Date: Sun, 02 Jan 2000 13:19:19 -0500
From: Ken Gilbert email@example.com
Subject: Cascode stuff (WAS selecting right tube for cascode circuit)
I hope you don’t mind me posting this on the list again–I think it may be information that others may find useful.
Now the thing that gets me is how you claim to get a higher amp factor from
a lower mu tube than from a hi mu tube.
Well the secret is in the cascode connection. When connected like this (tubes stacked on top of one another, top tube’s grid held at fixed potential, signal input to bottom tube’s grid, bottom tube’s plate directly connected to cathode of top tube, top tube plate loaded) then you are isolating the input and output circuits from one another.
In a normal, plate loaded tube, because the plate and grid are out of phase with one another, as the grid goes negative, the plate goes positive, and vice versa. You can see how the plate voltage will have an effect on the current through the tube, so that when the voltage is high the current will tend to rise, and vice versa. This is a measure of the internal plate impedance of the tube. A pentode, as you may well know, has a very high plate impedance due to the electrostatic screening effect of the screen grid. Thus the plate voltage has very little to do with the plate current. A triode, on the other hand, is not like this, exhibiting some lower level of plate impedance.
In a nutshell, the lower your plate impedance is, the lower your gain is going to be, since the plate voltage will thereby have a larger bearing on the plate current. There are other factors like transconductance to contend with, but remember that mu=gm*rp … the factors are inextricably dependent on one another. (Incidentally, if you stare at that equation long enough you’ll see why gm, or mutual conductance, or transconductance, is sometimes called a “figure of merit” for triodes… usually the highest mu and the lowest rp is desired, and the gm describes the degree to which that ideal is attained.)
The mechanism of plate impedance is not hard to understand intuitively, since as the tube tries to turn on harder, the plate voltage will drop, and thus the tube is NOT able to turn on quite so hard. It is a sort of degenerative feedback, this plate impedance, and it is what makes the triode alone as such a low distortion device. This mechanism does NOT happen in SS devices that I am aware of.
Now, in the cascode connection, as I’ve said, the input and output circuits are isolated from one another. The bottom tube is operating with a fixed voltage across it, since its plate is attached directly to the upper tube’s cathode, and the upper tube’s grid is fixed. The upper tube will keep a relatively stable voltage from grid to cathode, and that keeps the cathode’s potential very stable. So the plate voltage of the lower tube will be essentially unchanging.
When operated in this way, the lower tube is merely changing the CURRENT that it is passing. Since the plate voltage is fixed, the measure of mu never enters the equation. The lower tube is only concerned with transconductance, since a varying voltage on its grid will produce a changing plate current flow. This plate current is fed directly into the cathode of the upper tube.
Now, the upper tube does not really care what kind of mu it has either, since we’re not feeding it a varying voltage as a signal–instead we are feeding it a varying current. The grid/cathode voltage is not changing. The varying amount of electrons fed into its CATHODE have to go somewhere. When a greater number flow into the cathode, you can think of it almost as if the cathode is made more negative… there is now a surplus of electrons. Saying that the cathode is made more negative with a surplus of electrons is functionally the same thing as saying the grid has been made more POSITIVE. And as you know, a positive grid leads to an increase in plate current.
On the other half of the cycle, as the lower tube shuts off, there will be a deficit of electrons flowing into the top tube’s cathode. Therefore it will have fewer electrons, and the effect is similar to the cathode being made more positive, which is, of course, like the grid being made NEGATIVE, which shuts off the current flow through the tube.
Assuming the cascode is biased correctly, there is very little grid current flow in the upper tube. Therefore all of the plate current of the bottom tube is passed through and ends up going through the upper tube and on to the load impedance.
Now, if the upper tube’s plate voltage is allowed to droop too much during turn on, you will have a form of distortion, since the electrons that have collected on its cathode will not be drawn with such force towards the plate. But it will not be very drastic–as you can imagine you cannot have a continual build up of electrons without some kind of current flow occurring, since the effective grid bias on that top tube will continue to change (as I just described above).
Remember though that the ACTUAL voltage, on the upper tube, from its grid to its cathode, will not change much at all. I only mentioned the “effects” (i.e. the electron flow) to give a better understanding of what’s going on. By now, you should be able to see why they call the cascode a current mode of operation. Only the lower tube’s Vgk (as the input signal voltage), and the voltage on the upper plate (as the output voltage) really change during operation.
Some folks think of the two stage cascode circuit as if it were a single pentode. To a degree, the two connections are remarkably similar, especially if you look at the plate curves. Of course, they are subtly different. One difference that comes to mind immediately is that the upper grid of a cascode does not normally draw current, whereas the screen of a pentode does. Also, the plate of the cascode cannot really venture further negative than the upper grid, whereas the plate of a pentode can drop lower in potential than the screen. The two circuits definitely SOUND different from one another, as is expected.
I hope this helps someone. It’s not the clearest explanation, but it’s at least something!