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Copyright 2012 by Wayne Stegall
Created June 9, 2012.  See Document History at end for details.




One-Bend Amplifier

Part 3:  Modified circuit adds a predriver stage


Introduction

The quest for more slew rate seemed to suggest two paths:  find lower capacitance power MOSFETs or add a predriver stage.  Initial searches for faster transistor seemed a dead end,  the IRF510 already had low capacitance for its abilities, and there were seemingly no lower capacitance output devices than the IRFP240 and IRFP9240 in a large plastic TO247 package.  I was reluctant to add a predriver as well because I did not want to increase transfer curve complexity or lose the large phase margin that I apparently had.  However, in the end I tried the predriver anyway. 

Circuits

The predriver adds a buffer stage M3 and M4 to drive the 5000nF of total gate capacitance of the output transistors.  After verifiying the circuit with resistor bias for the predriver, I decided that current bias consisting of M5, Q4, and their related components would be a more certain way to set the bias in the face of expected VT variations in M3 and M4.

Figure 1:  One-bend amplifier with predriver stage added.
ampmoscfb3

Adding the predriver circuit posed to take away 3 more volts of voltage headroom from the amplifier, an undesirable outcome.   To compensate, I added another transformer to the power supply to raise the first, second, and predriver stage supplies above that of the output stage.

Figure 2:  New power supply.
pwrsupply2


SPICE Results

SPICE model of amplifier
SPICE model of power supply

Power Bandwidth


Attempts to measure slew rate resulted in the difficulty of deciding which of the multiple slopes actually was representative.  So I followed the recommendation of a TI technical document that suggested that power bandwidth be determined as the frequency at which the distortion rises to a certain level, possibly 1%.  I experimented with the previous circuit to find powerbandwidth calculated from slew rate at about 1.2% distortion.  By experiment, the power bandwidth of this circuit at approximately 1% distortion to be ≈ 45kHz.

Fourier analysis for vout:
  No. Harmonics: 16, THD: 1.07739 %, Gridsize: 200, Interpolation Degree: 3

Harmonic Frequency Magnitude  
Norm.Mag  
Percent  
Decibels
-------- --------- ---------
---------
---------
---------
1 45000 21.0952
1
100
0
2 90000 0.161286
0.0076456
0.76456
-42.3318
3 135000 0.121504
0.00575976
0.575976
-44.7919
4 180000 0.0784135
0.00371712
0.371712
-48.5959
5 225000 0.0450847
0.0021372
0.21372
-53.4031
6 270000 0.0312462
0.00148119
0.148119
-56.5878
7 315000 0.0265608
0.00125909
0.125909
-57.9989
8 360000 0.0201131
0.000953441
0.0953441
-60.4141
9 405000 0.0138871
0.000658306
0.0658306
-63.6314
10 450000 0.0111787
0.000529915
0.0529915
-65.5159
11 495000 0.00993006
0.000470725
0.0470725
-66.5447
12 540000 0.00838333
0.000397404
0.0397404
-68.0154
13 585000 0.00711102
0.000337091
0.0337091
-69.4451
14 630000 0.00640845
0.000303786
0.0303786
-70.3487
15 675000 0.00572887
0.000271572
0.0271572
-71.3223

slew rate =
dv
dt
(peak) = 2πfvpk = 2π 45kHz 21.0952V = 5.96453V/us


Stability Analysis


Figure 3:  Loop gain shows dominant pole still greater than 10kHz and phase margin greater than 80 deg.
loopgain

Distortion Results


Fourier analysis for vout @ 1kHz:
  No. Harmonics: 16, THD: 0.0155797 %, Gridsize: 200, Interpolation Degree: 3

Harmonic Frequency Magnitude  
Norm.Mag  
Percent  
Decibels









1 1000 21.1936
1
100
0
2 2000 0.00329427
0.000155437
0.0155437
-76.1689
3 3000 0.000223848
1.05621E-05
0.00105621
-99.525
4 4000 1.80022E-05
8.49415E-07
8.49415E-05
-121.418
5 5000 1.69424E-06
7.9941E-08
7.9941E-06
-141.945
6 6000 2.00519E-07
9.46127E-09
9.46127E-07
-160.481
7 7000 5.91641E-08
2.7916E-09
2.7916E-07
-171.083
8 8000 6.0557E-08
2.85732E-09
2.85732E-07
-170.881
9 9000 5.80361E-08
2.73837E-09
2.73837E-07
-171.25
10 10000 6.74031E-08
3.18035E-09
3.18035E-07
-169.951
11 11000 3.71128E-08
1.75113E-09
1.75113E-07
-175.134
12 12000 7.98147E-08
3.76598E-09
3.76598E-07
-168.482
13 13000 1.25026E-08
5.89922E-10
5.89922E-08
-184.584
14 14000 8.48198E-08
4.00214E-09
4.00214E-07
-167.954
15 15000 1.27269E-08
6.00508E-10
6.00508E-08
-184.43


Fourier analysis for vout @ 20kHz:
  No. Harmonics: 16, THD: 0.00777008 %, Gridsize: 200, Interpolation Degree: 3

Harmonic Frequency Magnitude  
Norm.Mag  
Percent  
Decibels









1 20000 21.1799
1
100
0
2 40000 0.00158615
7.48892E-05
0.00748892
-82.5116
3 60000 0.000403248
1.90392E-05
0.00190392
-94.407
4 80000 0.000165509
7.81444E-06
0.000781444
-102.142
5 100000 4.78777E-05
2.26052E-06
0.000226052
-112.919
6 120000 1.20349E-05
5.68224E-07
5.68224E-05
-124.91
7 140000 3.56298E-06
1.68225E-07
1.68225E-05
-135.482
8 160000 1.07192E-06
5.06101E-08
5.06101E-06
-145.915
9 180000 6.60838E-07
3.12012E-08
3.12012E-06
-150.117
10 200000 3.74125E-07
1.76642E-08
1.76642E-06
-155.058
11 220000 3.26926E-07
1.54357E-08
1.54357E-06
-156.229
12 240000 2.98762E-07
1.41059E-08
1.41059E-06
-157.012
13 260000 3.00341E-07
1.41805E-08
1.41805E-06
-156.966
14 280000 2.65863E-07
1.25526E-08
1.25526E-06
-158.025
15 300000 2.33183E-07
1.10096E-08
1.10096E-06
-159.165

Transfer Error Curves


Figure 4:  Small signal DC transfer error curve. Figure 5:  Large signal DC transfer error curve.
trdcss
trdcls

C7 removes the predriver from the transfer curves above a pole frequency set by C7 and the sum of the transconductances of M3 and M4.  Above this pole there is a different ac transfer error curve.

Figure 6:  Small signal AC transfer error curve. Figure 7:  Small signal AC transfer error curve.
tracss
tracls

If the feedback along the inner feedback loop is increased by reducing RC from 220kΩ to 100kΩ  the AC transfer curves above become more symmetrical parabolas at the expense of somewhat greater overall distortion.


Conclusions





Document History
June 9, 2012  Created.