John Curl's Blowtorch preamplifier part II

[john curl]

I again challenge Syn08 to come up with equivalent quality complementary matched fets that are available today, except for Linear Systems, which are relatively expensive at the moment, and hard to get, as well. It is not professional to criticize without any proof, but it is done with impunity, here, that's for sure.

[PMA]

Originally Posted by syn08

Great, now apply input signal and swing the output towards the rails.

Output is unable to get to rails. Look once more at the circuit and operating point voltages.

[john curl]

Interesting, but first everyone please realize that the CTC Blowtorch circuit, and similarly PMA's approximation, have a differential gain of about 16 dB, or about 8 dB on each side. This is not a unity gain buffer, as it would be pointless to put this topology in the circuit, if it were.
Please correct me if I am wrong, but isn't Syn08 presuming that this circuit has negative or at best, unity gain?

[john curl]

Thanks PMA, I simply approached it from the input angle, but everyone please realize that INTERNALLY this circuit operates from +/- 15V and the input devices can't even swing that far, unless you hit them with lightning, or some such, as they are clamped at 15 volts on their drains, no matter what the output swing.

[PMA]

They can see max. Vds like 18V when overdriven hard.

[john curl]

Oh, now I see what Syn08 is saying: He thinks that the input noise does not matter, because the cascode connection is 'compromised with the 220 ohm resistors? Well, it is partially true, especially at very low frequencies, but I still need complementary matched pairs, the highest transconductance I can get in the input pairs, and they have to be quiet, in order not to add to the second stage.
Now, for everyone else: It might be a good time to show what is happening.
First, a cascode connection is always composed of 2 active devices.
The classic cascode is usually the same polarity devices (for example, 2N channel fets) and the top fet, is biased at a voltage between the input (usually zero volts) and the power supply. This connection gives almost all the noise contribution to the FIRST or input device. Now, what happens when we add a parasitic resistor (for some reason) of 220 ohms at the junction between the first and second fets and capacitively couple it with a large cap, to ground?
Well, this changes things, somewhat. Now the top fet can have actual voltage gain of its own, and this noise can inject itself into the output.
I think that this is what Syn08 is hinting at. Of course, I make the resistor as large as possible to reduce this noise, is why we use 30V instead of 18V or so. Make more sense, now?

[syn08]

Originally Posted by john curl

Oh, now I see what Syn08 is saying: He thinks that the input noise does not matter, because the cascode connection is 'compromised with the 220 ohm resistors? Well, it is partially true, especially at very low frequencies, but I still need complementary matched pairs, the highest transconductance I can get in the input pairs, and they have to be quiet, in order not to add to the second stage.
Now, for everyone else: It might be a good time to show what is happening.
First, a cascode connection is always composed of 2 active devices.
The classic cascode is usually the same polarity devices (for example, 2N channel fets) and the top fet, is biased at a voltage between the input (usually zero volts) and the power supply. This connection gives almost all the noise contribution to the FIRST or input device. Now, what happens when we add a parasitic resistor (for some reason) of 220 ohms at the junction between the first and second fets and capacitively couple it with a large cap, to ground?
Well, this changes things, somewhat. Now the top fet can have actual voltage gain of its own, and this noise can inject itself into the output.
I think that this is what Syn08 is hinting at. Of course, I make the resistor as large as possible to reduce this noise, is why we use 30V instead of 18V or so. Make more sense, now?

From this perspective, you finally got it almost right. Add that, within the schematic that was posted as BT gain stage, the input JFET transconductance doesn't matter a iota. In the first approximation it is the ratio between the drain resistor and the source degeneration equivalent resistor that gives the input stage voltage gain. Good bye very large JFET transconductance requirement.

And no, as long as the impedance the first stage sees in the MOSFET source is very large, the input stage is NOT a transconductance stage and the MOSFET is NOT a I/V converter. The situation would be completely different if you would use bipolars in the output stage. Which, in fact, could be a better solution here.

Gotta run...

[john curl]

What does anyone want to predict that the input impedance of the second stage mosfet is when operated between 35 and 50ma? Now, the input impedance is defined as 1/Gm. The representative device is the IRF510.

[john curl]

Times up. It is about 5 ohms. Now, 5 ohms is a bit less than 200-220 ohms, does this change the situation?
After all, lets say that there is NO resistor but an ideal current source or nothing at all. Then what is the 'voltage gain of the first stage? Does anyone see the complexity of the situation? How could a cascode work in the first place, if you thought of it as 2 separate gain stages?

[PMA]

It makes almost no difference, if 220R resistors, or current sources.

And output mosfets have no current gain, output voltage is created on 1k load resitors.

It behaves like voltage output with 1k output impedance, so I do not know why syn08 complains to "gain dependent on load" - same as many common output circuits. Frankly speaking, I would rather see 100 ohm output impedance, 1k is a way too much to me.