Question about Bootstrap AC-coupled voltage follower

bootstrap capacitor in ac voltage follower

Can we cancel those two caps C1 and C2 to convert Bootstrap AC-coupled voltage follower to Bootstrap DC-coupled voltage follower?

ac coupled op-amp bootstrap

Good question.
I think a bootstrapped DC amplifier will work without the capacitors.

bootstrapping input impedance

In principle it works....It makes R1 look huge...(R2 is just a load and can be removed) But it is useless...
The impedance provided by the opamp is very high (contributed only by bias and leakage currents) and any additional path can only reduce it.
The stability will depend on what impedance is on the input side.., if the input is removed., it can ring heavily.., If the opamp has high offset and/or bias current the follower may not work at all.....Moreover the resistor will increase the noise at the output.

ac coupled

The AC coupled voltage follower is normally used in applications, where it has a purpose. In this applications, it generally can't be canceled. In all applications, that don't need AC coupling on input, a simple voltage follower is more suitable. In some cases, input current or offset voltage compensation may be a solution.

The frequency characteristic and noise behaviour of the shown circuit is worse enough to avoid it, wherever you can.

If you have an application, that seems to be of a third kind, please let me know.

capacitor coupled voltage follower

FvM said:

..............
..............
If you have an application, that seems to be of a third kind, please let me know.

Click to expand...

Yes, there is an application which is used frequently.
If you remove both capacitors you have a normal unity gain amplifier (with a load R2) with a resistor across its input.
This resistor lowers the loop gain and, therefore, increases the stability margin.
Such a kind of "input compensation" is one of several methods to compensate any decrease of stability properties caused by a heavy capacitive load.
Of course, as a consequence the noise as well as the output offset slightly increases.

capacitor coupled voltage follower op amp

I see. It's effect obviously depends on the source impedance.

However, I wouldn't discuss the circuit under the bootstrapped voltage follower topic, which mainly deals with input impedance.

unity gain voltage follower

Yes, agreed.

To complete the description of "input compensation" circuitry:
- the bandwidth is reduced somewhat because of the reduced loop gain
- also the input resistance goes down
- in most cases, the resistor between both inputs is replaced by a R-C-series combination with the advantage, that at least at low frequencies (including dc) the loop gain remains high (better offset properties and PSRR) .

boot strap inverting amplifier

LvW said:

Yes, there is an application which is used frequently.
If you remove both capacitors you have a normal unity gain amplifier (with a load R2) with a resistor across its input.
This resistor lowers the loop gain and, therefore, increases the stability margin.
Such a kind of "input compensation" is one of several methods to compensate any decrease of stability properties caused by a heavy capacitive load.
Of course, as a consequence the noise as well as the output offset slightly increases.

Click to expand...

I have doubts about this LvW.., what you said is right (about input-lag compensation) when you have a resistor between output and inverting input and one of the input is gnd....There the included resistor reduces the loopgain at high frequencies (controlled by the series cap) improving the stability. That cant be applied to dc. Here.., there are two paths from output. One goes to the inv terminal with zero resistance and the other to the non-inv terminal with a finite resistance. So if the input is open or has high driving impedance., the gap between positive and negative feedback becomes narrow causing the output to ring....
In my opinion the circuit plays havoc to the stability rather than enhancing it...
Expecting your views and possible correction

reduce noise voltage follower

Quote: I have doubts about this LvW.., what you said is right (about input-lag compensation) when you have a resistor between output and inverting input and one of the input is gnd....There the included resistor reduces the loopgain at high frequencies (controlled by the series cap) improving the stability.


Yes, you are right. This method requires a voltage divider in the feedback path, but for the unity gain follower (as FvM has stated) the stabilization effect occurs only in conjunction with the source impedance.

That cant be applied to dc. Here.., there are two paths from output. One goes to the inv terminal with zero resistance and the other to the non-inv terminal with a finite resistance.

Sorry. I don´t understand. I don´t see a path between the output and pos. terminal.(By the way - this input compensation scheme is rather common.)
The feedback factor is reduced due to R1-C1 between both terminals; and the "feedforward" factor is reduced exactly by the same amount. Thus, the closed loop gain remains the same with improved stability.

ac coupled voltage follower

This is how I see it...
Stability IMO hinges on parasitic resistance of the short., input capacitances at both the terminals and source impedance...
Also I dont understand you reasoning for the first point even though we both concur ....just a bit too complicated for me

voltage follower ac installation

In your previous post, I hear about DC and ringing. That sounds confuse to me.

Generally, a voltage follower cant have a defined output signal with inifinite source impedance. This is the case either with or without R1 present in the circuit, the behaviour may be different in detail, but the signal meaningless anyway. You don't need to care about possible ringing in this situation.

If R1 is sufficient high, it's effect can be neglected with a finite source impedance, if it's low, the loop gain is reduced. You may be able to calculate a complex source impedance, that results in a reduction of phase margin in combination with R1, but it's more a theoretical consideration.

ac coupled inverting amplifier stability

My questions in the following links force to ask this topic





In this company's design, the op-amp they are using almost has the same input bias current as LT1806/7.

we are using the same sensor.

