LTP w/CCS tail and plate loads - diyAudio
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Old 19th January 2008, 03:05 AM   #1
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Default LTP w/CCS tail and plate loads

I was reading the Morgan Jones book and am having trouble understanding one aspect of a circuit. On page 136 he shows a LTP configuration with CCS tail and plate loads.

In a configuration such as this, what fixes the plate voltage? Just looking at the circuit and plate curves it seems that since there is no fixed resistance value for a plate resistor or cathode resistor, there would be no fixed operating point.

Please help me understand this if I am missing something here.
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Old 19th January 2008, 02:55 PM   #2
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Let me better explain my confusion. The tail CCS can set the bias voltage on the grid to whatever it wants to. However, the plate load CCSs can adjust their resistance to whatever it needs to be at any given bias. Don't know if that makes any sense. I'll work on posting an example picture so that those who can't picture this circuit can look at something.
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Old 19th January 2008, 03:03 PM   #3
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Here's a picture for an example.
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Old 19th January 2008, 04:05 PM   #4
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Morgan gave the answer: "Fine adjustment of the tail current sets anode voltages". In other words, it's determined by trial.
In this application, the single transistor plate loads have lower impedance than the cascaded cathode load. They are intended to act as relatively high impedance plate loads when the B+ is not high enough for a high value resistor. You don't want a perfect CCS in both plate and cathode - that would be an immovable object meeting an irresistible force. So the cathode load dominates and the adjustment is made there.

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Old 19th January 2008, 06:02 PM   #5
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I also have difficulty with this concept. CCS tail and plate loading is a good idea, to get the best balance in the LTP and the most linear performance from the triodes. In fact, it's hard to imagine anyone ever using resistors by choice, even if they have plenty of B+ available.

But how do you know what the plate voltage is going to be? It can be important if you're using DC coupling to the next stage.

Well, one way out of the dilemma is to set each plate CCS to slightly less that 50% of the current in the tail CCS and connect a high value resistor in parallel with the plate CCS. The difference in current is forced to go through this resistor, so its value multiplied by the current difference determines the voltage drop across it. If the difference in current is small (say, 50uA), then the parallel resistor value can be high (much higher than an ordinary plate load resistor would be without a CCS), so you still get the benefit of having a really high value plate load.

It would all have to be adjusted and set by experimentation to get equal plate voltages, because of the tolerance of the components involved. I can't see it being used in commercial amps - every case would be different!
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Old 19th January 2008, 09:48 PM   #6
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Ray:

I've always had the same difficulty with this circuit concept, and I agree with the resistor idea. I call it a ballast resistor, not sure if that's appropriate or not. You could also connect it from plate to common, I would think, and ohm's law determines plate voltage by imbalanced current.
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Old 19th January 2008, 10:25 PM   #7
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Quote:
Originally posted by SpreadSpectrum
Here's a picture for an example.

I'm afraid of a moderator to comment such ideas, sorry...

But you may use my CCS with servo to make this thingy usable:

Click the image to open in full size.

Upper PNP transistor may be replaced by a FET to get as high as possible dynamic resistance (degraded because base current is needed!)
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Old 19th January 2008, 11:15 PM   #8
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Thanks, Wavebourn. It makes sense, if we're going to use controlled current imbalance between tail CCS and plate CCSs, with 'ballast resistors' (I like that term) to take up the current 'slack', then the plate CCSs might just as well use good PNP transistor cascodes, as opposed to an inferior single transistor, since we're no longer relying on the dominance of the tail CCS.
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Old 19th January 2008, 11:35 PM   #9
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Quote:
Originally posted by ray_moth
Thanks, Wavebourn. It makes sense, if we're going to use controlled current imbalance between tail CCS and plate CCSs, with 'ballast resistors' (I like that term) to take up the current 'slack', then the plate CCSs might just as well use good PNP transistor cascodes, as opposed to an inferior single transistor, since we're no longer relying on the dominance of the tail CCS.

The idea is not just a transistor cascode; the upper transistor adjust the current such a way that the voltage on the plate is determined by ratio between resistors R2 and R3. It acts as a combined AC current source and a DC voltage regulator. With such a load you may vary the cathode current as you want, but voltages on anodes will be always the same and stable.
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Old 20th January 2008, 12:42 AM   #10
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As I understand the circuit, it is a current source that prefers to set its resistance to a certain value under DC conditions, thus assuring that the plate voltage has a quiescent value. I will have to build one of these and try it out.

Looking back at the example in the Jones book, total current in the plate CCSs is .775mA and tail current would be .62mA, so I am guessing that somehow this CCS imbalance produces some stable plate voltage. However, I don't understand the mechanism for this and how to predict what the plate voltage would be. I guess that's what he means by, 'Fine adjustment of the tail current sets anode voltages.' I hadn't run the numbers and notice that there was an imbalance here before. I haven't built one of these yet. Maybe I will will have to soon...

I am thinking that building one with equal plate and cathode CCSs would result in strange behavior. Again, I will have to build one and see.
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