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Old 28th December 2010, 01:32 PM   #1181
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Default contradictio in terminis

Quote:
Originally Posted by michaelkiwanuka View Post
With TPC C1 should be larger than C2, ..............
So, according to you, C1 < C2 is non-optimal, right?

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Originally Posted by michaelkiwanuka View Post
There is no such thing as an optimum ratio. .......
Maybe my sense of logic falls short, but I would think if there are non-optimal ratios, then there must also exist optimal ratios. Or do you mean all ratios are non-optimal anyhow?
Given the performance of TPC vs TMC, I would opt for the latter.
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Last edited by Edmond Stuart; 28th December 2010 at 01:41 PM.
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Old 28th December 2010, 03:21 PM   #1182
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Default obvious requirement

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Originally Posted by AndrewT View Post
I think everyone is agreed on that requirement.
I suppose you meant it sarcastically, didn't you?
Regarding the other requirement (C1>C2), I think it's 'utter nonsense' (his language, not mine). See, if not already done: Bob Cordell's Power amplifier book and this one: Bob Cordell Interview: Negative Feedback Clearly, C1<C2 gives better results.

Cheers,
E.
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Old 28th December 2010, 04:10 PM   #1183
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Quote:
Originally Posted by Bob Cordell View Post
TMC transitions the Miller pick-off point from the output stage back to the VAS at high frequencies.
If this were true then it would mean the resistor connected to the output provides feedback transmission that falls with frequency. An imposibility.
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Old 28th December 2010, 04:28 PM   #1184
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Quote:
Originally Posted by Edmond Stuart View Post
Hi Bob,

Same minds, same thoughts. see: HEEEELLLPPP : M. Randy Slone Mirror Image Topology Construction - Troubles.

What do you think about my latest sims, post 1169 and 1174?

Cheers,
E.
Hi Edmond,

I think your results are demonstrative that TMC is often better than TPC and that there seems no reason to use TPC when an amplifier with a good IPS/VAS is used.

For reference below, I have included a zip file that has two simulations of the amplifier I quoted results on earlier. Note that this file uses the models that I have posted on my website and which I believe to be better than most of those out there.

One simulation uses TMC and yields the results I quoted earlier (THD20 =0.0017%, 7th=3e-6). The other uses TPC that is set sufficiently agressive (by making R1 appropriately small) to achieve the same results for both THD-20 and 7th.

Note further that C2=5C1 for the TMC and C1=C2 for TPC. I have found that the use of identical C1, C2, R1 for TPC and TMC does indeed usually yield the same distortion results, but that TPC and TMC have different preferences for the ratio of C1 to C2. For example, if the TMC amplifier here is made into TPC by connecting R1 to ground, the same distortion performance is achieved but there is an anomoly in the closed loop frequency response that I cannot at this time explain.

Similarly, if you do TMC with C1=C2 with the same agressiveness in R1 to achieve 0.0017% distortion, there is also an anomoly in the closed loop frequency response.

So, TPC and TMC give about the same distortion result with identical components, but this is academic because each technique wants to have a different ratio for best performance. This is what I have been trying to say all along, perhaps not very well. It is thus silly to require the use of the same set of components for an honest apples-apples comparison. It is more appropriate to compare the techniques when each is set agressively enough to yield the same distortion performance, if possible.

Notably, the TPC amplifier with equally low distortion as the TMC amplifier exhibits 67% small-signal squarewave overshoot and +3dB closed loop gain peaking. Large signal squarewave response exhibits ringing.

Cheers,
Bob
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File Type: zip Cordell TMC TPC.zip (8.2 KB, 85 views)
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Old 28th December 2010, 04:38 PM   #1185
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Quote:
Originally Posted by michaelkiwanuka View Post
If this were true then it would mean the resistor connected to the output provides feedback transmission that falls with frequency. An imposibility.
no, it means that at high frequencies, the cap connected
to the vas output has lower impedance that the said resistor...

Isn t this obvious??????....
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Old 28th December 2010, 05:05 PM   #1186
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Thanks wahab. Clearly i misunderstood bob.
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Old 28th December 2010, 05:13 PM   #1187
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Originally Posted by Bob Cordell View Post
Notably, the TPC amplifier with equally low distortion as the TMC amplifier exhibits 67% small-signal squarewave overshoot and +3dB closed loop gain peaking. Large signal squarewave response exhibits ringing.
67% small-signal squarewave overshoot & ringing suggests your amp. is borderline stable. This implies component values for degeneration & compensation may well be incorrect. Check your loop gain. Also use phase lead comp.

Last edited by michaelkiwanuka; 28th December 2010 at 05:16 PM.
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Old 28th December 2010, 05:19 PM   #1188
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It looks like the phase margin for the TPC version is a bit low in the loop around the output stage. It also has a lower crossover frequency at 1.2 MHz compared to 1.7 MHz for the TMC version.

The gain in the loop is the same at 60 kHz though at about 46 dB.
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Old 28th December 2010, 08:11 PM   #1189
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Quote:
Originally Posted by michaelkiwanuka View Post
67% small-signal squarewave overshoot & ringing suggests your amp. is borderline stable. This implies component values for degeneration & compensation may well be incorrect. Check your loop gain. Also use phase lead comp.
Hi Mike,

This is just what it takes to obtain TPC performance equal to TMC performance - you know as well as anybody that TPC inevitably results in overshoot and frequency response peaking (assuming no phase lead crutch is used in the feedback path). The more agressive you make TPC to get lower distortion, the higher price you pay in transient behavior and stability. The large-signal ringing, though small, results from the fact that margins do decrease in the real world due to signal swing, especially when driving a 4 ohm load.

Mind you, this amplifier with TPC is not anything I would build or recommend; it is merely for comparison to show what you get when you try to get distortion performance from TPC that is as good as that easily obtainable from TMC.

The TMC version is rock stable, with no overshoot, no frequency response peak, and good phase and gain margins.

I urge you to choose different compensation values for this amplifier and simulate it to see if you can do better.

I also will be happy to see what you can do with this if you put in some lead compensation. Once again, however, I remind you that the TMC amplifier needs no lead compensation to perform really well.

Cheers,
Bob
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Old 28th December 2010, 08:31 PM   #1190
wahab is offline wahab  Algeria
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Indeed ,TPC overshoot can almost be suppressed, but one
has to consider the high value of the lead cap, not to
mention ,as pointed by Bob, that TMC has better looking
step response without ressorting to annoying arrangements...
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File Type: zip STEP RESPONSE.zip (29.4 KB, 39 views)
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