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Copyright 2018 by Wayne Stegall
Created May 15, 2018.  See Document History at end for details.




One-Bend Amplifier

Part 5: Set bias for class B/AB and evaluate results

Introduction

Because class-a amplifiers are so inefficient and often low-powered, I thought it would be interesting if the one-bend amplifier would produce similar results in an efficient class b.  Now in class-b more power is possible so I sought to raise the power supply voltage to the output stage to 60V and the driver stages to 75V to support 250W into 4Ω with 50% headroom.  Note that this is an evaluation not a redesign, if the results are acceptable the input and driver stages would possibly be adjusted to ensure that no components are out of their safe ranges in a later exercise.

Circuits

Note:  Component values can be obtained from the SPICE models for now.

Figure 1:  Amplifier Schematic
ampmoscfb4.jpg
SPICE model


Figure 2:  Regulated power supply
pwrsupply2.jpg
SPICE model




Spice Results


Forgo hum analysis for now.  Instead begin with error transfer curves.

Figure 3:  Small signal error curve.
Shows dominance of second harmonic and some third.
 
Figure 4:  Large signal error curve.
Shows dominance of second harmonic then third.
trerrsss.jpg

trerrls.jpg

These error curves plotted with convergence difficulties.  I think the following distortion analysis would suggest that they are smoother than they appear.
Continue with full-power distortion analysis at 1kHz.

Fourier analysis for vout:
  No. Harmonics: 16, THD: 0.0408884 %, Gridsize: 200, Interpolation Degree: 3

Harmonic 
Frequency  
Magnitude  
Norm. Mag  
Percent  
Decibels










1 1000
44.7200000
1.0000000
100.0000000
0.00
2 2000
0.0181631
0.0004062
0.0406151
-67.83
3 3000
0.0019576
0.0000438
0.0043776
-87.18
4 4000
0.0003447
0.0000077
0.0007708
-102.26
5 5000
0.0004991
0.0000112
0.0011161
-99.05
6 6000
0.0000647
0.0000014
0.0001448
-116.79
7 7000
0.0003209
0.0000072
0.0007176
-102.88
8 8000
0.0000362
0.0000008
0.0000808
-121.85
9 9000
0.0002477
0.0000055
0.0005538
-105.13
10 10000
0.0000288
0.0000006
0.0000644
-123.83
11 11000
0.0001972
0.0000044
0.0004410
-107.11
12 12000
0.0000240
0.0000005
0.0000537
-125.40
13 13000
0.0001604
0.0000036
0.0003588
-108.90
14 14000
0.0000208
0.0000005
0.0000465
-126.64
15 15000
0.0001320
0.0000030
0.0002952
-110.60

Certainly we are interested in frequency response

Figure 5:  Bode shows frequency response down 3dB at 524kHz
bode.jpg

For stability the open loop phase margin at the closed-loop -3dB frequency should be more than 45.

Figure 6:  Stability analysis  showing 80.9 of  phase margin.
stabilityol.jpg








Conclusions

The desired predominance of second harmonic expected as a result of the inner feedback loop is the same as the previous class--a result.  After that the harmonic profile does not taper off like class-a but rather odd-harmonics are preferred perhaps an expected result.  Perhaps a less pure class-a sound would result than previously.  I think the class-b version is worth further investigation.



1Note:  Raw Fourier analysis data has been processed in spreadsheet to calculate related results and reformatted.
3See related article:  Floating Source-follower Regulator.

Document History
May 15, 2018  Created.