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Old 2nd October 2012, 02:46 PM   #1
catalin is offline catalin  Romania
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Join Date: Jun 2007
Thumbs up CFB-CFP amp development

Hello ,
I want to develop this project in next time so all advices will be helpful .
From my point of view this topology is the best possible .I want to have a good design of the resistors firstly and then the capacitor feedback value .
Now the circuit is stable in transient response with 6.8pF.But if there are some good advices I will be happy .
The gain is 72 db linear until 10 Mhz without compensation 6.8pf .Can be decreased to 58-60db without problems .
With comp ,the gain is linear untill 2Mhz which is enough. The phase is also gorgeous like any other amp .
I simulated also transient response ,thd ,and there is very good results .
The slew rate I will let you to see ..
The group delay is also the best .
Schematic :
Click the image to open in full size.
OLG without 6.8pF compensation :
By incresing the resistors we can have also phase reserve without capacitor but we will see ..
Click the image to open in full size.

The ltspice schematic attached and my model library :

.MODEL 2N5551 NPN (Is=2.511f Xti=3 Eg=1.11 Vaf=100 Bf=242.6 Ne=1.249 Ise=2.511f Ikf=.3458 Xtb=1.5 Br=3.197 Nc=2 Isc=0 Ikr=0 Rc=1
Cjc=4.883p Mjc=.3047 Vjc=.75 Fc=.5 Cje=18.79p Mje=.3416 Vje=.75 Tr=1.202n Tf=560p Itf=50m Vtf=5 Xtf=8 Rb=10)

.MODEL 2N5401 PNP (Is=21.48f Xti=3 Eg=1.11 Vaf=100 Bf=132.1 Ne=1.375 Ise=21.48f Ikf=.1848 Xtb=1.5 Br=3.661 Nc=2 Isc=0
Ikr=0 Rc=1.6 Cjc=17.63p Mjc=.5312 Vjc=.75 Fc=.5 Cje=73.39p Mje=.3777 Vje=.75 Tr=1.476n Tf=641.9p Itf=0
Vtf=0 Xtf=0 Rb=10)

.MODEL 2SC3423 NPN(IS=9.98627F BF=2K NF=967.67M VAF=100 IKF=49.6929M ISE=1.04163F NE=1.07574 BR=601.257M IKR=462.798U ISC=32.904P RC=899.97M
CJE=2P MJE=500M CJC=6.42174P VJC=749.999M MJC=499.509M TF=713.346P XTF=500M VTF=10 ITF=9.9976M TR=10N)

.MODEL 2SA1360 PNP(IS=10F BF=134.853 VAF=100 IKF=109.96M ISE=221.874F NE=1.66575 BR=10 IKR=880.176M ISC=187.58P NC=1.90472 RE=1 RC=15.5104
+ CJE=2P MJE=500M CJC=6.24728P VJC=692.028M MJC=340.013M TF=1.08385N XTF=16.9293 VTF=9.36211 ITF=670.025M TR=10N)

.MODEL Q2SA1930 PNP(
+ IS=10.000E-15
+ BF=210
+ VAF=78
+ IKF=10.000E-3
+ XTB=1.5
+ BR=.1001
+ VAR=100
+ IKR=10.000E-3
+ ISC=10.000E-15
+ CJE=3.252E-12
+ CJC=63.196E-12
+ MJC=.33333
+ TF=83.239E-12
+ XTF=10
+ VTF=10
+ ITF=1)

.MODEL Q2SC5171 NPN(
+ IS=10.000E-15
+ BF=210
+ VAF=100
+ IKF=10.000E-3
+ XTB=1.5
+ BR=.1001
+ VAR=100
+ IKR=10.000E-3
+ ISC=10.000E-15
+ CJE=2.0000E-12
+ CJC=38.866E-12
+ MJC=.33333
+ TF=83.239E-12
+ XTF=10
+ VTF=10
+ ITF=1)

.MODEL BD139 NPN (
+ IS =2.3985E-13
+ BF =244.9
+ NF =1.0
+ BR =78.11
+ NR =1.007
+ ISE =1.0471E-14
+ NE =1.2
+ ISC =1.9314E-11
+ NC =1.45
+ VAF =98.5
+ VAR =7.46
+ IKF =1.1863
+ IKR =0.1445
+ RB =2.14
+ RBM =0.001
+ IRB =0.031
+ RE =0.0832
+ RC =0.01
+ CJE =2.92702E-10
+ VJE =0.67412
+ MJE =0.3300
+ FC =0.5
+ CJC =4.8831E-11
+ VJC =0.5258
+ MJC =0.3928
+ XCJC =0.5287
+ XTB =1.1398
+ EG =1.2105
+ XTI =3.0 )

