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Old 17th January 2012, 01:27 PM   #591
Botte is offline Botte  United States
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Quote:
Originally Posted by Kamel View Post
Because I ask unnecessary questions I worked a little bit on an AC Analysis of the BA-3

Note for the PushPull BA-2 Output Stage you need C3...

As you can see on the Pictures Voltage Gain mostly depends on transconductance of the 2SK170/2SJ74 and of course R3/R4 and R6/R7 for the first Stage, which is: A1= R6/(R3+(1/Yfs))
Jfets with higher Idss would have higher Voltage Gain!

The Voltage Gain of the Second Stage is: A2= R13/R10+(1/Yfs))
With higher BIAS Voltage Gain can be increased slightly.

Overall Gain is: A= A1*A2*(Rload/(R12+Rload))

I didn't observe fluctuations of Yfs which differs in large Signal conditions!
In this point of view the Bandwidth is infinite

Best regards Felix
Kamel,

You had it correct and it works out, you just dropped the R5.

So A1=R6/R5+(R3||(P3/2))+(1/Yfs)

First stage 100/100+8.3+45 =.65 gain for first stage and I verified it with my amp.

A2=R13/R10+(1/Yfs)

Second stage 330/22+3.3=13 gain.

For an overall gain of .65*13= 8.45 and that is what we are getting.
Now if you are running balanced you would get 8.45*2 =16.9 or 24.5dB
I’m now satisfied we can calculate the gain correctly.
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Old 17th January 2012, 04:24 PM   #592
Nelson Pass is offline Nelson Pass  United States
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My copy of the article sez:

The voltage appearing at the Gate of Q3 is amplified by something less than
the ratio of R10 divided by R8, and with the same happening at Q4 and
considering the transconductance of the Mosfets, comes out at about 15.
Both of them added make a system voltage gain of about 30X, or 30 dB.


Obviously it's not clearly written. The voltage gain contributed by Q3 is
about 15 and the voltage gain contributed by Q4 is about 15 and the
result at the output is about 30.

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Old 17th January 2012, 04:48 PM   #593
Zen Mod is online now Zen Mod  Serbia
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as I wrote in comment #15 of article itself : Burning Amplifier BA-3 , there is some discrepancy between text and parts nomenclature ;
Papa , I wrote you about that , also regarding same issue in article on FW site , but it's completely logical that you didn't read that e-mail , considering that I'm sending you approx. 15-25 e-mails/day


anyway - here is , at least by my knowledge , revised text with proper parts nomenclature ( Papa sez , Babelfishing by ZM ) :

Quote:
This is familiar enough. Q1 and Q2 are JFETs which self-bias into resistors R3 and R4 at currents around 8 mA. R1 is chosen to avoid oscillatory interaction with whatever source impedance you might have, and R2 provides a DC reference to ground in the event that the source does not, and also establishes the nominal input impedance.

Q1 and Q2 are largely degenerated by R5, setting the amount of AC current which flows through them for a given input voltage. The voltage gain of this initial stage is approximately

the value of Drain load resistors R6 plus R7 divided by R5. In this case we have roughly unity gain – the Jfets are used as unity gain DC level shifters to Q3 and Q4.


Coming off the Drain of Q1 is the loading network of R6, C1 and P2, and there is a comparable network of R7, C2, and P1 attached to the Drain of Q2. R6 clearly sets the AC load for Q1, but the DC requirements to bias up Mosfet Q3 are higher than that, so P2 in parallel with C1 provides a higher resistance value below about 0.5 Hz, and gives the approximately 3 volt DC drop required to bias the Mosfets.

P1 and P2 are adjusted so as to set the DC bias of Q3 and Q4. You will want to set them at zero when you first fire up the circuit, and increase their resistance to achieve the correct bias voltages across R10 and R11 (about 1 volt) while also keeping the output DC offset voltage at a minimum. This circuit is capacitively coupled at the output, but low offset measured at the Drains of Q3 and Q4 will maximize your output voltage swing.

