I think the circuit is a highly modified LTP phase inverter, with an extra stage tacked on.
The plate load of the driven triode grid of the LTP is Zero Ohms. That makes it a cathode follower.
The cathode follower drives the 2nd cathode of the LTP.
The LTP 2nd plate drives the 3rd triode (SRPP?).
True Grounded Grid Amplifiers always have the following 2 characteristics:
The Grid is DC terminated at Zero Ohms.
The Grid is AC terminated at Zero Ohms (And at RF too).
Anything else is either:
A Marketing Ploy to get you to purchase the product;
Or,
Has been named Grounded Grid by a Marketeer who is technically challenged.
Grounded Grid? . . .
Check out a 1956 ARRL Radio Amateurs Handbook.
Or Check out the 30MHz IF amplifier of a 10GHz Surface Search Radar that was on Year 1943 Fletcher Class Destroyers.
Some of those who do not know history are bound to repeat it . . . and might make things worse than they originally were.
The plate load of the driven triode grid of the LTP is Zero Ohms. That makes it a cathode follower.
The cathode follower drives the 2nd cathode of the LTP.
The LTP 2nd plate drives the 3rd triode (SRPP?).
True Grounded Grid Amplifiers always have the following 2 characteristics:
The Grid is DC terminated at Zero Ohms.
The Grid is AC terminated at Zero Ohms (And at RF too).
Anything else is either:
A Marketing Ploy to get you to purchase the product;
Or,
Has been named Grounded Grid by a Marketeer who is technically challenged.
Grounded Grid? . . .
Check out a 1956 ARRL Radio Amateurs Handbook.
Or Check out the 30MHz IF amplifier of a 10GHz Surface Search Radar that was on Year 1943 Fletcher Class Destroyers.
Some of those who do not know history are bound to repeat it . . . and might make things worse than they originally were.
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Not necessarily. The grid of the RH triode of an LTP phase platter isn’t DC grounded, and that’s a grounded-grid stage. What’s important is the AC grounding and the cathode drive.
ejp,
Good point!
A Grounded Grid amplifier's Grid:
Is Not always at DC Ground.
But that grid IS Always at AC / RF / and Signal Ground.
Grounded Grid stage definition:
Input signal to the Cathode
Output signal from the Plate
The Grid is Common to signal In and signal Out. (the grid does not "move", it is tied to Signal Common).
Common Cathode stage definition:
Input signal to the Grid
Output signal from the Plate
The Cathode is Common to the signal in, and the signal out.
And If an Un-bypassed: resistor; Un-bypassed current source; or another Un-bypassed cathode plus a current source / or LTP (*) is connected to the first cathode that is called Degeneration.
* That defines a coupled cathode CCS or LTP phase splitter.
A cathode coupled phase splitter has 1/2 the gain versus the stage gain of each stage if it had a bypass capacitor (of course we do not bypass, because we want the stage to split the phase).
Common Plate stage, Cathode Follower definition:
Input signal to the Grid
Output signal from the Cathode
The plate is Common to the signal in, and the signal out.
If you look carefully at the schematic in Post # 10, there is No grounded grid stage.
The first triode grid is connected to signal in.
The second triode grid is tied to the junction of a series resistor output signal voltage divider. Those series resistors divides the 2nd triode's plate signal.
The 2nd triode's grid moves when any signal is applied to the 1st triode's grid. The 2nd triode is not a grounded grid amplifier stage.
The top tube's grid also does move with signal.
It provides a low impedance output, and also provides the negative feedback signal to the 2nd triode grid of the phase splitter.
Conclusion: There is No grounded grid in this circuit
I guess the negative feedback to the 2nd triode's grid confused some people; they thought it was a grounded grid stage, it is not.
Which makes the title of this thread completely incorrect.
Other than that, the thread is a good learning tool!
Good point!
A Grounded Grid amplifier's Grid:
Is Not always at DC Ground.
But that grid IS Always at AC / RF / and Signal Ground.
Grounded Grid stage definition:
Input signal to the Cathode
Output signal from the Plate
The Grid is Common to signal In and signal Out. (the grid does not "move", it is tied to Signal Common).
