain increases, the frequency decreases and the error rate increases."

What Ron Mancini says is:

"As shown in prior chapters, when frequency increases, the op amp gain decreases and errors increase."

You've negated only two of his terms. If we take his sentence and complement the words "increases", "decreases" and "increase", we would get:

"As shown in prior chapters, when frequency decreases, the op amp gain increases and errors decrease."

To be consistent with what Ron says, your statement would have to be:

"Indeed, as the gain increases, the frequency decreases and the error rate decreases."

Do you agree?

 

I got it.

In post #42, you said:

"Indeed, as the gain increases, the frequency decreases and the error rate increases."

What Ron Mancini says is:

"As shown in prior chapters, when frequency increases, the op amp gain decreases and errors increase."

You've negated only two of his terms. If we take his sentence and complement the words "increases", "decreases" and "increase", we would get:

"As shown in prior chapters, when frequency decreases, the op amp gain increases and errors decrease."

To be consistent with what Ron says, your statement would have to be:

"Indeed, as the gain increases, the frequency decreases and the error rate decreases."

Do you agree?

Agreed. I overlooked the last part, thanks for the reminder.