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Old 19th December 2010, 05:12 PM   #1061
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Originally Posted by YWN View Post
................
By inserting a gain probe in series with the feedback resistor (I am using a Tian probe implemented in LTSpice) I got the low frequency loop gain of about 95dB and the uncompensated unity loop gain frequency of about 9MHz. These are reasonable values for a Blameless, much lower than the 130dB and 30-40MHz unity loop gain frequency, claimed by wahab and Edmond.
.................
Aargh! We were talking about open loop gain, thus not the loop gain at the FB resistor. Read again: Bob Cordell's Power amplifier book where Wahab explicitly stated: 'not loop gain'. Furthermore, I've already explained the difference between these two gains here: Bob Cordell Interview: Negative Feedback
For a typical audio power amp the difference is about 30dB!

Next, I was talking about a typical blameless amp. As such an amp, compared to Bob's fig 3.10, has much lower degen. resistors in the IPS and is supplied with a much higher tail current, the open loop gain is (obviously) much higher, hence a UOLGF of 30MHz or so and hence about 130dB DC gain.
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Old 19th December 2010, 05:58 PM   #1062
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Originally Posted by jacco vermeulen View Post
More specific TPC : www.acoustica.org.uk/DPA/images/100S-3.jpg

(introduction : 1986)
Many thanks for making that page accessible; I have not had time to dig my old HFNs out of the cellar.

The circuit does not look like TPC as we know it, Jim; the R1-C1 network looks like a lead-lag network of the sort used in the notorious Otala amplifier. The previous website reference to three gain stages could be more evidence that the circuit might be an Otala, errm, tribute.

I suppose no-one has the schematic? I've not been able to find it.
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Old 19th December 2010, 06:01 PM   #1063
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Originally Posted by WuYit View Post
Hi,
from Doug's book it becomes clear that bipolar devices in CFP configuration must be biased in class A.
I don't know how you got that impression. It is not true.
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Old 19th December 2010, 06:55 PM   #1064
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Originally Posted by Edmond Stuart View Post
Aargh! We were talking about open loop gain, thus not the loop gain at the FB resistor.
Hi Edmond, you are probably right (although the schematic with the gain probe would always do a much better job in clarifying the author's intention, compared to stray/isolated graphs, language semantics and words phrasing). However, assuming it's about open loop gain and open loop unity gain frequency, I fail in understanding the relevance of those, in the context of a Miller-TPC-TMC comparison.

Last time I've checked, both the linearity and Bode stability criteria (phase and gain margins) are relative to the loop gain and unity loop gain frequency. The open loop parameters (gain, unity gain frequency), as used by wahab and yourself are, at best, only indirectly relevant. A higher open loop gain for the TMC, compared to TPC, (if that's possible, I haven't checked) doesn't mean that TMC creates more (useful for linearizing) loop gain compared to TPC.

Last edited by YWN; 19th December 2010 at 06:58 PM.
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Old 19th December 2010, 07:45 PM   #1065
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[snop] A higher open loop gain for the TMC, compared to TPC, (if that's possible, I haven't checked) doesn't mean that TMC creates more (useful for linearizing) loop gain compared to TPC.
I thought that 'the usefull loop gain for linearization' is simply the difference between closed loop gain and open loop gain. So, for the same closed loop gain, if you get more open loop gain (at a certain freq) you have more gain you can sacrifice for linearization.

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Old 19th December 2010, 09:24 PM   #1066
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Originally Posted by DouglasSelf View Post
does not look like TPC as we know it
Mr Self,

question is if the DPA-50S model is All that different, you indicated that Deltec Precision merely "claimed" they used 2-pole compensation.
The DPA-50S is a development from/out of the 100S, the step from double-pole to two-pole is in the margin of semantics.
Seems odd that DPA does not mention it anywhere currently, while sticking to their Sziklai output stage arrangement.

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The mists clear...According to the review, it used "double-pole/zero phase compensation" with three differential cascoded gain stages.


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Old 19th December 2010, 10:21 PM   #1067
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Originally Posted by janneman View Post
I thought that 'the usefull loop gain for linearization' is simply the difference between closed loop gain and open loop gain. So, for the same closed loop gain, if you get more open loop gain (at a certain freq) you have more gain you can sacrifice for linearization.

jan didden
Hi Jan, yes and no.

What you call "the sum" (in the time domain) is in fact (in the frequency domain) the product between the transfer functions (Laplace transforms) of the feedback network and the amplifier open loop gain. That's not necessary as simple as it may look, because the feedback network may add to, or modify (by (un)loading the amplifier), the pole-zero distribution. If the feedback network is strictly passive, then only loading effects (if any) are to be considered. If the loading effects can be neglected, and the feedback network is strictly passive, only then the loop gain is a scaled down (by a constant) version of the open loop gain.

An analysis on a specific schematic is the only way to find if any of the above approximations are valid - that's why I was asking wahab for the schematic, and stated that an open loop gain graph in isolation doesn't tell much about the loop gain itself.

As an example, if the feedback network has a phase lead cap in parallel, then the loop gain is totally different to the open loop gain.
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Old 20th December 2010, 01:09 AM   #1068
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The Deltec's output stage may bave been inspired by the Elektor's Edwin but this one had emitter followers as current dumpers, a scheme seen on Phase Linear, SAE and Graham Nalty's amp.
It's neither a Sziklai, it looks more like a Threshold Stasis.
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Old 20th December 2010, 01:09 AM   #1069
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Originally Posted by AndrewT View Post
or adding an EF follower after the CFP.
The CFP part is the pre-driver and driver.
The output part is the Follower or pairs of Followers.

Do the pre-driver and driver of the CFP run in true ClassA all the time, when not overloaded?
And only the output EF runs in ClassAB?
Hi Andrew,

You raise a couple of good points here. One of the nice things about the Locanthi Triple is that the predriver and driver always run in class A. Indeed, one of the disadvantages of the CFP output stage is that the driver cannot run in class A.

However, when a CFP is used as pre-driver and driver, the CFP can be run in class A, just like the Darlinton emitter follower pre-driver and driver in the Locanthi Triple if the outputs of the top and bottom CFP are connected together with a single emitter resistor.

Cheers,
Bob
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Old 20th December 2010, 05:42 AM   #1070
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Hi Doug,
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I don't know how you got that impression. It is not true.
Well, it is true to me. Minority carriers cause trouble leading to severe crossover distortion; ordinary switching distortion spiced with cross-conduction, a very annoying combination even for a hardened ear. Bipolars are poor transconductors, have less favorable input impedance, large base-to-collector capacitance, exhibit inferior switching characteristics, and numerous disturbances originated from the bipolar structure, there`s no good reason to use them as slave drivers anyway.
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