Re: Re: Re: Re: No reason?????
Since your analysis above ignores the intrinsic nonlinear emitter (or source) resistances of the diff pair (the half of the circuit which I take to be "out of the loop"), do you believe that their influence on the feedback signal cancels out due to symmetry? If so, how do you reconcile this with the necessarily unequal base currents in a BJT implementation?mikek said:
Apparently Mikek misunderstood my reply. Of course, ONLY a folded cascode connection would lower gain, BUT with a normal base connection to the second stage as well, will overwhelm the folded cascode connection with added gain. This is the condition initially stated in the schematic published earlier on this thread. Overreaction like this, with a circuit expert is not recommended in future. 
lets get both quotes out there, shall we?
Again....No....the folded cascode connection is not 'trivial', and does not 'give a little more' gain.....
Even when combined with the cascode connection, the connection to the base of the second stage will not 'overwhelm' the cascode connection.....derive the transfer function, or easier still, run a simulation of both circuits........
Note that nominal transadmittance Ym for both must be the same.
cheers.
john curl said:
Again....No....the folded cascode connection is not 'trivial', and does not 'give a little more' gain.....
john curl said:Apparently Mikek misunderstood my reply. Of course, ONLY a folded cascode connection would lower gain, BUT with a normal base connection to the second stage as well, will overwhelm the folded cascode connection with added gain. This is the condition initially stated in the schematic published earlier on this thread. Overreaction like this, with a circuit expert is not recommended in future.
Even when combined with the cascode connection, the connection to the base of the second stage will not 'overwhelm' the cascode connection.....derive the transfer function, or easier still, run a simulation of both circuits........
Note that nominal transadmittance Ym for both must be the same.
cheers.
Re: Re: Re: Re: Re: No reason?????
My 'analysis' does not 'ignore' the dynamic non-linearity introduced by re.......This simply was not the subject of your earlier post.
At any rate such non-lineariarity is negligible....and with both devices in the global feedback loop, should not even apear over the horizon.
The 'necessarily unequal base currents' with a BJT diff. pair are vastly negligible, compared to the inequality in Vgs in FET's....the use of which often requires a servo...
All we need do in respect of mitigating inequality in base current draw, is equalise resistance to ground from each input. Keep the value of said resistance as low as possible, consistent with providing a sufficiently high input impedance to competently designed pre-amps. (4K7~6k8 input resistance should suffice). Use as high Hfe BJT's as possible.....and most important, use a current mirror.
Joe Berry said:
My 'analysis' does not 'ignore' the dynamic non-linearity introduced by re.......This simply was not the subject of your earlier post.
At any rate such non-lineariarity is negligible....and with both devices in the global feedback loop, should not even apear over the horizon.
The 'necessarily unequal base currents' with a BJT diff. pair are vastly negligible, compared to the inequality in Vgs in FET's....the use of which often requires a servo...
All we need do in respect of mitigating inequality in base current draw, is equalise resistance to ground from each input. Keep the value of said resistance as low as possible, consistent with providing a sufficiently high input impedance to competently designed pre-amps. (4K7~6k8 input resistance should suffice). Use as high Hfe BJT's as possible.....and most important, use a current mirror.
Mike,
I am sorry to say that most modern designs today use fet's for inputs these days. It's only the cheaper designs that use bipolars. I can say this after examining hundreds of modern day designs from Pass, Curl..........to Yamaha. The main exception to this seems to be most British designs.
As for Self, he might put down something nice on paper but unfortunately it is usually medicore at best on real world speakers. I have built several of his amps but no prize.
Theory is one thing ...........but in the real world a lot of it goes out the window.
Regards,
Jam
I am sorry to say that most modern designs today use fet's for inputs these days. It's only the cheaper designs that use bipolars. I can say this after examining hundreds of modern day designs from Pass, Curl..........to Yamaha. The main exception to this seems to be most British designs.
As for Self, he might put down something nice on paper but unfortunately it is usually medicore at best on real world speakers. I have built several of his amps but no prize.
