ok, the tube being a voltage driven device while the BJT being a current driven device would account for in circuit differences regarding feedback. But Beta has to be constant with transistor action for linear amplification. To my (very limited) experience Beta highly depends on temperature.
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Since it is a current amplifier circuit, you need to consider the linearity of the current gain from emitter to collector. For a high gain device, pretty much all the input (emitter) current comes out at the collector unaltered, minus some base current.
That current through the (linear) collector resistor provides the output voltage. Note that the Vout is taken off referenced to the +Vc supply so you need a good regulated supply (or take it off differentially).
For best linearity: 1 - make sure your input signal is a current, from a large impedance (current) source like a current-output DAC as was the case in my app, this takes care of any Vbe issues and non-zero input impedance at the emitter; 2 - use a high gain device to minimize ibase, and set it up with a DC standing current larger than the maximum expected signal current amplitude, so it remains in class A all the time.
Jan
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As DF96 has already pointed out, the LTP configuration being discussed is more correctly termed a coupled-cathode. A coupled-cathode pair functions in an interesting manner as an open-loop compound amplifier stage. The front (input) half of the LTP functions as a cathode-follower which directly drives the cathode of the back (output) half of the pair functioning as a grounded-grid! The grid of the back half of the pair must, of course, be A.C. grounded. So, no feedback signal can be accepted, such as is usually applied to a differential input LTP. This strikes me as an particularly elegant configuration when implemented with a dual-triode tube.
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Well explained.
To add, it is common practice to analyze circuits using the superposition principle. This is in fact what you do here, setting the 'other side' input to zero and then analyzing how it works with an input signal.
The next step would be to use an input on the feedback side, setting the input signal to zero. Then, the superposition principle says that the full operation is the combination of the two, meaning here that the output is the difference of the two inputs.
It is a powerful technique of divide-and-govern.
Jan
If by "balanced" you mean, differential, then no, a coupled-cathode LTP does not operate as a differential amplifier. Rember, one half of the pair has it's grid grounded, so, it cannot accept a differential input signal. There's only a single input to, and a single output from, the compound pair. So, there can be no differential error feedback loop processing, as would be the typical case when utilizing the LTP for differential/balanced operation.
A feedback loop could, of course, be implemented via a couple of resistors at the single input grid (in inverting amplifier fashion) but that then would require a phase-inversion be implemented on the feedback signal somewhere downstream of the coupled-cathode pair.
The main point is that in a differential amplifier, the joint cathodes are not refered to ground. Whether a signal is present or not is factually and definitionally irrelevant. A phase inversion is not accomplished by signals. To develop an amplitude, the cathode-follower requires the presence of a resistive element.
N101N,
Do you call the Cascode amplifier a Cascode amplifier?
Or, do you call the Cascode amplifier a Common Cathode amplifier Plus a Common Grid amplifier?
What is in a name?
How about a LTP or differential amplifier?
A differential amplifier may, or may not, have a signal at one, none, or both inputs.
And those signals may be in opposite phase; or they may be in phase.
The amplifier is differential, the signals may be different, or they may not.
Do you call the Cascode amplifier a Cascode amplifier?
Or, do you call the Cascode amplifier a Common Cathode amplifier Plus a Common Grid amplifier?
What is in a name?
How about a LTP or differential amplifier?
A differential amplifier may, or may not, have a signal at one, none, or both inputs.
And those signals may be in opposite phase; or they may be in phase.
The amplifier is differential, the signals may be different, or they may not.
Indeed. You could argue that grounding one input of a diff amp is just feeding that input with a signal that happens to be zero. The output is still the difference between the two inputs.
And what is zero signal? It's normally understood to be zero measured to ground - measure it to any other node and it isn't zero.
Understanding concepts gets you much farther than staring at individual circuits ;-)
Jan
And what is zero signal? It's normally understood to be zero measured to ground - measure it to any other node and it isn't zero.
Understanding concepts gets you much farther than staring at individual circuits ;-)
Jan
I'm a bit uncertain about what exactly you are hoping to illustrate with that. A coupled-cathode LTP will exhibit greater bandwidth than an otherwise equivalent differential LTP. Each half of a proper differential LTP has an anode load and operates as an grounded-cathode amplifier. The cathodes are referenced to ground, A.C. ground. As a consequence, the pair will suffer Miller capacitance.
A coupled-cathode does not suffer from Miller. Grounding the grid of one half of an LTP and removing the anode load in the other half changes the operating mode of the pair. What was two grounded-cathode amplifiers becomes one cathode-follower feeding one grounded-grid amplifier. Because a coupled-cathode visually appears similar to a differential-pair doesn't mean they operate the same.
Yes. One output and one input not being refered to ground, technically it is a balanced topology consisting of a transconductance amplifier and a transimpedance amplifier. I have approving attitude towards the Cascode.
We deal with the signal propagation medium, not the signal. It is absolutely essential to differentiate between these notions. We can only describe the medium. The medium properties hugely determine the characteristics of generated signals.
Ken,
the single-stage amplifier topology denotes devices operating with a single-polarity power supply, the three terminals refered to ground through a passive element, such as wire, resistor, choke. Do you stick to this definition?
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