JFET cascoded with tube gain calculation

[Joshua_G]

How can the gain of JFET cascoded with tube, like in attached schematic, be calculated?

[Allen Wright]

Why fuss around with calculations? Build it and MEASURE! I very seldom caculate, but I do a LOT of measuring, and then I can be 100% certain.

Regards, Allen

[sawreyrw]

We need to see more of the circuit in order to do the calculation.

[davidsrsb]

It's just like a all jfet cascode, the gm is determined by the jfet.
The valve has very close to unity current gain
R2 gives series feedback

[Joshua_G]

Thanks, Allen and David.

[Michael Koster]

Originally Posted by Allen Wright

Why fuss around with calculations? Build it and MEASURE! I very seldom caculate, but I do a LOT of measuring, and then I can be 100% certain.

Regards, Allen

Hi Allen,

Although I generally agree with that, I do think there is a place for calculation, (modeling, simulation...)

Sometimes I mess around with calculations to predict how a circuit will behave before building it, mainly to get the circuit closer to it's desired operation before building. It helps in deciding which parts and part values to select and sometimes rules out impractical circuit approaches.

It's also very instructive in cases like this to work through the calculation as a way to teach the circuit theory.

When a question like this comes up, it's an indication that the poster doesn't fully understand the operation of the circuit (otherwise he or she could easily construct the formula themselves).

It's also not clear in this case whether Joshua is building this or studying for learning purposes. In any case, building it is definitely the only way to be sure, but maybe not always necessary. Maybe if 3 circuits are analyzed, only one of them need be built.

So I think a good response would be to either teach the circuit theory through example or refer the poster to some reference material that teaches about cascode circuits.

There is enough of the circuit shown to teach the stage gain analysis. Several clues have been given.

Who can teach the analysis? Can Joshua now demonstrate how to estimate the gain of a cascode amplifier stage?

[Joshua_G]

Hi Michael,
The question was not for studying purpose, I'm going to build a tape playback amp with the front end after Allen's phono stages, like in the schematic I posted (which is taken from Allen's website).

I admit I don't understand completely how cascoded stage work. Any explanation will be welcomed.

[Gordy]

Originally Posted by Allen Wright

Why fuss around with calculations?

Because people often have limited time, and therfore can not afford to build on the off-chance that it might be right.

Because people often have limited money, and therfore can not afford to buy a stock of components just to experiment.

Because people often have limited time and money, and therfore can not afford to have circuits explode because they were designed wrong.

Because calculating is a proven professional way of taking a step towards a successful design. (And I don't just mean in audio, but in any technical subject).

Because calculating goes hand-in-hand with understanding, and understanding often leads to improved design, easier modification, easier fault-finding, and a good chance of success, and hence ultimately leads to a sense of intellectual reward.


If you want to do it the other way then best of luck to you, however I would encourage folks to understand and calculate before 'buying and trying'.

[dia thien thai]

Originally Posted by Allen Wright

Why fuss around with calculations? Build it and MEASURE! I very seldom caculate, but I do a LOT of measuring, and then I can be 100% certain.

Regards, Allen

I guess nobody in this forum knows exactly how to calculate the gain or to analyze the circuit. People here normally judge a circuit by ear. Except me, of course.

[SY]

Originally Posted by dia thien thai

I guess nobody in this forum knows exactly how to calculate the gain or to analyze the circuit.

You guess incorrectly. A = gm of the FET times the AC load resistance of the tube. The effective gm of the FET is modified by the source degeneration resistor.