Noiseless Preamplifier

181

02:35:10

A preamp with a theoretical minimum THD+N value of 0.00064%. In reality due to thermal noise (as it is most prominent at these levels) and nonlinearities of the semiconductor devices the value would be something like 0.005% or so, maybe even a little higher, depending on the components used, the layout of the topology, the stability of the supply rails etc. Basically this result is best case scenario, without all the deterioration factors. Use ultra low noise transistors like BC550 or BC547 and their complementary counterparts. The ripple rejection ratio is also very good at -120dB.  I found out that keeping the 6.8kOhm resistor greatly improves the ripple rejection and the overall noise. It gives the input a ground reference, which is usually prone to picking up noise from all sorts of places

published 9 years ago

hurz

9 years ago

Again, within EC there is no 'thermal noise' ask Igor. There is some numerical noise as in any computational evironment, but no extra thermal one! And you would be surprised how much thermal noise this circuit has if you build it in real. The numerical noise is several decads below the real thermal noise! To many junctions to be "noiseless" . What you propably mean is "low noise". All components have thermal noise. There are good reasons to make preamps as simple as possible and use maybe upto 2 BJTs maximum to stay in lownoise. But having tens of them with high openloop gain together and many high ohmic resistors doesn't really make sense.

Robert_Kidd

9 years ago

Granted!

thebugger

9 years ago

As I said thermal noise is prominent in reality, not here, but even then the 33k resistor has a noise level at 7uV for a bandwidth of 100Khz and 4uV for a bandwidth of 20KHz. Combine that will all the other resistors and you still get a noise level of less than 40uV. Even if this gets amplified the overall gain is very low and the value will not exceed 150uV, which is still a very acceptable value. 2 transistor designs can almost never reach that value, and require either a bootstrap or a constant current source to bring their linearity close to these levels (both of which are present here). That's why people usually use ultra low noise preamps for their designs, because they're as close to perfect as they can, and thia is basically a copy of the internal schematic of one such op amp.

thebugger

9 years ago

http://everycircuit.com/circuit/6369553241604096

thebugger

9 years ago

One more thing i wanna add. These values I've chosen are only like that so that EC doesn't show me no solution. Typically I advise to keep all feedback resistor values low, but not low enough to load down the output. Moreover I try to use as little resistors as i can in preamp circuits

hurz

9 years ago

One of your comments from 2. of july 2016 "The app is actually pretty close to reality. It has some thermal noise that starts the oscillations, ..."

thebugger

9 years ago

It's as close to a thermal noise as a simulation can be. It's actually numerical noise, I made an example of it, a few months ago. I placed 5 or 6 op amps at maximum gain and they brought it to a visual level. I think it was some -150dB - -200dB. Anyway the last comment has no meaning to what we're discussing here. Give me an example why a two stage preamp is better than a full on op amp preamp? I've given enough examples to the contrary

Robert_Kidd

9 years ago

Can't wait for the answer .....

thebugger

9 years ago

Won't get one. The lowest THD of 2 stage preamps can be no less than 0.1%, and in Hi-Fi amps this is unacceptable. Basically when it comes to choosing, the power stage is desirable to have the higher distortion, the preamp stage should be as clean as possible, because it carries the distortion for further amplification.

hurz

9 years ago

Wrong. Noise is more important! THD for a preamp is less.

thebugger

9 years ago

Yes, that's true, but all is transfered for further amplification to the power amp, so really it doesn't matter. I calculated the noise from the resistors and it doesn't exceed 150uV. Even so, after the power amp the noise level transfered by the preamp should be no more than 1mV (depends on power amp gain) so it willbe negligible

hurz

9 years ago

hurz

9 years ago

no references but bullshit comments. troll.

hurz

9 years ago

You work hard to downgrade EC niveau. Keep up!

hurz

9 years ago

@robby-kiddi, you removed again by accident your comments!

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