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Old 10th January 2007, 07:39 PM   #1
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Default unusual design of input stage

I havn`t see something similar to this before. Thanks to IvanGrig from vegalab phorum.
So, what do you think about this? It can be easily combined with darlingtons or mosfets output stage for power amplifier application.

Click the image to open in full size.

Fully symmetric, low parts count, very good performer at simulators.
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Old 11th January 2007, 12:16 AM   #2
lineup is offline lineup  Sweden
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Default Re: unusual design of input stage

Quote:
Originally posted by Kuzmenko

I havn`t see something similar to this before. Thanks to IvanGrig from vegalab phorum.
So, what do you think about this? It can be easily combined with darlingtons or mosfets output stage for power amplifier application.

http://images.people.overclockers.ru/106940.gif

Fully symmetric, low parts count, very good performer at simulators.
Fully symmetric ... well, there is no such 'free lunch'
and PNP and NPN transistors are not made in the same way.
So they are always different in several parameters.


If this circuit performs better than our known and less complicated ones,
then good!
At least this one may have some ideas we could take, and make something better out of.
I have stated, I like to see peole who can THINK OUT OF THE BOX.


I never use cascading.
I always try for other solutions, like find transistors that operates better without cascade.
Why use 2 transistors, to do a 1 transistor job ??????

I can see this one circuit, in my opinion, is a bit sensitive to supply noise.
Which is almost the same as low PSRR (Power Supply Rejection Ratio ).
-------------------------------------------------------------


Questions:
Is this in use in any Amplifier?
If so, what was the result?


Regards
lineup
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Old 11th January 2007, 07:46 AM   #3
Elvee is offline Elvee  Belgium
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Hi

There is something I fail to understand: basically, the circuit is made of two differential amplifiers, but with complementary transistors in a series configuration, instead of the usual identical transistors in parallel. The inverting input is at the bases of Q7/11, and the non-inverting at the bases of Q10/9. But for the amplifier to operate correctly, the NI input has to be kept at the ground potential; here the only coupling to the ground is via the bias resistors R34/35, and is quite loose. Is this intentional? If not, the open loop gain of the circuit could be greatly improved by substituting two capacitors (or two strings of diodes) to the ground in place of C8.
LV
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Old 11th January 2007, 08:01 AM   #4
lineup is offline lineup  Sweden
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Quote:
Originally posted by Elvee

There is something I fail to understand: basically, the circuit is made of two differential amplifiers, but with complementary transistors in a series configuration, instead of the usual identical transistors in parallel. The inverting input is at the bases of Q7/11, and the non-inverting at the bases of Q10/9. But for the amplifier to operate correctly, the NI input has to be kept at the ground potential; here the only coupling to the ground is via the bias resistors R34/35, and is quite loose. Is this intentional? If not, the open loop gain of the circuit could be greatly improved by substituting two capacitors (or two strings of diodes) to the ground in place of C8.
LV
yes, it might look like that.
But I am sure in this case the feedback is for an inverting operation.
This makes input impedance 4.7K and R32 / C12 is the feedback.

The input will create voltage/current changes in R26 and R27.
These resistors R26/R27 are connected to a defined operation voltage
by the divider R34, R36, R35 which is between V+ and V-.

So this would work.
But question is how good?


V+ and V- should be at very precise voltages.
This is why I wonder about the PSRR of this circuit.
I would say we need very good regulated supply for this amplifier.
But let's hear from some other people here ..



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Old 11th January 2007, 08:13 AM   #5
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Default Re: Re: unusual design of input stage

Quote:
Originally posted by lineup


Fully symmetric ... well, there is no such 'free lunch'
and PNP and NPN transistors are not made in the same way.
So they are always different in several parameters.


If this circuit performs better than our known and less complicated ones,
then good!
At least this one may have some ideas we could take, and make something better out of.
I have stated, I like to see peole who can THINK OUT OF THE BOX.


I never use cascading.
I always try for other solutions, like find transistors that operates better without cascade.
Why use 2 transistors, to do a 1 transistor job ??????

I can see this one circuit, in my opinion, is a bit sensitive to supply noise.
Which is almost the same as low PSRR (Power Supply Rejection Ratio ).
-------------------------------------------------------------


Questions:
Is this in use in any Amplifier?
If so, what was the result?


