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Richard Lee's Ultra low Noise MC Head Amp
Richard Lee's Ultra low Noise MC Head Amp
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Old 14th July 2019, 04:01 PM   #651
scott wurcer is offline scott wurcer  United States
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Quote:
Originally Posted by gerhard View Post
Spice 3 does not seem to have it:
We didn't have it either, partially because, "designer forgets noiseless flag and circuit tapes out with "noiseless" resistors."
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Old 14th July 2019, 04:09 PM   #652
Mark Johnson is offline Mark Johnson  United States
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Richard Lee's Ultra low Noise MC Head Amp
Set the tempco of the resistor to zero ppm per Kelvin. Set the temperature of the resistor to zero Kelvin. Done. Yes LTSPICE allows you to individually set the temperature of every device.
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Old 14th July 2019, 04:17 PM   #653
scott wurcer is offline scott wurcer  United States
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Originally Posted by Mark Johnson View Post
Set the tempco of the resistor to zero ppm per Kelvin. Set the temperature of the resistor to zero Kelvin. Done. Yes LTSPICE allows you to individually set the temperature of every device.
But they already have a noiseless flag.
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Old 14th July 2019, 04:24 PM   #654
gerhard is offline gerhard  Germany
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I'm not sure if that has an effect on noise. That was already in Spice 2,
but looking into the manuals, I had the impression that it was only for TC.
But methinks that LTspice gets it right.

While I'm at it:
Node collapsing is very tempting in Spice-like programs, as execution
time explodes with conductance matrix size.
When I have +60 Ohms in series with -60 Ohms, what noise do I get
for the complete branch? 1.4nV/rt(Hz) or 0?
I'll have to try that.

BTW, a 50 Ohm 12 dB attenuator with the other side open has about 60R.
A nice plug-on 1nV/rtHz calibrator.

cheers, Gerhard
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Last edited by gerhard; 14th July 2019 at 04:35 PM.
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Old 16th July 2019, 08:31 PM   #655
Hans Polak is offline Hans Polak  Netherlands
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Playing around with Richards Duraglit, I found that the circuit can just as well be seen as a Common Base but also as a Common Emitter.

Fig 1 below shows the well known topology
Fig 2 is exactly the same, but with all Gnd connections drawn together.
Fig 3 is the same as Fig 2, but with the Gnd connection now shifted to both emitters.
After all, since everything is floating when battery fed, this makes no difference in the functioning, other than that the circuit is now inverting from input to output.
Fig 4 is exactly Fig 3, but now with all components redrawn.

Thus the only difference between Fig 1 and Fig 4 is resp in non inverting and inverting. Apart from that, everything is still exactly the same: noise, distortion and gain with identical resistors R3 and R4.
I have meticulously checked that.

One could say that the topology represents an amp where the junction of the emitters is Vin+ and the junction of C1 and C2 is Vin-
So by just shifting the Gnd connection, the circuit morphs from a Common Emitter into a Common Base, without further changing anything to its performance.


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Old 16th July 2019, 08:57 PM   #656
as_audio is offline as_audio
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excellent insight ..
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Old 16th July 2019, 09:34 PM   #657
Bonsai is online now Bonsai  Europe
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Nice work Hans.

BTW I have updated the presentation with a few more graphs and MC amps
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Old 16th July 2019, 10:34 PM   #658
syn08 is offline syn08  Canada
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Quote:
Originally Posted by Hans Polak View Post
Playing around with Richards Duraglit, I found that the circuit can just as well be seen as a Common Base but also as a Common Emitter.

Fig 1 below shows the well known topology
Fig 2 is exactly the same, but with all Gnd connections drawn together.
Fig 3 is the same as Fig 2, but with the Gnd connection now shifted to both emitters.
After all, since everything is floating when battery fed, this makes no difference in the functioning, other than that the circuit is now inverting from input to output.
Fig 4 is exactly Fig 3, but now with all components redrawn.

Thus the only difference between Fig 1 and Fig 4 is resp in non inverting and inverting. Apart from that, everything is still exactly the same: noise, distortion and gain with identical resistors R3 and R4.
I have meticulously checked that.

One could say that the topology represents an amp where the junction of the emitters is Vin+ and the junction of C1 and C2 is Vin-
So by just shifting the Gnd connection, the circuit morphs from a Common Emitter into a Common Base, without further changing anything to its performance.
Looking at Fig. 4 there's an interesting perspective: the circuit can be considered as an inverting negative feedback circuit, however the loop gain is not easily there; the open loop gain is 2*gm*R4 which is not necessary large enough to approximate the closed loop gain to -R4/R3. The condition would be (2*gm*R4)*(R3/R4)>>1 or 2*gm*R3>>1 or gm>>1/(2*R3) and for R3=10ohm gm~500mS or a collector current over 12mA. The larger the Rs=R3 the lower Ic could be. I wonder how loop gain would look and any feedback loop stability condition, this needs spice. However, intuitively I would think this is unconditionally stable for all possible loop gains.
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Old 16th July 2019, 11:06 PM   #659
syn08 is offline syn08  Canada
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BTW, in the common base view, this is actually what Scott already mentioned: Rs=R3 is an emitter degeneration, and the gain could be, if gm is high enough Rload/Rs=R4/R3.
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Old 16th July 2019, 11:15 PM   #660
as_audio is offline as_audio
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Quote:
Originally Posted by syn08 View Post
Looking at Fig. 4 ... the open loop gain is 2*gm*R4 ...
Open loop gain is not 2*gm*R4.
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