.MODEL bd136 pnp
+IS=1e-09 BF=681.414 NF=0.85 VAF=10
+IKF=0.196957 ISE=1e-08 NE=1.57381 BR=56.5761
+NR=1.5 VAR=0.975138 IKR=0.952908 ISC=1e-08
+NC=3.58666 RB=40.4245 IRB=0.1 RBM=0.106663
+RE=0.00034585 RC=1.31191 XTB=22.4074 XTI=1
+EG=1.05 CJE=1e-11 VJE=0.75 MJE=0.33
+TF=1e-09 XTF=1 VTF=10 ITF=0.01
+CJC=1e-11 VJC=0.75 MJC=0.33 XCJC=0.9
+FC=0.5 CJS=0 VJS=0.75 MJS=0.5
+TR=1e-07 PTF=0 KF=0 AF=1
* Model generated on Feb 22, 2004
* Model format: PSpice

.MODEL 2SA1943 PNP (
+ IS=1.30E-10 BF=91.42 VAF=100 IKF=4.480 ISE=1.02E-10 NE=2.0 VAR=100 ISC=5.0900E-9 NC=1.5 BR=0.882 IKR=2.9015 RE=0.0011
+ RC=0.0553 RB=140.05 RBM=0.0041 IRB=8.5e-9 CJE=2.00E-10 FC=0.5 CJC=9.45E-10 VJC=0.48 MJC=0.28 TF=9.250E-10 XTF=10 VTF=10
+ ITF=1 TR=1.00E-8 EG=0.76 XTB=2.68 mfg=Toshiba)

.MODEL 2SC5200 NPN (
+ IS=3.0463E-11 BF=96.20 VAF=100 IKF=15.04256 ISE=5.6190E-11 NE=2.0 BR=4.849 IKR=1.05012 VAR=100 ISC=7.18E-8 NC=1.5 RE=0.0025
+ RB=20.18 RBM=0.0014 IRB=1.0E-7 RC=0.01137 CJE=4.5000E-10 CJC=8.4915E-10 VJC=0.68977 MJC=0.54081 TF=6.8583E-10
+ XTF=9.5721 VTF=10.425 ITF=6.8697E-2 TR=1.000E-8 XTB=1.45 EG=0.82 FC=0.5 mfg=Toshiba)

*2SC3281 MCE 5/13/95
*Si 150W 200V 15A 30MHz pkg:TO-247 2,1,3
.MODEL 2SC3281 NPN(IS=10.000E-15 BF=155.65 VAF=100 IKF=9.2028 XTB=1.5
+ISE=54.325E-15 NE=1.3056 BR=10.787 VAR=100 IKR=1.8561 ISC=106.69E-15
+NC=1.6728 RC=26.745E-3 CJE=2.0000E-12 CJC=534.41E-12
+MJC=.33333 TF=8.0821E-9 XTF=3.1968 VTF=21.461E-3 ITF=169.59 TR=187.91E-9 )

*2SA1302
*Si 150W 200V 15A 30MHz pkg:TO-247
.MODEL 2SA1302 PNP( IS=21.479E-12 BF=136.48 VAF=100 IKF=19.980 ISE=21.504E-12
+NE=1.3784 BR=329.48 VAR=100 IKR=19.980 ISC=4.3670E-9 NC=1.4264 RC=93.301E-3
+CJE=755.31E-12 MJE=.33333 CJC=1.1417E-9 MJC=.33333 TF=1.2802E-9 XTF=10 VTF=10
+ITF=1 TR=10.000E-9)