Theory is one thing ...........but in the real world a lot of it goes out the window.
Regards,
Jam
jam said:
Hi jam....good to hear from you...
.....i suspect that this trend is down to marketing..... jam said:
Actually, i do not consider self's designs to be optimal.....his output stage designs are not sufficiently robust in my view....and his SOA protection is likely to activate intermittently during normal use....not good.
But...this has nothing to do with not having FET's in his diff. stage..
...Moreover, Halcro and Krell, both considered manufacturers of the best power amps. in the world by subjectivist periodicals such as stereophile....Hi-Fi World...etc.....use BJT inputs......
Do you have any more unusuall schematics?
Mike.
Many fet input amps do not require servos (no base current) with bipolars to get away with this you need the nasty input cap. ( screams of horror from the cap of the month club members). So offset in not likely to drift much once set.
No much in the way of RFI problems.
Matched fets are available.
High input z available without bootstrapping the input.
Low noise.
Nelson Pass, John Curl............and Fred use them.
What more could you ask for?
Regards,
Jam
Many fet input amps do not require servos (no base current) with bipolars to get away with this you need the nasty input cap. ( screams of horror from the cap of the month club members). So offset in not likely to drift much once set.
No much in the way of RFI problems.
Matched fets are available.
High input z available without bootstrapping the input.
Low noise.
Nelson Pass, John Curl............and Fred use them.
What more could you ask for?
Regards,
Jam
You are incorrect. Fets are now, and have been for the last 25 years, as quiet as bipolar devices in almost every application, if you use Toshiba input fets. Bipolar transistors, because they have somewhat higher Gm, can have slightly lower short circuit noise, but at realistic impedance levels and currents, they are usually just as noisy or noisier than discrete fet input stages.
That is easy, a 2SK146 will do the trick. It will also keep low noise at all reasonable impedances, up to 1 meg ohm or so. Of course, these devices are not easily available, so you could parallel a pair of 2SK389's and get the same performance. I work at 0.4nV/rt hz with complementary paralleled fets in my Vendetta Research design. I have achieved a 10 ohm overall equivalent noise level with this circuit for the last 20 years. Before that, I used bipolar transistors to achieve the same result, approximately 30 years ago, with the introduction of the Mark Levinson JC-1 pre-preamp.
The problem with transistors is their NOISE CURRENT which is very high when the NOISE VOLTAGE is low. It is virtually impossible to get both very low noise voltage and noise current with a bipolar device with an inductive source, such as a tape head or a moving magnet phono cartridge, and they can be very noisy with source impedances above 1K ohm or so. Fets can be operated at higher currents for low voltage noise, and still have almost unmeasurable current noise.
The problem with transistors is their NOISE CURRENT which is very high when the NOISE VOLTAGE is low. It is virtually impossible to get both very low noise voltage and noise current with a bipolar device with an inductive source, such as a tape head or a moving magnet phono cartridge, and they can be very noisy with source impedances above 1K ohm or so. Fets can be operated at higher currents for low voltage noise, and still have almost unmeasurable current noise.
.........No need to parallel BJT's....so i think not...
http://www.analog.com/UploadedFiles/Data_Sheets/410572853MAT02_e.pdf
.....No need to parallel BJT's....so i think not...http://www.analog.com/UploadedFiles/Data_Sheets/410572853MAT02_e.pdf
"The defence rests." if only the offensive would! I think the defensive is restless
Well... is it?
"Yes... i think D. Self has done audio engineering a great service with his work....."
OR?
"Actually, i do not consider self's designs to be optimal.....'
"Moreover, Halcro and Krell, both considered manufacturers of the best power amps. in the world by subjectivist periodicals such as stereophile...."
OR?
"high end community=subjectivist community
..marketing...old chap...to the subjectivist community.."
"Using degeneration allows you trade input stage trans-admittance for increased linearity over a broader differential input range.
OR?
"At any rate such non-lineariarity is negligible....and with both devices in the global feedback loop, should not even apear over the horizon."
