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Old 27th March 2012, 03:27 PM   #11
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Thanks for your confirmation, Edmond.

Looking at the circuit again, I should re-phrase my question. It's not the matching of Q2 and Q3 but rather the current sources I1 and I2 that I'm concerned with. They need to be exact in order to derive the full benefits of this symmetry concept.

With opamp manufacturing, it is not an issue. However, using discretes, I have encountered difficulties in getting both current sources to be equal. Is there an easy solution.

By the way, thanks for sharing your design. I think there would be a real reduction in THD20. Very elegant.
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Old 27th March 2012, 03:28 PM   #12
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Quote:
Originally Posted by AndrewT View Post
how do I1 & I2 get current from/to the supply rails?
The only route I can see is through the shorted CtoB of Q2 & 3
Hi Andrew,

Good point. Indeed through the shorted CtoB of Q2 & 3.
How exactly I will show in one of the following updates of my website.

Cheers,
E.
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Old 27th March 2012, 03:50 PM   #13
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Quote:
Originally Posted by Michael Chua View Post
Thanks for your confirmation, Edmond.

Looking at the circuit again, I should re-phrase my question. It's not the matching of Q2 and Q3 but rather the current sources I1 and I2 that I'm concerned with. They need to be exact in order to derive the full benefits of this symmetry concept.
Hi Michael,

Now understand your question better. I1 and I2 don't need to be exactly equal, though a difference does have a profound on the voltage- and current-offset.

Quote:
With opamp manufacturing, it is not an issue. However, using discretes, I have encountered difficulties in getting both current sources to be equal. Is there an easy solution.
Don't worry. The circuit still works if I1=2*I2 ! If you keep it within a few percent, everything is fine.

Quote:
By the way, thanks for sharing your design. I think there would be a real reduction in THD20. Very elegant.
Thank you!

But my question was: Is it new; has anybody seen it before?

Cheers,
E.
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Last edited by Edmond Stuart; 27th March 2012 at 03:57 PM. Reason: red
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Old 27th March 2012, 03:55 PM   #14
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Quote:
Originally Posted by Bonsai View Post
So What?
Is that all you have to say?
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Old 27th March 2012, 04:34 PM   #15
PChi is offline PChi  United Kingdom
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Hello,
I have seen similar front ends.
Q1 to Q6, Q9 and Q12 are similar to the complimentary differential input stage with current mirror loads shown in Figure 7.9 of 'Designing Audio Power Amplifiers by Bob Cordell. Q7, Q8, Q10 and Q11 are different. The double differential input configuration is also shown in the 'Audio Power Amplifier Design Handbook' , Fifth edition, figure 5.12 by Douglas Self.
I believe that rail to rail input operational amplifiers use the NPN and PNP differential pairs for the commom mode range.
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Old 27th March 2012, 06:09 PM   #16
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it looks like one of the circuits developed by Hawksford.
see his distortion correction for audio amplifier circuits paper.
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Old 27th March 2012, 07:16 PM   #17
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Quote:
Originally Posted by PChi View Post
Hello,
I have seen similar front ends.
Q1 to Q6, Q9 and Q12 are similar to the complimentary differential input stage with current mirror loads shown in Figure 7.9 of 'Designing Audio Power Amplifiers by Bob Cordell. Q7, Q8, Q10 and Q11 are different.
Hi PChi,

That's the precisely the point: "Q7, Q8, Q10 and Q11 are different."

Quote:
The double differential input configuration is also shown in the 'Audio Power Amplifier Design Handbook' , Fifth edition, figure 5.12 by Douglas Self.
I believe that rail to rail input operational amplifiers use the NPN and PNP differential pairs for the common mode range.
I've checked those books (of course) but nothing comes close. It's not about the double (i.e. complementary) input stage, rather the Baxandall super pair in the TIS (or VAS if you like, a misnomer btw)

Cheers,
E.
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Old 27th March 2012, 07:30 PM   #18
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Quote:
Originally Posted by myhrrhleine View Post
it looks like one of the circuits developed by Hawksford.
see his distortion correction for audio amplifier circuits paper.
Hi myhrrhleine,

You are on the right track. Indeed, the Baxandall super pair closely resembles the Hawksford cascode (hence the reference to his article on my website), but a crucial difference is that my TIS has a current gain of just 1x (instead of beta or even beta^2).

Cheers,
E.
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Old 27th March 2012, 07:40 PM   #19
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E.

It has lots of similarity to a number of circuits. Cherry at first came to mind, but the exact design with your intent seems like a new twist. (The adage of enough chimps pecking at typewriters will eventually produce Shakespeare comes to mind, preventing an absolute claim at exclusivity.)

So go for it!

ES
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Old 27th March 2012, 08:15 PM   #20
Elvee is online now Elvee  Belgium
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I have never seen or done something exactly identical.

I have used variants, mainly without the cascodes, and with the collectors of Q5/Q6 directly tied to R15/R16.
A sort of an afterthought, a way to recycle an active current that would otherwise be wasted.

But anyway, I am not very fond of double LTP's and their closed translinear loop, I didn't mind very much, and I didn't notice anything extraordinary.
I may have missed something.....
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