Common Cathode stage definition:
Input signal to the Grid
Output signal from the Plate
The Cathode is Common to the signal in, and the signal out.
And If an Un-bypassed: resistor; Un-bypassed current source; or another Un-bypassed cathode plus a current source / or LTP (*) is connected to the first cathode that is called Degeneration.
* That defines a coupled cathode CCS or LTP phase splitter.
A cathode coupled phase splitter has 1/2 the gain versus the stage gain of each stage if it had a bypass capacitor (of course we do not bypass, because we want the stage to split the phase).
Common Plate stage, Cathode Follower definition:
Input signal to the Grid
Output signal from the Cathode
The plate is Common to the signal in, and the signal out.
If you look carefully at the schematic in Post # 10, there is No grounded grid stage.
The first triode grid is connected to signal in.
The second triode grid is tied to the junction of a series resistor output signal voltage divider. Those series resistors divides the 2nd triode's plate signal.
The 2nd triode's grid moves when any signal is applied to the 1st triode's grid. The 2nd triode is not a grounded grid amplifier stage.
The top tube's grid also does move with signal.
It provides a low impedance output, and also provides the negative feedback signal to the 2nd triode grid of the phase splitter.
Conclusion: There is No grounded grid in this circuit
I guess the negative feedback to the 2nd triode's grid confused some people; they thought it was a grounded grid stage, it is not.
Which makes the title of this thread completely incorrect.
Other than that, the thread is a good learning tool!
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The second stage is a grounded grid stage. The first stage is a cathode follower, and together they form a cathode-coupled amplifier.
The name 'grounded grid' is a cause for confusion, because it does not have to mean the grid is literally connected to ground. It is better to say 'common grid' stage, which means the same thing but makes it clear that the grid is merely 'common' to the IP and OP ports in AC terms. Feedback can be returned to the grid but it is still a common grid (grounded grid) stage, just like feedback can be returned to the cathode of a common-cathode stage.
Calling it a 'grounded grid preamp' is a bit of a stretch, however. When I hear those words it makes me assume the input stage (maybe the only stage) is the grounded grid stage, which it almost never is. By their logic, any preamp that contained an LTP could be called a grounded grid preamp, which would be kinda silly.
The name 'grounded grid' is a cause for confusion, because it does not have to mean the grid is literally connected to ground. It is better to say 'common grid' stage, which means the same thing but makes it clear that the grid is merely 'common' to the IP and OP ports in AC terms. Feedback can be returned to the grid but it is still a common grid (grounded grid) stage, just like feedback can be returned to the cathode of a common-cathode stage.
Calling it a 'grounded grid preamp' is a bit of a stretch, however. When I hear those words it makes me assume the input stage (maybe the only stage) is the grounded grid stage, which it almost never is. By their logic, any preamp that contained an LTP could be called a grounded grid preamp, which would be kinda silly.
Merlinb,
Are we talking about the schematic in Post # 10?
If you look at the 3 triodes that are all connected together, one thing the 3 grids have is . . .
Each grid moves when signal is applied to the input grid.
All grounded grid stages have one characteristic, their grids do Not move when signal is applied (by definition).
Just my knowledge of ancient vacuum tube history, and my present opinion too.
Are we talking about the schematic in Post # 10?
If you look at the 3 triodes that are all connected together, one thing the 3 grids have is . . .
Each grid moves when signal is applied to the input grid.
All grounded grid stages have one characteristic, their grids do Not move when signal is applied (by definition).
Just my knowledge of ancient vacuum tube history, and my present opinion too.
"All grounded grid stages have one characteristic, their grids do Not move when signal is applied (by definition)."
The definition is which terminal is common to the IP and OP ports from an analytical point of view. There are only three ways to arrange a tube as an amplifier, after all, it has to be one of them.
The definition is which terminal is common to the IP and OP ports from an analytical point of view. There are only three ways to arrange a tube as an amplifier, after all, it has to be one of them.
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Merlinb,
Thanks for responding.