"Completely untrue...i fear.....the transistor to which the negative feedback network is returned merely acts as a very linear unity-gain buffer to said network...., with the subtraction of feedback from input signal effectively taking place at the (+) input transistors base-emitter, (or gate-source), as the case may be....."
The rest of us learned that the negative feedback is connected to the inverting input (-), I think that is why it is called negative feedback. Of course I can't be positive about that and don't want accused of negativity.
Fred
"Foolish consistency is the hobgoblin of little minds". - R. W. Emerson
Well... is it?
"Yes... i think D. Self has done audio engineering a great service with his work....."
OR?
"Actually, i do not consider self's designs to be optimal.....'
"Moreover, Halcro and Krell, both considered manufacturers of the best power amps. in the world by subjectivist periodicals such as stereophile...."
OR?
"high end community=subjectivist community
..marketing...old chap...to the subjectivist community.."
"Using degeneration allows you trade input stage trans-admittance for increased linearity over a broader differential input range.
OR?
"At any rate such non-lineariarity is negligible....and with both devices in the global feedback loop, should not even apear over the horizon."
"Completely untrue...i fear.....the transistor to which the negative feedback network is returned merely acts as a very linear unity-gain buffer to said network...., with the subtraction of feedback from input signal effectively taking place at the (+) input transistors base-emitter, (or gate-source), as the case may be....."
The rest of us learned that the negative feedback is connected to the inverting input (-), I think that is why it is called negative feedback. Of course I can't be positive about that and don't want accused of negativity.
Fred
"Foolish consistency is the hobgoblin of little minds". - R. W. Emerson
I am feeling great! Your post cheer me up like you would not believe. Don't Stop!
And still, no end is near;
He just won't face the final curtain.
My friend, I'll say it clear,
Mike states his case, and knows he's certain.
He's seldom runs Spice or builds.
He's read about most all tried ways;
He knows, much more than you,
He knows it! …. Mike K
Regrets, he's had so few.
In fact he thinks too small to mention.
He did what we thought he'd do
And plowed right through without redemption.
Unplanned, no charted course;
Post on the information highway.
But more, much more than this,
He did it …. Mike K.
Yes, there were times, I'm sure you knew
When he bit off more than he could chew.
But through it all, Mike had no doubt,
He ate us up and spit us out.
Disgraced by all. He still won't fall.
He knows it, Mike K.
I've read, I've laughed and cried.
I've had my thrills; my share of losing.
And now, as laughs subside,
Still find it damned amusing.
To think he said all that;
And may I say - not in a shy way,
"No, it wasn't me,
I t was old Mike K.
For what is a man, what has he got?
He's not Doug Self, at least Self taught.
To say the things he truly thinks;
And not the views, of us who shrink..
The mighty knows that jfets blow
Bipolar! Mike K.
Fred
You'll still be here when I get out of the sin bin won't you?
And still, no end is near;
He just won't face the final curtain.
My friend, I'll say it clear,
Mike states his case, and knows he's certain.
He's seldom runs Spice or builds.
He's read about most all tried ways;
He knows, much more than you,
He knows it! …. Mike K
Regrets, he's had so few.
In fact he thinks too small to mention.
He did what we thought he'd do
And plowed right through without redemption.
Unplanned, no charted course;
Post on the information highway.
But more, much more than this,
He did it …. Mike K.
Yes, there were times, I'm sure you knew
When he bit off more than he could chew.
But through it all, Mike had no doubt,
He ate us up and spit us out.
Disgraced by all. He still won't fall.
He knows it, Mike K.
I've read, I've laughed and cried.
I've had my thrills; my share of losing.
And now, as laughs subside,
Still find it damned amusing.
To think he said all that;
And may I say - not in a shy way,
"No, it wasn't me,
I t was old Mike K.
For what is a man, what has he got?
He's not Doug Self, at least Self taught.
To say the things he truly thinks;
And not the views, of us who shrink..
The mighty knows that jfets blow
Bipolar! Mike K.
Fred
You'll still be here when I get out of the sin bin won't you?
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