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
SPICE model

 Figure 2:  Regulated power supply
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.

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

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

 Figure 6:  Stability analysis  showing 80.9º of  phase margin.

### 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.