Impedance-bootstrapping opamps

[Fancy Lime]

Hi there,

I've bee wondering why I have never seen this topology in use in a guitar pedal:

Taken from https://www.nutsvolts.com/magazine/article/op-amp-cookbook-part-2

Seems like this would allow to use low input resistance opamps like the NE5532 as high impedance front ends, which may be handy at times. Sure, bootstrapping has it's own problems like low frequency resonant peaks if one isn't very delicate with the capacitor choices, and for most purposes, a TL072 will do and does not need bootstrapping. But sometimes bigger output current, phase-reversal safe input, and potentially lower noise are factors. And if we could get over the input bias current issue that may impose an untenable DC shift when using a 1M bias resistor by using bootstrapping, the humble old 5532 is all of a sudden equal or superior to a lot of very expensive low noise JFET input opamps. So what gives? One major obstacy, I imagine, may be noise? Does anyone know how to properly analyze voltage and current noise of this circuit and or of the simpler bootstrapped buffer? I suspect we would need to lower the resistor values to actually make this worth our while in terms of noise.

Yes, I know, lipstick on a pick in a guitar pedal. But I am actually thinking about a different application, namely a flow meter that works with a magnet in a rotor that passes by a little coil. So basically a guitar pickup but with a very weak signal. I would like to use a robust and cheap bjt opamp for the preamplifier that then goes to the frequency meter. Noise may or may not be an issue, I will have to test that first, so best to know what to expect beforehand.

Andy

[Gus]

From 2011
https://www.diystompboxes.com/smfforum/index.php?topic=93793.msg808042#msg808042

[Fancy Lime]

Hi Gus,

Very interesting read, thanks for the link! Did your experimentation ever go further than documented there?

Andy

[Rob Strand]

Bootstrapping has its limitations - it only works while the opamp isn't clipping.  If the opamp clips the input impedance drops to a low value.   

You can use that to advantage by choosing the input cap to roll-off the lows (which causes clipping).   That's not a new idea either,
https://patents.google.com/patent/US5509080A/en

[Vivek]

My company is trying to get agency rights to sell ultrasonic water flow meters. They have no moving parts, are very linear across huge range of flow rates, don't get affected by sand in the line.

They have LORAWAN transmitter to send flow data per 15 minutes.

[antonis]

I've bee wondering why I have never seen this topology in use in a guitar pedal:

https://www.diystompboxes.com/smfforum/index.php?topic=127881.msg1228436#msg1228446
Reply #19 

[iainpunk]

i think the drawbacks and the design limitations are a bigger factor than paying 30 cents more for a better opamp if you really need one.

LF resonances are a big drawback, and weird and unexpected stuff will happen when they clip.

i have to admit i am biased against bootstraps, since the circuit simulator in my head cant compute them very well.

cheers