Cornish Buffer Help

[yeeshkul]

So, if i rephrase my question - do i need the bootstraping at all if i need a buffer with roughly about 100k input impedance? I guess i can use just a standard topology and drag the input impedance down by something like a 100k pulldown across the input, right? Then i can keep the standard base bias divider at something like 470k/220k, bacause those resistors count as in parallel together and in parallel with the pulldown when counting the input impedance.

[bonehead1972]

I went through the posts and now it looks a bit more clear (to me) why part values are crucial in this case. I measured all the resistors and found that R4 and R5 read 118k instead of 120k. Would you suggest to replace them for 120k ones, or it can go fine as being "part tolerances" it's been already soldered:) by the way I tried 2.2M resistor across the input and it sounds a lot better to me..
Thank you guys!

[antonis]

I measured all the resistors and found that R4 and R5 read 118k instead of 120k. Would you suggest to replace them for 120k ones, or it can go fine as being "part tolerances"

Let them there, as they are..

Base bias voltage difference should be about 80mV (both for voltage divider higher point and Base bias resistor lower voltage drop) meaning Emitter should be biased at a level 80mV higher than theoretical value..
(no big deal at all..)

As for input impedance, it should be less than 2% lower than the one estimated for 120k..
(no big deal again..)

[antonis]

@yeeshkul: Take into account that 10k series resistor should be added on signal source output impedance rather than on buffer input one..
(despite its placement after R1..)
It actually "lowers" buffer's input impedance in the mean of dominating voltage divider level (Z_buffer / [Z_source + Z_buffer])

[yeeshkul]

Thank you Antonis!

[antonis]

Thank you Antonis!

You're welcome..
But interchange R2 & R3 to have a happy biased buffer..

[daxliniere]

Hey Antonis and Garcho. Did R6 end up needing to be changed based on this discussion?
It's been a while, so the TL071 sch in in post #7 (https://www.diystompboxes.com/smfforum/index.php?msg=1177111)

Thanks in advance!
Dax.

R6 value should be in the range of 470R - 1K

Assuming max output of 20mA, and TL072 200 Ohms output, a 100 Ohms resistor would make maximum 30mA, which is maybe 10mA too much, am I slaughtering it again? 470 Ohms is ~15mA, 5 lower than max?

I've the bad habbit to always consider op-amps output impedance almost zero..

So, for full +/- 4.5V swing, output current should be limited to slightly less than 10mA for 470R load..
(of course, it could be double but many people don't feel comfortable designing an op-amp working at its maximun current rating..)

[antonis]

I presume neither Garcho nor me dealt further with that resistor..

[daxliniere]

Thanks Antonis! But does that mean the 100R drawing 30mA was okay? Your discussions seemed to suggest 470R would avoid blowing up the TL072 with overcurrent. Or it wasn't drawing 30mA?

Cheers!
-Dax.

[antonis]

It's a bit more complicated..

TL072 has an internal output current limit resistor (100 to 300 Ohm, if I recall it right..) for short-circuit prtotection so, without any additional resistor and in case of next effect  very low input impedance the signal would cancel negative feedback (most possible it should turn into an oscillator..)
(what you connect at the output will be fed back at the inverting input)
By adding extra series resistor (outside the feedback loop) you effectively raise next stage input impedance (in the cost of signal amplitude attenuation, of course..) reducing its effect on present stage NFB..

The above stand both for DC (almost GND) & AC (significant capacitance) heavy loads..