Bootstrapping an opamp opamp???

[Bill Mountain]

I don't know much about bootstrapping.  I can recognize it on occasion but it seems to be utilized differently for different applications or devices.  I also don't understand what items can be bootstrapped or what the primary function of bootstrapping is.

All of this was written to explain why I don't know the answer to this question:

Can you bootstrap an opamp?  What would be the purpose or benefits?

I am mainly concerned with increasing the input impedance of BJT opamps.

Thoughts?

Ideas?

Criticisms?

Funny jokes?

(Hey R.G. if you read this have you tried the VS-XO on bass?)

[BDuguay]

I got nuthin....constructive that is. I just want to know what it means to be honest so I'm subscribing....
B.

[electrosonic]

There is an example in the art of electronics of bootstrapping to raise the input impedance. There is a discussion of that here-

http://electronics.stackexchange.com/questions/77483/bootstrapped-resistor-a-current-source-or-open-circuit

Andrew.

[GibsonGM]

Yes, the purpose of bootstrapping is to increase input impedance.  You can probably find many examples of this related to tubes, like in the cathode follower.  Works VERY well with them and has a lot of interesting consequences.  I don't know if it's done (or considered to be) the same with opamps...never had occasion to try it!   If you want to increase the input Z, you could use a FET buffer ahead of it.  Or use a FET input opamp.  But if it is done, you'd probably have a lower parts count and may therefore be worth exploring.

For the sake of education, this is a pretty cool question, and I look forward to reading any guru replies if anyone has any info about it.

[R.G.]

I don't know much about bootstrapping.  I can recognize it on occasion but it seems to be utilized differently for different applications or devices.  I also don't understand what items can be bootstrapped or what the primary function of bootstrapping is.

All of this was written to explain why I don't know the answer to this question:

Can you bootstrap an opamp?  What would be the purpose or benefits?

Bootstrapping is using the output of something to make the input better, or more tolerant, or wider range or some such.

The commonest use is to use the output of an amplifier (opamp or not, doesn't matter) to effectively raise the input impedance of the amp by moving the source of the bias voltage around nearly as much as the input moves. With a smaller voltage across it, the biasing resistor looks much bigger than it is because less signal current flows through it. Note that this has to be done with a NONinverting amplifier. And that it involves positive feedback just below (!) the point of oscillation.

Two more instances of bootstrapping come from power amp practice. In old-SS amp days, a capacitor from the output was tied to a split supply resistor to the driver stage. When the output went high, the supply to the driver rode up on the capacitor voltage, and there was more voltage available to drive the outputs closer to positive saturation. Again, a form of selective positive feedback, although what's fed back is the power supply. Another use is to feed the output through a pair of zeners to be a +/- power supply to a low voltage opamp to let its output, which might be limited to +/-7-8V run an output stage that needs a +/-40V swing. Again, the output is moving something around to make it easier/possible.

I am mainly concerned with increasing the input impedance of BJT opamps.

No problem. The first version works. Split the resistor from (+) input to bias, feed the split from the output through a capacitor and perhaps resistors making @^&$*#^( sure the gain never reaches unity around the loop.

(Hey R.G. if you read this have you tried the VS-XO on bass?)

I haven't - I've told so many bad "my bass player did this... " jokes that the ICBPRABS (International Commitee for Bass Players' Rights And Benevolent Society) won't let me touch a bass any more. But two of the guys at the office are bass players, and they really like bass through it.  

[Gus]

Link from 2011
http://www.diystompboxes.com/smfforum/index.php?topic=93793.0

[Bill Mountain]

Link from 2011
http://www.diystompboxes.com/smfforum/index.php?topic=93793.0

Of course Gus has a circuit!

I should have known.

Thanks for the help everyone.

I'm a little confused by RG's statement not to let it approach unity gain but I'll read it a few more times to see if it clicks.

[merlinb]

"Bootstrapping" is another word for impedance multiplication. It is a method of making an impedance appear larger -or indeed smaller- than its 'out of circuit' value (we make it appear smaller by multiplying it by less than 1).
Commonly we only use the word 'bootstrapping' when we do it deliberately, to increase a load impedance for example, but there are numerous other examples of impedance multiplication. The Miller effect is a kind of bootstrapping where the input capacitance is made to appear larger, for example. Bridging two amplifiers bootstraps the loudspeaker, making it appear smaller in impedance to either amplifier , so more current can be dumped into it.

[Bill Mountain]

So...like this???



Note:  I accidentally took a screen grab with the buffer switch closed.  I basically just wanted to show my idea of a switchable buffer/booster.

I read about gain issues so I don't think this would work for a high gain distortion but is this application doable?

[Bill Mountain]

I just thought I'd pump this to get some thoughts on my schematic.

[PRR]

> So...like this???

No. That has a gain of +3.5. It wants to be un-stable. If the source impedance is less than 3.9K it is stable. However everything gets open-inputted at some time or another.

I don't swear it WILL oscillate. Snakes don't always bite, guns don't always blow your toe off. But better just to not play with snakes or guns.

What is the UNITY gain point in a non-inverting opamp? The "-" input *follows* the "+" input. It is unity-gain.

It is often a high impedance. We can often choose the "-" input node impedance at will. Here we wish it to be low, much-much lower than the bias resistor.



BUT.

> increasing the input impedance of BJT opamps.

Rarely a problem. You do not need DC accuracy. You can accept quite large DC shift. Additionally in DIY you can afford to cast-out the few worst-case chips.

NE5532 is the only really greedy opamp in our drawers. Bias current is rated 0.2uA typical 1uA worst-case. Try a 1Meg bias resistor. You will typically have a 0.2V offset, 1V worst-case.

You are under a 9V (or more) power rail. Your target idle point is +4.5V. You will typically end up at +4.7V; +5.5V worst-case. That's still well within the rails. Overload is a bit less, but if you are that close to the edge then any over-enthusiatic player will blatt the thing anyway. And at $0.55/each, you can toss a leaky chip and try another.

Whether 4.5V or 4.7V or 5.5V, you are still goiong to use a coupling-cap to lose that bias level before you go outside the box or to any ground-referenced network (tone/vol pots). No difference here.

Yes, there is a number on the NE5532 data sheet showing input impedance as 30K worst-case. Not an issue. Under NFB this increases by the excess gain. NE5532 GBP is 10MHz, so it has gain of 1,000 at 10KHz, the top of the guitar band. Say you close-loop at gain of 3.5. You have 300:1 of excess gain. 30K*300= 9Megs intrinsic input impedance.

Before you go nuts with bipolar inputs on HIGH impedance sources... transistors have significant input current. Everything has a randomness. There is input current noise (hiss). NE5532 says 0.7pA/rtHz. That's 70pA across 10KHz. Assume a 250K ohm source. We have 0.000,02V or 20uV of hiss voltage. The cheaper TL072 will do 2uV hiss voltage at any audio impedance. And is quite fine about 10Meg bias resistors.

'5532 is optimum for hiss for source impedances near 7K ohms. The optimum is wide; '5532 is often a great pick for sources from 1K to 50K. However guitars are one thing which *may* have impedance over 50K. '5532 *may* not be the best choice.