And negative feedback response with compensation normalized to 0db
Click the image to open in full size.
Attached Files
File Type: asc joy_cfb_ac.asc (7.6 KB, 35 views)
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Last edited by catalin; 2nd October 2012 at 03:03 PM.
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Old 2nd October 2012, 03:19 PM   #2
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Thats a typical circuit, there is a lot you can do to improve it.
For starters connect the collectors of Q7 and Q8 to the emitters of Q9 and Q10. R22 and R21 can be current sources, either jfet or Bjt or current mirror which can be used to current boost the inputstage in case youd want much higher slewrates. Cascode Q9 and Q10 with their reference the input pair emitter current. For better vas performance use a hawksford cascode instead. Put a resistor of say 100ohms accross c7 or you might run into stability problems. You can bootstrap the input pair to get rid of the inverting input capacitance to extend bandwith even more or you could do it by using a three stage diamond buffer but this would mean another 2 transistors although it also reduces distortion. There is a lot that can be done with the circuit but performance is pretty good just as is.
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Old 2nd October 2012, 08:30 PM   #3
catalin is offline catalin  Romania
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Join Date: Jun 2007
Quote:
Originally Posted by homemodder View Post
Thats a typical circuit, there is a lot you can do to improve it.
For starters connect the collectors of Q7 and Q8 to the emitters of Q9 and Q10. R22 and R21 can be current sources, either jfet or Bjt or current mirror which can be used to current boost the inputstage in case youd want much higher slewrates. Cascode Q9 and Q10 with their reference the input pair emitter current. For better vas performance use a hawksford cascode instead. Put a resistor of say 100ohms accross c7 or you might run into stability problems. You can bootstrap the input pair to get rid of the inverting input capacitance to extend bandwith even more or you could do it by using a three stage diamond buffer but this would mean another 2 transistors although it also reduces distortion. There is a lot that can be done with the circuit but performance is pretty good just as is.
Thank you homemodder ,if there is a typical circuit can you show the same topology with the values ?
By connecting the collectors of q7 and 8 to emitters q9,10 thd is increased.
Of course that I will add current generators at the first stage or at least a voltage source,zenner etc ..
I don't need higher slew rate ,exactly this configuration without input filter has almost 2000V/us so it is really the fastest I ever seen .
I will try to add the resistor in series with C7 to create an zero and increase the phase in the feedback loop thought phase reserve is 70 degrees and rock stable in square wave signal also with 1uf capacitive load .
By adding cascodes I will increase the gain but I don't want this .The gain is big enough , as you can see in pictures .
What do you want to say by bootstraping the input ?

I want to leave the topology like it is because it is simple and has less components and the same performance can't be achieved with the same amount of components .The thing what I am looking for is to determine the optimum values of the resistors by setting the optimum currents and gain in stages .
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Last edited by catalin; 2nd October 2012 at 08:56 PM.
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Old 3rd October 2012, 10:06 AM   #4
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Cool schematic.
Why so big caps in the servo circuit?
Try capacitor in parallel with r25 for faster switch off. I agree about current sources.
Next:the pcb.
Good luck.Keep progressing.
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Old 3rd October 2012, 10:36 AM   #5
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If the the distortion increases theres something wrong somewhere, it might just be the way you designed it, the resistor values you use on the input section are way higher than what I commonly use. As an example R8 and R9 I never use larger than 33 ohms but I havent studied all the consequences of the high values. I get bandwith at 2.5Mhz at 88 degrees and thats without any additions and THD of .009 at 20khz fullpower 100w rms with just a simple single vas. Better figures are easily attained with improvements to the vas.

Sorry, when I said accross I actually meant in series is correct.
Bootstrapping Ill have to check whether I can show the circuit first.
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Old 6th December 2012, 11:27 AM   #6
catalin is offline catalin  Romania
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Hello ,I want to downgrade the number of parts of the previous design so now I have this thing .I will build it ,listen it and if the results are good I will think to a solution for the constant current into the first stage .Thd at 1-3watts is 0.006% but the bandwith at 0db is 24 mhz .I will have a look at TIM. The OLG is 60db and the phase at 20khz in olg is only -10degrees .
Attached Images
File Type: jpg simplecfb.jpg (50.6 KB, 275 views)
File Type: jpg simplesim_cfb_acOL.JPG (162.7 KB, 253 views)
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Old 6th December 2012, 03:00 PM   #7
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Catalin, have alook at SSA thread, this design of yours is the basic one. SSA is wrongly worded though, that design is certainly not so simple, this is and can result in a good amp but youll have to look at temp compensation here of the input transistors. It can be done quite simply. There is another thread by Garbobela of the same design exactly like yours which should be of interest, he claims its the best amp he has ever listened to.
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Old 6th December 2012, 03:23 PM   #8
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I am amazed by all those best amplifiers all over this forum.....
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Old 6th December 2012, 03:28 PM   #9
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Quote:
Originally Posted by mchambin View Post
I am amazed by all those best amplifiers all over this forum.....
Ive been for several years now
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Old 6th December 2012, 05:59 PM   #10
AndrewT is offline AndrewT  Scotland
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Some folk can't help themselves by stopping exaggerating everything they do/hear/say.
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