SIT measurements, Mu Follower, and amplifier build
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lhquam
diyAudio Member

Join Date: Mar 2010
Location: Olalla, Oregon: Land of the 100 Valleys
Quote:
 Originally Posted by grimberg Wouldn't that be 75W per heatsink, left and right, for a total of 150W for both?
You are correct, 150W for both heatsinks.

The fan cooled amplifier I show in post #16 dissipates about 170 Watts per heatsink and provides 25W class-A with about 3% THD.

aspringv
diyAudio Member

Join Date: Sep 2010
Location: Sydney, NSW
Quote:
 Originally Posted by lhquam My mu-follower push-pull information is about ready to post. Unfortunately it is full of math, but I will (eventually) use my the amplifier channels as an example of the math equations.
Looking forward to it, including the math! It's all a good learning, and interesting as well! I like the rigor of having a well defined 'why' for the choices we make on amps.

lhquam
diyAudio Member

Join Date: Mar 2010
Location: Olalla, Oregon: Land of the 100 Valleys
More Sweet Spot theory

Earlier posts presented the sweet-spot relationship:

Id = A*Vd + C*Vd/RL

where the parameters A and C are computed from meaasurements made with a test jig using a single-ended resistor load topology shown in post #2.

The objective of the next few posts is to show how to apply the sweet-spot relationship to the common-sourcemu follower. There is a notational change in the following posts: the quiescent SIT drain voltage Vd becomes X0 and the quiescent SIT drain current Id becomes I0.

Before presenting the circuit analysis of the mu follower, it is useful to examine the behavior of a single-ended resistor load and a 3-terminal generic load, combined with an amplification device such as a SIT, JFET, NFET, NPN transistor, or triode.

This image shows a simple single-ended common-source amplifier similar to the FirstWatt SIT-1. The maian parameter of interest here is RL, the load impedance on the drain of the SIT, and its relationship to β, the current gain (loss) of the due to the load circuit.
Attached Images
 Single-ended-circuit-1.asc.jpg (107.0 KB, 233 views)

Last edited by lhquam; 19th February 2020 at 02:41 AM.

lhquam
diyAudio Member

Join Date: Mar 2010
Location: Olalla, Oregon: Land of the 100 Valleys

The first image shows a generic 3-terminal load circuit, and similar expressions for β and RL. Since the circuit is unknown, β and RL are derived from ΔI, ΔI(Rload), and ΔX which can be measured external to the generic load.
The second image introduces a sense resistor Rsen between the SIT drain and the bottom terminal of the generic load circuit, which allows for the direct measurement of ΔI. Unfortunately, that circuit modification will slightly change the load impedance seen by J1.
The third image introduces a sense resistor Rsen, between the top of the generic load and the rail supply. Since the "generic load" is a 3 terminal device, using KCL (Kirchhoff Current Law),

thus providing the value of ΔI from AC voltage measurements across Rsen and Rload.
With a 1 Watt amplifier output level, an Rsen value of 0R25 (3 Watt, 1% or better) appears provide enough AC voltage drop to enable accurate measurements with a decent multimeter.

Caveat: All of the above computations for β and RL are based on RMS voltage measurements and are correct only if the load circuit does not introduce significant phase shifts, in which case RL would have to be represented by a complex (reactive) impedance and a totally different measurement approach would be needed.
Attached Images

lhquam
diyAudio Member

Join Date: Mar 2010
Location: Olalla, Oregon: Land of the 100 Valleys
Common Source Mu Follower Circuit Parameters

The (simplified) mu follower circuit in shown in the first image below. For simplicity, the actual bias circuit for the mu follower NFET M1 has been replaced by a battery symbol. The circuit on the right is a further simplification where the NFET M1 and resistor Rhi are replaced by the undegenerated NFET M1e. M1 has the transconductance gm at the operating point, but is degenerated by Rhi. M1e has the transconductance gme=1/(1/gm+Rhi), which is the effective transconductance of M1 degenerated by Rhi.
The circuit equations for the schematic on the right are simple to solve, with the results shown in the second image. For the brave, the set circuit of equations and solutions is shown in the third image.

The net result is that we obtain the simple equations:

β = Rmu*gme+1

We have multiple ways to compute the values for β and RL:

Circuit Equations using known parameter values. The mu follower NFET transconductance (gm) is dependent on bias current and to a lesser extent the Vds of the NFET. For best results, gm should be computed based on calibration of the individual NFET at the desired values of I0 and Vds.

In-circuit measurements, measuring ΔI based on RMS measurements across either Rmu as in the 2nd image of the previous post, or Rhi as in the 3rd image in the previous post.

I will be continuing this thread with mu follower design approaches and examples. It is taking longer than I expected to get things right.
Attached Images
 SIT-mu-follower-M1-simp-2a.asc.jpg (156.3 KB, 238 views) SIT-mu-follower-M1-simp-2a-solns.jpg (44.7 KB, 71 views) SIT-mu-follower-M1-simp-2a-eqns-solns.jpg (83.0 KB, 48 views)

 19th February 2020, 04:17 AM #66 grimberg   diyAudio Member     Join Date: Oct 2002 Location: California, USA lhquam, I would like to use the modified version of the Franken Tracer as you showed in post #4, but I haven't been able to find the 500W Photo Flood lamp. Where did you get yours?
lhquam
diyAudio Member

Join Date: Mar 2010
Location: Olalla, Oregon: Land of the 100 Valleys
Quote:
 Originally Posted by grimberg lhquam, I would like to use the modified version of the Franken Tracer as you showed in post #4, but I haven't been able to find the 500W Photo Flood lamp. Where did you get yours?

Search for: Eiko ECT 120v 500w Photo Flood Lamp

 19th February 2020, 07:14 PM #68 kannan_s   diyAudio Member   Join Date: Jun 2009 these 300W lamps are in Home Depot
 21st February 2020, 02:44 PM #69 alexberg diyAudio Member     Join Date: Oct 2009 I believe this wil do as well: https://images-na.ssl-images-amazon....L._SL1000_.jpg
 21st February 2020, 03:23 PM #70 kannan_s   diyAudio Member   Join Date: Jun 2009 Not Halogen lamps for this purpose

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