The voltage appearing at the Gate of Q3 is amplified by something less than the ratio of R13 divided by R10, and with the same happening at Q4(R11) and considering the transconductance of the Mosfets, comes out at about 15. Both of them added make a system voltage gain of about 30X, or 30 dB.


R10 and R11 help set the voltage gain, and they also help stabilize the bias of Q3 and Q4, else it would tend to drift upwards as the parts warm up. The bias current here is about 50 mA, and it will deliver peaks of approximately 100 mA. Q3 and Q4 require heat sinks.

Of course you can bias this circuit higher if you wish – 100 mA bias is perfectly OK as long as you properly heat sink Q3 and Q4, and if you are crazy (like me) you can experiment with higher bias, remembering that the parts are rated at 25 watts, and that it costs you voltage losses across R10 and R11. If you want to play with even higher bias, you can consider lowering the values of R10 and R11 and also R13, all in proportion.

The supply voltage is only critical with respect to the voltage rating of the input JFETs, which are nominally 25 volts. In actual testing, they break down around 40 volts. I wouldn't worry about running them as high as 30V. Hot-rodding this circuit would likely involve cascoding the input Jfets to allow higher voltages.
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Last edited by Zen Mod; 17th January 2012 at 04:51 PM.
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Old 17th January 2012, 04:56 PM   #594
Botte is offline Botte  United States
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Quote:
Originally Posted by Nelson Pass View Post
My copy of the article sez:

The voltage appearing at the Gate of Q3 is amplified by something less than
the ratio of R10 divided by R8, and with the same happening at Q4 and
considering the transconductance of the Mosfets, comes out at about 15.
Both of them added make a system voltage gain of about 30X, or 30 dB.


Obviously it's not clearly written. The voltage gain contributed by Q3 is
about 15 and the voltage gain contributed by Q4 is about 15 and the
result at the output is about 30.

I would have thought that Q3 and Q4 being out of phase wouldn’t add together and then the rest of the math works.

My amp has a gain of 8.5X and sounds great. Thank you.
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Old 17th January 2012, 05:42 PM   #595
Zen Mod is online now Zen Mod  Serbia
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imagine elastic string between two hands ;

move one hand out and back

move other hand out and back

move both hands out and back

move both hands left and back

move both hands right and back
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Old 17th January 2012, 05:49 PM   #596
Kamel is offline Kamel  Germany
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Assumed Gain of first Stage is (R6+R7)/R5 = 2 and of second Stage is (R10+R11)/R13 = 7.5... Overall Gain is 15.

Why adding Gain of upper and lower part of second stage?
When we split it up, so first Stage Gain is R6/R5 = 1 and of second Stage Gain is R13/R10 = 15... overall Gain is still 15?!
The upper half of the Stages only amplify the upper half of the Sine Wave and so second Stage the lower half...

Picture follows...
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Old 17th January 2012, 05:51 PM   #597
Botte is offline Botte  United States
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Quote:
Originally Posted by Zen Mod View Post
imagine elastic string between two hands ;

move one hand out and back

move other hand out and back

move both hands out and back

move both hands left and back

move both hands right and back
Oops, you are correct they are in phase, but they still don’t add together, they are push-pull. We are all still getting a gain of 8.5X.
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Old 17th January 2012, 06:14 PM   #598
Kamel is offline Kamel  Germany
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Real Gain is about 8.5

Now it must be correct!

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Old 17th January 2012, 06:27 PM   #599
bobodioulasso is offline bobodioulasso  Europe
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Quote:
Why adding Gain of upper and lower part of second stage
Neither i get it.
let a 2V pp input signal.
positive half (1v) is amplified 15 times. thats 15v.
Negative half(1v) is amplified 15 times. thats 15v.
output is 30v pp.
That is 15 times 2volts.

Last edited by bobodioulasso; 17th January 2012 at 06:35 PM.
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Old 17th January 2012, 06:37 PM   #600
Zen Mod is online now Zen Mod  Serbia
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Quote:
Originally Posted by Kamel View Post
.......

Why adding Gain of upper and lower part of second stage?
......
Quote:
Originally Posted by bobodioulasso View Post
........
Quote:
Originally Posted by Zen Mod View Post
........

move both hands out and back
.....
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