But why they do not have output offset?

voltage follower ac

Ok lets discuss in detail

FvM said:

This resistor lowers the loop gain and, therefore, increases the stability margin.

Click to expand...

I cant comprehend this.., The entire voltage is fed-back irrespective of anything making beta one (just taking the negative feedback loop's beta). In the classic input compensation., the beta is reduced at high frequencies thus increasing the stability. How is that relevant here when there is a short between output and inv input...

FvM said:

Generally, a voltage follower cant have a defined output signal with inifinite source impedance.

Click to expand...

True., but you dont use a follower when it's input side has zero impedance. Infinite impedance is the case where we can have sustained oscillations (well almost)., but much before that for moderate driving impedances the stability degradation can be phenomenal...the major purpose of using a follower is just this isnt it....
People can get overzealous about this boot-strapping and use low values for R1 (trying to reduce noise and offset) which can have detrimental effect and the possibilities of ringing increase for a low R1.

FvM said:

if it's low, the loop gain is reduced

Click to expand...

How can the dc loop gain reduce without affecting the closed loop dc gain? Are you coming to say that R1 decides the dc gain of the voltage follower?
R1 does not affect the negative feedback loop's loopgain., but it affects only the positive feedback's loopgain

FvM said:

You may be able to calculate a complex source impedance, that results in a reduction of phase margin in combination with R1, but it's more a theoretical consideration.

Click to expand...

All this for the positive fb loop and the stability implications are clearly different.

bootstrap a voltage divider

Hi saro_k_82,

the circuit you have posted helps to clarify - now I see what you mean.
However, the situation is quite clear:

- if the source impedance is zero, the gain is +1 and R1 has no influence.
- if the source impedance is Ro, the gain is still +1 - however, at a reduced loop gain which improves stability. The loop gain now is Hloop=-(R1/(Ro+R1))Ao.

This scheme of mixed feedback (pos. and neg.) is used temporarily to increase stability margin.
And it is interesting that we have derived it from the circuit with "input compensation" for the special case of a unity gain amplifier. Thanks.

Added after 5 minutes: This was written NOT as a reply to the above reply from saro_k_82, as both posts did appear about at the same time. I have to try to unerstand the content.

instability voltage follower

Ah great!.....
LvW., But my point is that the presence of R1 can only decrease stability...See if my reply to FvM makes sense

bootstrapped voltage follower

saro_k_82 said:

Ah great!.....
LvW., But my point is that the presence of R1 can only decrease stability...See if my reply to FvM makes sense

Click to expand...

No, the effect is as follows:
The net feedback factor is lowered as it is now
Hreturn=-1+Ro/(Ro+R1)=-R1/(Ro+R1) (Ro=source impedance).

Therefore the loop gain is lowered and the frequency at the point where the loop gain is 0 dB is reduced with the consequence of improved stability. Right ?

bootstrap voltage follower

I dont think that is so simple LvW.
If you think the loopgain is what you have quoted then why should the closed loop gain be 1 at low frequencies?
The pos fb section must be analysed like we analyse the inverting amplifier block diagram....,
I'll try to derive this and simulate as well..., but I too tired to do this now

Thanks,
Saro

voltage follower offset

saro_k_82 said:

I dont think that is so simple LvW.
If you think the loopgain is what you have quoted then why should the closed loop gain be 1 at low frequencies?
The pos fb section must be analysed like we analyse the inverting amplifier block diagram....,
I'll try to derive this and simulate as well..., but I too tired to do this now
Thanks,
Saro

Click to expand...

OK, you can simulate it or you can explicitely calculate - in both cases the result will be: closed loop gain is 1.

However, there is a simple way to see WHY it is +1:
As the output voltage equals the voltage at the neg. terminal., the voltage at the pos. terminal is practically also identical to the output. That means: no current through R1.
But when there is no current through R1 there will be no current through R0 and, hence, the voltage at the pos. terminal (which is the output) is also equal to the input voltage. That means: Gain of 0 dB.
OK ? Regards.

Yes I understand that..., I dont have doubts about the closed loop gain being 1., but with your expression for feedback factor how do you see that?
If you believe that your expression is right, can you derive and show that the low frequency gain is 1 (Intuition is fine).., I just said that the loop-gain and subsequent stability implications are much more profound than this simple equation...,

saro_k_82 said:

Yes I understand that..., I dont have doubts about the closed loop gain being 1., but with your expression for feedback factor how do you see that?
If you believe that your expression is right, can you derive and show that the low frequency gain is 1 (Intuition is fine).., I just said that the loop-gain and subsequent stability implications are much more profound than this simple equation...,

Click to expand...

Hi saro_k_82,

I have prepared a little pdf document in which I have derived the closed loop gain.
Hope it helps.
Regards.