You said " It is better to say 'common grid' stage, which means the same thing but makes it clear that the grid is merely 'common' to the IP and OP ports in AC terms".
I think that the only common point of input and output signals is non-moving ground. These are single ended inputs and outputs, not balanced inputs and outputs.
If Bell Labs had never invented negative feedback, all grounded grids would be at AC / Signal / RF, and Microwave Ground.
In the case of the schematic in Post # 10, none of the 3 Triode elements are common to both signal in, and signal out.
Signal In is common to ground. Signal Out is common to groound.
Schematic, again, Post # 10:
Both signal in & signal out, are common to the bottom end of R12, which is . . . Ground.
But the grid is not at signal ground; instead it is at the Junction of R12 and R10, which is Moving.
The moving grid voltage is the method of creating negative feedback. Good!
The point is, the grid voltage is not common to either signal in, nor signal out, they are at Non-Moving Ground, but the grid Moves.
I would argue that the grid is common to Negative Feedback signal (not the non-moving negative feedback signal; instead the single ended moving negative feedback signal.
Call that circuit whatever you want, it is not historically correct to call it either grounded grid, or to call it common to input and output.
Just my imperfect circuit analysis, and my imperfect opinion.
Thanks to all for reading!
Thanks for responding.
You said " It is better to say 'common grid' stage, which means the same thing but makes it clear that the grid is merely 'common' to the IP and OP ports in AC terms".
I think that the only common point of input and output signals is non-moving ground. These are single ended inputs and outputs, not balanced inputs and outputs.
If Bell Labs had never invented negative feedback, all grounded grids would be at AC / Signal / RF, and Microwave Ground.
In the case of the schematic in Post # 10, none of the 3 Triode elements are common to both signal in, and signal out.
Signal In is common to ground. Signal Out is common to groound.
Schematic, again, Post # 10:
Both signal in & signal out, are common to the bottom end of R12, which is . . . Ground.
But the grid is not at signal ground; instead it is at the Junction of R12 and R10, which is Moving.
The moving grid voltage is the method of creating negative feedback. Good!
The point is, the grid voltage is not common to either signal in, nor signal out, they are at Non-Moving Ground, but the grid Moves.
I would argue that the grid is common to Negative Feedback signal (not the non-moving negative feedback signal; instead the single ended moving negative feedback signal.
Call that circuit whatever you want, it is not historically correct to call it either grounded grid, or to call it common to input and output.
Just my imperfect circuit analysis, and my imperfect opinion.
Thanks to all for reading!
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I also do not agree and a differential pair (long tailed pair) where both
inputs are used should be described as a differential pair (post 19).
Some differential pairs are set up as follower driving a grounded grid
stage, yet this is not the case here. The other grid has "the same" input
signal. In fact I can not understand the confusion.
inputs are used should be described as a differential pair (post 19).
Some differential pairs are set up as follower driving a grounded grid
stage, yet this is not the case here. The other grid has "the same" input
signal. In fact I can not understand the confusion.
Those who do not know the original history are bound to change the meanings of those old definitions.
A grounded 416A grid RF amplifier at 108 MHz is never going to apply negative feedback to the grid.
The stage is already Super Linear, why add negative feedback?
The grounded grid amplifier was so intrinsically linear, that an engineer came up with the idea of a Cascode amplifier,
where the top triode was guess what? . . . a grounded grid amplifier.
There were cascode RF amplifiers on all the early TV sets, probably more cascode stages than all of the rest of tube electronics.
Those who are solid state fans, should look at history too.
The original transistor amplifier (Bell Labs again) was a Grounded Base amplifier.
Lots of people know the term ft, the cutoff frequency of a grounded emitter amplifier.
But did you know this one . . .
. . . The Alpha frequency cutoff of a grounded base amplifier is 1.2 x ft of the transistor.
You can build oscillators at 1.2 x ft.
'Nuf History for now!
A grounded 416A grid RF amplifier at 108 MHz is never going to apply negative feedback to the grid.
The stage is already Super Linear, why add negative feedback?
The grounded grid amplifier was so intrinsically linear, that an engineer came up with the idea of a Cascode amplifier,
where the top triode was guess what? . . . a grounded grid amplifier.
