Talk:Cascode

History

This article should begin with a description of the original cascode circuit using valves (vacuum tubes) and the reasons it was developed —Preceding unsigned comment added by 78.32.152.248 (talk) 12:03, 9 September 2009 (UTC)

Reducing distortion

Cascode's are also useful for reducing distortion. Might be worth a mention. http://www.passlabs.com/downloads/articles/cascode.pdf Rogerbrent 04:39, 11 March 2006 (UTC)

Mosfet cascodes? hah

NOTE: The following comments (in the brackets )are directed at schematics that have been removed from the article and not at the schematics currently in the article! Alfred Centauri 15:30, 1 May 2007 (UTC)

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Those are not cascodes at all. The first one, as it's been noted, it's just a source degenerated common source amplifier. The second one is obviously the same thing, using a transistor instead of the resistor, but analytically equivalent. It would be a cascode if Vb2 were the input, and Vb1 a constant voltage. 22 February 2007

I agree that there are significant problems with the diagrams and such in this article. To be a cascode, the input is to the base/gate/grid of one active device whilst the collector/drain/plate of that same device is connected to the emitter/source/cathode of a second device where (most importantly), the base/gate/grid of the second device is at AC ground such that the second device operates as a common base/gate/grid amplifier. When connected this way, the first device is a trans-conductance amplifier (input voltage gives an output current) which feeds a trans-resistance amplifier (input current gives an output voltage. The key advantages being linearity and large bandwidth. I'll think about how to fix this article. Alfred Centauri 02:03, 8 March 2007 (UTC)

Wouldn't the best fix be to simply REMOVE the first section on "Mosfet Cascodes"? 14-03-2007

It's going to take more than that. The first two diagrams are not cascodes or, at the least, its can't be said that they are cascodes. For example where is the output to be taken from? Are both of the inputs used or just one? If just one, which one? I'll delete those images and the MOSFET section for now. I just don't have the time right now to fix this article completely. Alfred Centauri 20:34, 14 March 2007 (UTC)

>

I am looking at this page on 26 April 2007 (my first time looking at this article) and will say that the schematic for the MOSFET cascode circuit is absolutely correct.

Please look at the history to see the previous versions of the pages in order to understand the context of the comments before yours. Alfred Centauri 21:04, 26 April 2007 (UTC)

I agree - the MOSFET circuits are not cascodes - they are source degenerated FETs. They do have a higher output imepedance, as a cascode does, but the input signal is to the upper FET, not the lower one as in a cascode.

Secondly, while cascodes are used to reduce distortion, that is a general result of them being 'better' -- the output current is more accurately defined by jsut the input signal, and less dependent on the output biasing or loads.

I'm not sure why you think http://en.wikipedia.org/wiki/Image:Cascode-FET.jpg isn't a cascode, but nevertheless I updated the schematic. Roger 05:18, 1 May 2007 (UTC)

I'm not sure why you think we are referring to http://en.wikipedia.org/wiki/Image:Cascode-FET.jpg in the discussions above, Roger. Please look at the history to see where I deleted the images in question. Alfred Centauri 14:44, 1 May 2007 (UTC)

Well http://en.wikipedia.org/wiki/Image:Cascode-FET.jpg was the current schematic when the previous poster made his comment, so naturally I assumed he was referring to it. I changed the schematic after his comment. Why would he be commenting on an old version of the schematic? Roger 14:56, 1 May 2007 (UTC)

He wasn't commenting on an old version of the schematic. He believed, as you did, that my comments from a month ago referred to the Cascode-FET.jpg schematic when in fact, my comments were targeted at schematics that were in the article last month and which I promptly removed. Alfred Centauri 15:26, 1 May 2007 (UTC)

"He wasn't commenting on an old version of the schematic". Exactly why I made my first comment! I knew the discussion was about the older image, which is why his comment surprised me. Anyway, case closed now. Roger 17:31, 1 May 2007 (UTC)

BJT example from article

I removed the following because it needs cleaning up and may be unnecessary (there's already a discussion of FET cascodes). If you'd like to reformat and perhaps shorten it, feel free. Roger 20:21, 14 July 2007 (UTC)

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BJT CASCODE AMPLIFIER:

                         A cascode amplifier has one transistor on top of(in series with) another.,as shown in fig.Q1 and its associated components work as a common emitter input stage,while the circuit of Q2 functions as a common base output stage.The arrangement is designed to provide a high input impedance with low voltage gain to ensure that the input miller capacitance is minimum with common base stage providing good high frequency operation.
                       File:Cascode bjt amplifier.jpg

                         For the dc bias condition of the circuit.it is seen that the emitter current for transistor Q1 is set by VE1 and R4.Collector current IC1 is approximately equal to IE1 and IE2 equals IC1.Thus IC2 is approximately equal to IE1.The current remains constant regardless of the level of VB2,as long as VCE1 remains large enough for correct operation of transistor Q1.
                         The input impedance of the emitter transistor Q2 constitutes the ac load in the collector circuit of transistor Q1.

