How it works - Bootstrap circuit in audio output stage
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- เผยแพร่เมื่อ 7 ต.ค. 2018
- A look at how this circuit performs its magic. Demonstration of the circuit on a breadboard will be shown in a followup video. I know the question will be asked, "Will I use this in the discrete amplifier project circuit?" The answer is no.
I want to clarify the point about capacitor distortion: I should have said that the voltage change across the capacitor is very small with the AC signal while in the circuit's pass band. The charge on the capacitor will remain about the same (~5v in this example).
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This channel is a basic electronics encyclopaedia. Simple, practical, easy - what more could someone want from a basic electronics course? I had never seen an explanation of what the boot-strapping circuit did before - and for that matter, I didn't even know what it was until now.
Thanks for the good lesson. Simple and clear.
Good video explaining bootstrapping. Instructor has chosen a simple circuit to make it understandable. Well done !
Crazy how someone came up with this. I guess if you spend time thinking about current flow it eventually would occur to you to be able to do stuff like this
Good, simple to understand explanation of bootstrapping! Thanks, John!
Great explanation! Had seen this described in Douglas Self’s work but didn’t quite understand it. Thanks for posting these, it’s a really great channel 😁
Great videos. Really helps demystify what is happening in audio amplifiers.
SUBSCRIBED!
Just ran across you channel and am wanting to homebrew some audio amps for analysis and deeper understanding, so appreciate your video!
Thanks & 73...
Howdy again.
Bootstrapping is like adding a "servo" to the input ...
Regards.
Thank you very much for precious information!!!
Dude, thanks so much for this video!
It will surely help me a lot with an amplifier I'm working on!
Nice work John and thanks for the nice explanation- as always
8:23 at this time cap is charged so its left plate should be #Negative instead of positive , in next stage of its operation when upper Bjt are try to be in active condition its output voltage ≈ + 10 ( or 10 volt center ground ) will add up with 5 volt change stored in cap giving overall +15 volt ,
#but......
You have not included Diode just below the positive supply rail so that Bootstrap boosted voltage have two way to go ..
One towards Base of upper bjt
And second it should also go towards +10 V since there's Potential difference
Of 15 - 10 = 5 volt !
Plzz review and confirm it .
M i right ?
Excellent explanation
Great video !!
How to calculate the value of the bootstrapping cap?
That took alot of mystery out of the whole bootstrapping business.
Thanks sir nice videos God bless you
thank you for share youw knowledge,
I'm thankful!
Very Clear...
1) Thanks for another great video! 2) Your technique for showing the circuit riding the sine wave created excellent visual imagery. I really got it! 3) I can get easily confused, trying to sort out the difference between an AC signal and an oscillating DC signal. But, I think that you may have misspoke, when you said that a boot strap circuit requires an AC signal. For a single, positive going supply, doesn't the operation of the boot strap just require an oscillating DC signal? Meaning that the signal must fluctuate, but it does not have to cross below 0 (zero) volts? I look forward to you straightening me out.
I was hoping that you were going to drive the circuit into clipping, showing us on the oscilloscope, then connect the bootstrap cap to the output to demonstrate the effect in practice
8:23 at this time cap is charged so its left plate should be #Negative instead of positive , in next stage of its operation when upper Bjt are try to be in active condition its output voltage ≈ + 10 ( or 10 volt center ground ) will add up with 5 volt change stored in cap giving overall +15 volt ,
#but......
You have not included Diode just below the positive supply rail so that Bootstrap boosted voltage have two way to go ..
One towards Base of upper bjt
And second it should also go towards +10 V since there's Potential difference
Of 15 - 10 = 5 volt !
Plzz review and confirm it .
M i right ?
@JohnAudioTech good video, if you choose too large of a capacitor will oscillation occur, how do you choose the value? Do you calculate a rc filter using just the lower of the two resistors? Thanks
can u do a video about audio equalizers , and high , low , band pass filters , that will be great
In some pre assembled audio boards, the Bootstrap capacitor values are either reduced or increased than what's given in datasheets of the particular ICs used. What difference does it make by reducing or increasing the capacitor values ?
Howdy.
Blimey. I have been in error. (arrarrgh ...).
I thought what you explain was neutralization. Much obliged for the correction.
Neutralization evidently is applying negative feedback at the frequency the amp. has a tendency to oscillate.
Regards and many thanks.
Hey how do you find the appropriate value when creating amplifier , given just any transistor ! Where do you start from
I expected to see a transistor as a constant current source. Is it in another video?
will caps affect the slew rate and create phasing?
With global feedback monitoring the output after the DC filtering cap, it can cause the DC offset to drift upwards(before the DC filtering cap, not after it), but that can be fixed by adding some local feedback.
The capacitor isn't actually in the way of the signal, it just gives the supply rail extra current to work with. However, it can cause unwanted DC biasing of the output that gets hidden by the DC filtering cap if using global feedback, and that can cause premature/unbalanced clipping and inefficient operation, but a single resistor is all that it takes to fix it. Just add a resistor between the signal and output.
The issue has to do with the opamp in a global feedback arrangement 'chasing' the voltage droops of the output. It causes the output to become more and more DC offset behind the DC filtering capacitor.
12:57 at higher frequency cap impedance goes up !!!
Or impedance goes down ?
I think John’s referring to very high frequencies, much higher than audio, when an electrolytic capacitor becomes like an inductor and the impedance goes up.
@@jrausa1 it's seems Extraordinary
Some people bypass bootstrap capacitor with ceramic or film cap of 0.1u. Is there any benefit of doing so? Thanks
I see littler benefit in that as the resistors in the bootstrap circuit are orders of magnitude higher than the ESR of the bootstrap cap.
PLEASE USE ELECTRON FLOW, IT MAKES MORE COMMON SENSE TOO!
may be this capacitor is not required, if constant current source is used in place of resistor ? 🤔
He literally said that in the video.
Bostrap....isthe best...in idonesian
Will bootstrap circuit increase audio output power?
Gleeer tung glerrrrrr....