There were cascode RF amplifiers on all the early TV sets, probably more cascode stages than all of the rest of tube electronics.
Those who are solid state fans, should look at history too.
The original transistor amplifier (Bell Labs again) was a Grounded Base amplifier.
Lots of people know the term ft, the cutoff frequency of a grounded emitter amplifier.
But did you know this one . . .
. . . The Alpha frequency cutoff of a grounded base amplifier is 1.2 x ft of the transistor.
You can build oscillators at 1.2 x ft.
'Nuf History for now!
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So you would not call this a common-cathode amplifier, because the cathode 'moves' with the signal. I think you may be on your own there.
Attachments
Merlinb,
You are correct!
Common Cathode.
The Cathode IS the tube's common of tube input, and IS the tube's common of tube output.
And yes, the Cathode IS moving.
But now, let's talk about Signal Common . . .
The Cathode is NOT common to the signal in, and is Not common to the signal out.
That is because the Common of signal in is Ground; and the Common of signal out is Ground.
The top of the cathode resistor is Not common to signal in, Nor to signal out.
Definitions can be so tricky!
(They are Commonly mis-understood, and words are improperly interchanged)
Sign hanging at my old desk at work:
"Grounds are Commonly Mis-understood"
It still is hanging there, 14 years after my retirement.
Whoever replaced my, has to follow the same rules.
We had a customer who had a 6 foot "Ground" pipe.
The top of the pipe was the return for a Physics experiment of a Very fast rise multi-Ampere pulse.
1 foot above the bottom of the pipe, the custormer measured a pulse of 100V!
Any conductor, even a large diameter pipe is an inductor; inductive reactance in Ohms x Multi-Ampere pulse = many volts.
When are grounds not grounds.
When the conductor has resistance, and when the conductor has inductance.
All conductors have inductance.
But there are super conductors that do not have resistance (they still have inductance).
Whenever a conductor has Ground at one end, the other end is Not ground.
Parasitics Abound!
You are correct!
Common Cathode.
The Cathode IS the tube's common of tube input, and IS the tube's common of tube output.
And yes, the Cathode IS moving.
But now, let's talk about Signal Common . . .
The Cathode is NOT common to the signal in, and is Not common to the signal out.
That is because the Common of signal in is Ground; and the Common of signal out is Ground.
The top of the cathode resistor is Not common to signal in, Nor to signal out.
Definitions can be so tricky!
(They are Commonly mis-understood, and words are improperly interchanged)
Sign hanging at my old desk at work:
"Grounds are Commonly Mis-understood"
It still is hanging there, 14 years after my retirement.
Whoever replaced my, has to follow the same rules.
We had a customer who had a 6 foot "Ground" pipe.
The top of the pipe was the return for a Physics experiment of a Very fast rise multi-Ampere pulse.
1 foot above the bottom of the pipe, the custormer measured a pulse of 100V!
Any conductor, even a large diameter pipe is an inductor; inductive reactance in Ohms x Multi-Ampere pulse = many volts.
When are grounds not grounds.
When the conductor has resistance, and when the conductor has inductance.
All conductors have inductance.
But there are super conductors that do not have resistance (they still have inductance).
Whenever a conductor has Ground at one end, the other end is Not ground.
Parasitics Abound!
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I agree with your definition of a Grounded Grid 100% and the historical info you give is very interesting.
I'm not sure how the preamp circuit works, I would have to simulate it. That said the circuit looks like the so called differential pair and with V1B and V2A forming a so called Cascode that I call Cascade.
Looks like stress looks like.
Now Cascade does not fit formal definition of Common Grid or Grounded Grid but but people call it such.
More correctly the Cascade has a "virtual Grounded Grid" because with Cascode the bottom tube Plate current is changing but the bottom tube Plate voltage stays pretty much constant. Now I have seen the Grid of the upper Cascade tube wired either to a fixed voltage or via a resistor to the constant voltage of the lower tube Plate, so the Grid of the upper Cascode tube stays constant and hence people are calling it Common Grid.