So voltage gain of stage 1 is

                         Av1=-hfe*(Zin to Q2)
hie=-hfe*hie
hie(1+hfe=-1
                         With a stage gain of only 1,no miller effect occurs at transistor Q1.Voltage gain of stage 2,
                        Av2=hfb*(R3 parallel RL
hib

Converting to common emitter parameters,overall voltage gain of cascode circuit is same as that of CE amplifier.Overall voltage gain,

                        Av={-hfe(R3 parallel Rl
hie
                        Thus the cascode amplifier has high input impedance,the same voltage gain as CE amplifier but very low input capacitance.

---

Grammar: Cascode or Cascoded

Grammatically speaking, is it supposed to be "cascode amplifier/transistors" or "cascoded amplifier/transistors". My logic for using "cascoded" is similar to using "cascaded" in the case of "cascaded/cascading amplifiers". Lordgraviton Jul. 19, 15:09:49 UTC

Higher input impedance claim

Since the input impedance of the BJT cascode amplifier is r_π and the input impedance of the MOSFET is infinite, I really don't see how it can be claimed that the cascode amplifier has higher input impedance. The claim does not apply at all to the MOSFET because you can't go beyond infinity. I also don't see the increase in input impedance of the BJT cascode over a regular common-emitter amplifier with input impedance of r_π||R₁||R₂ where the last two are the biasing network for the BJT.

ICE77 (talk) 18:27, 15 May 2011 (UTC)

Perhaps the article isn't clear, but the cascode has a higher input impedance vs. a common base/gate stage. -Roger (talk) 19:22, 15 May 2011 (UTC)

Alright, thanks for the pointer. That makes much more sense.

ICE77 (talk) 01:11, 17 May 2011 (UTC)

BJT/FET hybrid cascode

Currently there is a sentence which says, "The cascode circuit can also be built using ... one FET (or MOSFET) and one BJT. In the latter case, the BJT must be the upper transistor; otherwise, the (lower) BJT will always saturate (unless extraordinary steps are taken to bias it)."

I'm no expert, but I can't see why a lower BJT would "always saturate". The only "extraordinary" biasing that I can see necessary is the standard biasing that all cascodes require to stop the lower transistor saturating, i.e. that the base/gate of the upper transistor be held at a high enough voltage that the emitter/source of that transistor, and therefore the collector/drain of the lower transistor, is at a high enough voltage to stop the lower transistor saturating. I'm looking at an example circuit in a datasheet right now that (if translated from PNP to NPN) essentially has an NPN BJT as the lower transistor and a N-channel MOSFET as the upper transistor, biased simply with a +5V DC zener voltage reference on its gate. Can someone more knowledgable than me please validate this sentence to see if it is correct. I've put in a [citation needed] for now. Cheers --KX36 (talk) 09:57, 13 August 2012 (UTC)

"Because at operating frequencies the upper FET's gate is effectively grounded". "efffectively grounded"? The schematic shows the upper FET's gate connected directly to ground, so what's this mystical nonsense about "effectively" grounded? Talk straight and talk sense. This stuff is physics; stop confusing readers with witchery. — Preceding unsigned comment added by 121.75.182.165 (talk) 10:08, 7 May 2017 (UTC)

It is grounded in the small signal approximation, which is what's shown in the circuit for clarity. Otherwise the schematic would be full of superfluous crud. For the same reason, the power source, connecting wires, screw terminals, etc, are omitted, as they are clutter which obscures the exposition. Obviously the base/gate needs DC biasing to be in the right operating region, and will be bypassed to ground only via a capacitor with low impedance at the relevant AC frequencies (or some similar arrangement). The word "effective" is present to remind you of this. It's possible to think of everything from interpretive dance to stamp collecting "as physics" with irritating conventions, but that's a bold move in terms of achieving practical effect! If you look on this site for "small-signal approximation" or "equivalent circuit" this should give you a start. 92.24.178.4 (talk) 16:42, 7 January 2023 (UTC)

Folded Cascode

One of the most used topologies of the cascode in operational amplifiers is the folded cascode. Unfortunatly, there is no mention of this in this page. Perhaps adding a section on them is worth considering? TheUnnamedNewbie (talk) 07:09, 14 June 2017 (UTC)

Classic usage: CRT Cathode drive in Television Receivers

Is it worth mentioning that this circuit element was very commonly used in Domestic CRT based televisions as the output stage (in triplicate for the three Red-Green-Blue guns) used to provide a relatively high bandwith (>10MHz) and high voltage swing (easily over 50-75 Volts) amplifier to drive the cathodes of the display tube? — Preceding unsigned comment added by SlySven (talk • contribs) 16:36, 11 July 2017 (UTC)