C B0: : B0:><C 2K2>4C ?@8H;8 2 5;; 01A 1>;55 70 ;5B =0704. K 2A53> ";8HL 45;05<, :0: =0?8A0=> 4> =0A"
>102;5=> G5@57 1 G0A 23 <8=CBK
These 60 nV/rtHz @100 K is good value and demonstrate low current noise - intrisinc thermal noise density of 100 K resistor is 40 nV/rtHz. Total noise - 60 nV/rtHz (Noise factor around 3 dB), and current noise can be estimated as SQRT(60^2-40^2-12^2)/100 ~ 0.4 pA/rtHz. Direct (and much more precision) measurements by mean of 5 M cooled resistors give results 0.25...0.28 pA/rtHz.
Virtually all noise of UL amp generated into
resistors in input filter-preamplifier, because input impedance couldn*t lowered than 10...12 K due to overloading of previous stages. Therefore opamp with lower voltage noise has no sense for this application.
Simple calculations:
Filter noise gain is 3, input/feedback parallel impedance is 8 K (24 || 12), series to opamp input - 10 K. Total noise resistance - 18K. Total filter output noise density (at shorted input) is
3*SQRT((18*(4^2))+(18*0.28)^2+12^2) = 64.2 nV/rtHz (RTI 32.1 nV/rtHz). Contribution of Op-Amp noise less than one-third by power.
Main amp noise gain is 6.76, total noise resistance 3K (3.47K || 20K).
Output noise density at shorted main amp input is
6.76*SQRT((3*(4^2))+(3*0.28)^2+12^2) = 94 nv/rtHz (16.3 nV/rtHz RTI)
Whole amp RTI noise is SQRT(64.2^2+16.3^2)/2 = 33.1 nV/rtHz.
This value equal to 4.67 uV lin, and 3.3 uV "IEC A".
Overall gain of this amp around 11.5 (2*5.76), or 21.2 dB.
Nominal output voltage at 45V supply is 28V eff, and corresponding input voltage is 2.44V (28/11.5).
2.44V/4.67uV = 522400, or 114 dB Lin
2.44V/3.3uVA = 739000, or 117 dB "IEC A"
Really measured values simply corresponds with abovementioned calculations.
If you can*t give good values, itsn*t impossibleness. Many measurement equipment has S/N ratio much more 120 dB.
Halcro DM58 has measured EIN around 0.42 uV "IEC A". In other point of view, values better than 110 dB haven*t techical reasons, only marketing
.
You didn*t take the principle of this protection - if current sense transistors begin works, trigger circuit on VT9-10 latched and off VAS completely. But when trigger circuit isn*t latched, no any influence to cignal chain.
This circuit slightly slow, but output current can*t rise too fast due to output inductors.
Bipolar output devices are virtually cheapest items on BOM, BD911/912 costs only around 40-50c (!). Therefore its very rational choice.
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