"Noise Floor" and S/N ratio in and with FFT graphs

[Terry Demol]

YES I am Jack , I am indeed looking for an extra LN, small signal amplifier as measurement amplifier as front end for the AP2322. just make a nice thread on it?

I experimented a bit and what I struggle with is that if you want very high input impedance like >> 1Mohm for precisely measuring filter response in
real life it depends soo much on the input capacitance of the FET (plus Miller effect) that you only have a high input impedance up to a few kHz....
The 100k Input impedance of the AP are sometimes a limiting factor

any one with ideas are welcome to share of course

Use a bootsrapped IP device. If the IP devices are discrete N channel Jfets then a J111 can make a great bootstrap with it's gate tied directly to IP jfet's
source as it will have at least 5V gate to source voltage with a few mA. It's a good idea to use a 'gate stopper' resistor for the bootstrap device.

I recommend having a play with Ltspice and testing the IP devices susceptibility to high impedance drive with various bootstraps.

If you can tweak it to have fairly flat distortion up to say 10k with a 1 meg driving impedance, the bootstrap is working perfectly.

If the bootstrap device, for whatever reason requires a much lower impedance as seen by the IP jfets drain, use a mixed Jfet / BJT bootstrap pair
such as J111 driving a BC337 (or equiv) in a 'darlington like' connection.

cheers

Terry

 

[dddac]

I ran some impedance tests this afternoon -- Groner did not have a chart in his LA article. See this thread. https://www.diyaudio.com/forums/equ...-audio-vol-3-spare-boards-29.html#post6431001

Thanks jack, I answered in the other thread

 

[dddac]

I see the point of your blog and agree that its dealing with a valuable issue here. Maybe its my nerdy-teacher background coming to the fore - I like to delve into what's behind people's misunderstandings and how they come about rather than simply correct them.

Carry on the good work anyway - I've changed my mind having seen your context - I think it would be less misleading to use 'plot' than 'histogram'.

no offense taken of course. I admit I like "plot" even better now - so I fixed it (again)

 

[dddac]

Use a bootsrapped IP device. If the IP devices are discrete N channel Jfets then a J111 can make a great bootstrap with it's gate tied directly to IP jfet's
source as it will have at least 5V gate to source voltage with a few mA. It's a good idea to use a 'gate stopper' resistor for the bootstrap device.

I recommend having a play with Ltspice and testing the IP devices susceptibility to high impedance drive with various bootstraps.

If you can tweak it to have fairly flat distortion up to say 10k with a 1 meg driving impedance, the bootstrap is working perfectly.

If the bootstrap device, for whatever reason requires a much lower impedance as seen by the IP jfets drain, use a mixed Jfet / BJT bootstrap pair
such as J111 driving a BC337 (or equiv) in a 'darlington like' connection.

cheers

Terry

Thanks Terry for contribution. I always use Spice simulations to do "what if" checks with nested curves and changing variables, so this is a good idea to see the impact of bootstrapping