Regards
lineup
It's called cascoding. Anyone can use what he/she likes.
I would separate the bases of Q7 and Q11 and apply feedback to base of Q7.
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Old 11th January 2007, 08:43 AM   #6
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The potentials of the bases Q8+Q9 and Q10+Q12 are NOT fixed to the ground to avoid flowing currents thru signal ground. So, in this design there is no smallsignal ground at all There is only 1 powerground node on amplifier`s pcb. Because of this you can`t do mistake when you would route pcb`s traces

About "poor PSRR": are they? It`s not poorer than traditional diff+va design.
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Old 11th January 2007, 08:44 AM   #7
lineup is offline lineup  Sweden
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Default Re: Re: Re: unusual design of input stage

Quote:
Originally posted by QSerraTico_Tico

It's called cascoding. Anyone can use what he/she likes.
If you understand better what I means you can call it anything.
There are plenty of others around, that can't comment on topic subject,
but only post about all us non-english spelling mistakes.
you are nott the only one, I am afraidly you are nott.

So how much better would anyone reader get my message
if I is spellingly one letters vrongg?
You tell me, QSerraTico_Tico


This is what I said:
I never use cascading.
I always try for other solutions, like find transistors that operates better without cascade.
Why use 2 transistors, to do a 1 transistor job ??????


And when did I say anyone should do things my way?
What would I be able to do to stop them from Kaskading transistors
when there are good alternatives to keep stages simple.

There are benefits with KASckading, but there are also several drawerbacks.
This you would know, too.
So to avoid these drawbacks I will live with a more Cleene And Shimply cirrkut.


I understand that you are a kaskading man
This is why you feelt I was critisissing jou.
But I did not critisise nobody. Why should I?
If they want to kassekuding everything, so let them ...
I only told my opinion, how I do things
and I asked:
Why use 2 transistors to do a 1 transistar's jobb?




Quote:
Originally posted by QSerraTico_Tico

I would separate the bases of Q7 and Q11 and apply feedback to base of Q7.
Splitting bases of Q7 and Q11 and apply feedback in another way?
Maybe it would work,
but this would make a different circuit.
And some of the point with this new idea would be lost.

Can ask the question in this way:

If it is better to separate Q7 and Q11
why did not this IvanGrig do this ....
If it is performing better.


Regards
lineup
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Old 11th January 2007, 09:15 AM   #8
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Technically lineup; the term cascading is used for several stages of gain while cascoding is.... you know.
The latter helps reducing the Miller effect as well as the Early Effect. With JFETs as inputs it reduces also the Gate's leakeage current.
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Old 11th January 2007, 10:56 AM   #9
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Quote:
Originally posted by Elvee
. But for the amplifier to operate correctly, the NI input has to be kept at the ground potential; here the only coupling to the ground is via the bias resistors R34/35, and is quite loose. Is this intentional? If not, the open loop gain of the circuit could be greatly improved by substituting two capacitors (or two strings of diodes) to the ground in place of C8.

This is crucial, input stage of inverting amp must be high in PSRR. The circuit needs redesigning the voltage references.
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Old 11th January 2007, 11:37 AM   #10
Elvee is offline Elvee  Belgium
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Quote:
Originally posted by Kuzmenko
The potentials of the bases Q8+Q9 and Q10+Q12 are NOT fixed to the ground to avoid flowing currents thru signal ground. So, in this design there is no smallsignal ground at all There is only 1 powerground node on amplifier`s pcb. Because of this you can`t do mistake when you would route pcb`s traces

About "poor PSRR": are they? It`s not poorer than traditional diff+va design.
Off course there is a small signal ground: it may be implicit, but it's definetely there, it sits at the junction of the + and - PSU filter caps. Making an amplifier without reference node is like lifting yourself in the air using your own bootstraps: impossible, except in the case of unity gain buffer.
Having the ground node at such an inconvenient place is certainly not an advantage: it means for instance you've to take your input ground from there; it also means there will be unwanted feedback paths via the supply leads impedances: when high currents are drawn, parasitic voltages will appear on the NI input.
If you're not convinced, try to include realistic impedances for each of your wires in your simulation: 0.1ohm/0.5H for example.
Also, have you tried to calculate your PSRR? It's easy: R34 and R35 having equal values, it is 6dB seen from the input, but this is then amplified by the non-inverting gain of the amplifier, 22.3 in this case; this means a resulting PSRR of 6-27=-21dB!! (yes, minus!).
LV
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