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SIT measurements, Mu Follower, and amplifier build
SIT measurements, Mu Follower, and amplifier build
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Old 3rd February 2020, 02:46 PM   #1
lhquam is offline lhquam  United States
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Location: Olalla, Oregon: Land of the 100 Valleys
Default SIT measurements, Mu Follower, and amplifier build

Here are the main topics I intend to discuss:
  • Sweet Spot Measurements of the Tokin 2SK182ES SIT.
  • The SIT mu follower and its circuit relationships. I found some very simple equations describing the behavior of the mu follower.
  • My build of an amplifier similar to the FirstWatt SIT-2.
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Old 3rd February 2020, 02:49 PM   #2
lhquam is offline lhquam  United States
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Default Sweet Spot Measurements of the Tokin 2SK182ES SIT

The "Sweet Spot" is a circuit operating point where H2 (the 2nd harmonic distortion) is minimized, as described in http://www.firstwatt.com/pdf/art_sweet_spot.pdf. Later in this thread I will describe how understanding the sweet spot is important to the design of common source, single-ended and mu-follower amplifiers such as the Pass SIT-1 and SIT2.

In the next few postings I present some measurements of a single Tokin 2SK182ES SIT. I have made similar measurements of two other 2SK182ES SITs that currently reside in a stereo amplifier similar to the SIT-2 that delivers more than 20W/ch into 8 Ohms at about 2% THD into 8 Ohms. More about that amplifier later.

The first image below is a schematic showing the SIT in common source mode driving a resistive load Rload connected to a variable rail voltage Vx. The SIT gate is driven by an adjustable DC voltage from a signal generator, modulated by an AC sine signal from a DAC driven by a software spectrum analyzer. The AC voltage from the SIT drain is connected to the ADC input to the spectrum analyzer.

For a given resistor value Rload and rail voltage Vx, the DC voltage Vgs is manually adjusted to obtain a minimum value for H2, the 2nd harmonic. The DC voltage Vd of the SIT drain is measured. The drain current is calculated as Id=(Vx-Vd)/Rload. This operating point (Vd, Id, Vgs) is a 'sweet spot".

Here is a set of measurements with Rload = 8 Ohms.

Vd . . . . Id . . . Vgs
16.83 0.74500 -3.79
24.03 1.01625 -4.25
33.23 1.36250 -4.72
37.90 1.53375 -4.92
42.10 1.67750 -5.10
52.20 2.09375 -5.52
58.40 2.31375 -5.83

The second image is a plot of Id vs. Vds for these measurements at 8 Ohms. It is interesting to note that all of the sweet spots fall on a straight line in the Id vs. Vds plot.

The third image is a plot of Vgs vs, Vds for these measurements, showing the same linear behavior.
Attached Images
File Type: jpg SIT-sweet-spot-measurement.jpg (109.6 KB, 696 views)
File Type: jpg Id-vs-Vd-Rload8-sweetspots.jpg (26.6 KB, 703 views)
File Type: jpg Vgs-vs-Vd-Rload8-sweetspots.jpg (26.9 KB, 679 views)

Last edited by lhquam; 3rd February 2020 at 02:54 PM.
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Old 3rd February 2020, 02:50 PM   #3
lhquam is offline lhquam  United States
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Location: Olalla, Oregon: Land of the 100 Valleys
Similar sweet spot measurements were made with 4R, 6R, and 10R loads. The first image below shows Id vs. Vds plots of all the four data sets. Again the data falls on straight lines.

The second image shows Vgs vs. Vd plots of the four same data sets. It is very interesting that the Vgs vs. Vd curves fall nearly on top of each other. Given uncertainty about the FET junction temperature of each measurement, and instrumentation errors, is it possible that Vgs vs. Vd is independent of the Id and Rload?

The third image shows a another view of the Id, Vds, Vgs space. (I haven't figured out a way to make the 3d plots available for adjustable viewing in the forum posts. Any suggestions would be appreciated).
Attached Images
File Type: jpg Id-vs-Vd-all-Rloads-sweetspots.jpg (39.2 KB, 685 views)
File Type: jpg Vgs-vs-Vd-all-Rloads-sweetspots.jpg (37.5 KB, 664 views)
File Type: jpg IdVgsVd-all-Rloads-sweetspots.jpg (45.2 KB, 104 views)
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Old 3rd February 2020, 02:52 PM   #4
lhquam is offline lhquam  United States
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Default FrankenTracer measurements

I also collected data using a varient of Michael Rothacher's FrankenTracer: Frankentracer Schematic | AudioMaker. The schematic of my test setup is shown in the first image, and the second image shows the Id vs. Vds curves for various Vgs values. The third image shows these plots combined with the sweet spot measurements.


In some following posts I will attempt to show the relevance of this data to the design of my amplifier.
Attached Images
File Type: jpg FrankenTracer.asc.jpg (88.7 KB, 227 views)
File Type: jpg FrankenTracer-plots.jpg (44.9 KB, 233 views)
File Type: jpg Sweet-Spot-FrankenTracer-plots.jpg (71.1 KB, 175 views)
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Old 3rd February 2020, 03:25 PM   #5
Zen Mod is offline Zen Mod  Serbia
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SIT measurements, Mu Follower, and amplifier build
good work , as always

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Old 3rd February 2020, 07:03 PM   #6
woofertester is offline woofertester  United States
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It is a pleasure to read your work. Inspiring as well.

I really like the IXYS depletion mode N-channel parts. I wish they offered them in a TO227 hockey puck. That package would be perfect for a Mu follower Tokin SIT build.
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Old 3rd February 2020, 07:21 PM   #7
lhquam is offline lhquam  United States
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Join Date: Mar 2010
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Quote:
Originally Posted by woofertester View Post
It is a pleasure to read your work. Inspiring as well.

I really like the IXYS depletion mode N-channel parts. I wish they offered them in a TO227 hockey puck. That package would be perfect for a Mu follower Tokin SIT build.

I am using the IXYS IXFN140N30P in the optocoupler based mu follower. No need for depletion mode.
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Old 3rd February 2020, 07:35 PM   #8
El Reino is offline El Reino  Finland
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SIT measurements, Mu Follower, and amplifier build
Good information, thanks
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Old 3rd February 2020, 09:47 PM   #9
Antoinel is offline Antoinel  United States
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SIT measurements, Mu Follower, and amplifier build
Thanks lhquam.
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Old 7th February 2020, 04:22 AM   #10
lhquam is offline lhquam  United States
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Join Date: Mar 2010
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Default SIT Sweet Spot Surface

I found the the sweet spot data for the form {Vd[i],Id[i],Rload[i]} have a fairly simple equation
involving only 4 coefficients [A,B,C,D].

Id = A*Vd + B/Rload + C*Vd/Rload + D

This surface fit to the data plotted in post #4 is a twisted plane, as shown in the plots below. The mesh is rectangular grid in Id and Rload, with the height of the surface representing Id. The least-squares fit has a standard deviation of only about 25mA, which very good considering the instrumentation accuracy.


The equation above can be solve for Rload:


Rload=-(C*Vd + B) / (A*Vd - Id + D)


The significance of this result is that the parameters of the mu follower can chosen for a sweet spot at the desired idle state Vd and Id. More about this in later posts.
Attached Images
File Type: jpg IdVdRload-surf-2.jpg (43.2 KB, 156 views)
File Type: jpg IdVdRload-surf-1.jpg (59.8 KB, 141 views)

Last edited by lhquam; 7th February 2020 at 04:34 AM.
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