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Old 28th January 2009, 10:59 PM   #1
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Default The Rush Cascode: Possible Wiki page

Hi all.

If you don't know about the Rush cascode I suggest you use Google and your other preferred resources to get some background information. Here are some threads and pages where it has been discussed:

Input stage idea

First of all, I need to add that I don't have much experience in electronic design. My comprehension and understanding at this point has come entirely from resources such as DIYAudio, Doug Self's site and many other free internet resources. Even though comprehension has come relatively easy for me, everything I know has still come from theoretical sources and not true time-tempered experience.

The Rush Cascode is a manifestation of the differential long-tailed pair. Same concept, different ways to implement it. It was first used by englishman Christopher Rush of THX fame, who is still designing today (info provided by AKSA).

In some posts here on DIYAudio, the Rush cascode has been dismissed quickly after theoretical and mental comparisons to the conventional long-tailed pair (henceforth referred to as the LTP) as an input scheme. I think more thought and work should be put into its understanding.

The Rush cascode can be difficult to get working because of its speed and gain, not to mention DC offset and temperature compensation if one intends to make a DC-coupled amplifier. However, there are things we can do to get around this, which I intend to expand upon.

The rush cascode can be difficult to make stable for the following reasons:

1: High gain.

2: High speed.

The common solutions to this are:

1: A degeneration resistor between the emitters to decrease gain.

Firstly, it seems there is a misconception that like the conventional LTP, the Rush Cascode benefits from using high-gain transistors. In my simulations I have found this to be inaccurate.

I have the following observations to present:

1: The distortion of the Rush Cascode is more dependent on the degeneration resistor than it is the transistors used. A low degeneration resistor will in my experience get lower distortion.

2: Stability decreases with higher-gain transistors.

I have the following suggestions when implementing the Rush Cascode:

1: Use low-gain devices, like the 2N5771/5769, with as small a degeneration resistor as possible.

2: Increase open-loop gain in other parts of the circuit instead of the cascode itself. This will result in better stability and distortion than the other way around.

3: Use both legs, rather than one, to increase OLG and VAS speed.

I believe that the Rush Cascode should have some 'net coverage, which is why I am doing this. More people should know about it.

So I want to make a Wiki page and/or PDF that talks about it along with some example circuits. I also have designed and amp on the sim that has a nifty thermal and DC offset compensation scheme, although it requires some calculations.

I want to see more circuits with it, and I want to see how others get past its shortcomings.

Attached are some ways I have used it in the simulator.

- keantoken
Attached Images
File Type: gif rushcascode.gif (10.8 KB, 520 views)
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