All that said this preamp is complicated to understand, it even has split HT supplies like a B+ 0V B- and I'm not going to admit to seeing a genuine Common Grid until proven so to be.
The Grounded Grid or Grounded Base is a magical configuration, have a look at the circuit of an Eddystone EC10 that I posted in one of my threads.
The thread is:
Magical Common Base configuration, why so little used?
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1. Triodes (pentodes, beam power tubes, JFETs, MOSFETs, BJTs, etc. can add more complications, so let's confine this discussion to Triodes).
The first problem with describing the preamp schematic of this thread, is it uses 3 triodes, not just 2 triodes.
So, do you describe:
The first 2 triodes as a unit, and the 3rd triode as a stand-alone unit;
Or do you describe the 1st triode as a stand-alone, and describe the 2nd and 3rd triodes as a unit;
Or do you describe each triode as 3 stand-alone units,
Or do you describe all 3 triodes together as one unit?
The way it is connected, plus the negative feedback, makes naming the function/mode of either the combinations of, Or of each of the individual 3 triodes somewhat nebulous.
That causes lots of circuit names, most of which are correct, but that are different circuit names; depending on the eye of the beholder.
2. Separate from the circuit of this thread's schematic, there is another commonly mis-understood pair of circuit names . . .
def: Cascade . . .
The most often used 2 triode topology that is called a "Cascade" amplifier . . .
Is when one common cathode triode's plate drives a second triodes grid (with the second triode also in common cathode mode).
The coupling is Either RC coupling, Or DC coupling.
def: Cascode . . .
The Cascode amplifier has signal driving the grid of a common cathode triode with its plate output - driving the cathode of a 2nd triode.
The second triode is in "grounded" grid mode, and the output of the 2nd triode's plate drives a plate load.
The two stages of the Cascode, can be DC coupled from plate 1 to cathode 2 . . .
Or can be Capacitor coupled from plate 1 to cathode 2 (as it was in the 30MHz IF amplifier of a 10GHz Radar).
Of course, with Capacitor coupling, either resistors, chokes, or CCSs have to provide the DC current to the 1st triode's plate, and provide the DC current to the 2nd triode's cathode.
I hope that clears up the differences.
As you can plainly see, the Cascade amp, and the Cascode Amp, are not the same. One is an Orange, the other is a Zebra.
The first problem with describing the preamp schematic of this thread, is it uses 3 triodes, not just 2 triodes.
So, do you describe:
The first 2 triodes as a unit, and the 3rd triode as a stand-alone unit;
Or do you describe the 1st triode as a stand-alone, and describe the 2nd and 3rd triodes as a unit;
Or do you describe each triode as 3 stand-alone units,
Or do you describe all 3 triodes together as one unit?
The way it is connected, plus the negative feedback, makes naming the function/mode of either the combinations of, Or of each of the individual 3 triodes somewhat nebulous.
That causes lots of circuit names, most of which are correct, but that are different circuit names; depending on the eye of the beholder.
2. Separate from the circuit of this thread's schematic, there is another commonly mis-understood pair of circuit names . . .
def: Cascade . . .
The most often used 2 triode topology that is called a "Cascade" amplifier . . .
Is when one common cathode triode's plate drives a second triodes grid (with the second triode also in common cathode mode).
The coupling is Either RC coupling, Or DC coupling.
def: Cascode . . .
The Cascode amplifier has signal driving the grid of a common cathode triode with its plate output - driving the cathode of a 2nd triode.
The second triode is in "grounded" grid mode, and the output of the 2nd triode's plate drives a plate load.
The two stages of the Cascode, can be DC coupled from plate 1 to cathode 2 . . .
Or can be Capacitor coupled from plate 1 to cathode 2 (as it was in the 30MHz IF amplifier of a 10GHz Radar).
Of course, with Capacitor coupling, either resistors, chokes, or CCSs have to provide the DC current to the 1st triode's plate, and provide the DC current to the 2nd triode's cathode.
I hope that clears up the differences.
As you can plainly see, the Cascade amp, and the Cascode Amp, are not the same. One is an Orange, the other is a Zebra.
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