The Art of Electronics speaks gushingly about the Cascode Configuration in pretty much every chapter that involves transistors. :) Recommended.

Interesting !! I learned two things today. Cascode optocoupler and wiggly in gets wiggly out. Many thanks

Good morning! Thank you once again for making things visual, versus all mathematical. I really appreciate your approach. I deal with cascode topology quite often on mosfet amplifier circuits (no optoisolators, but still very relevant) and using your oscilloscope in the compare mode just really added the "icing on top of he cake" for me, as to why cascode topology is so important to increase the slew rate of the output stage of an amplifier. Thank you IMSAI Guy! Fred

The size of the collector-base parasitic capacitor is effectively multiplied by the transistors voltage gain which makes the miller effect even worse. Great video, thanks for sharing.

@paulcohen1555

3 жыл бұрын

Not by the Hfe, by the voltage gain of the circuit.

@vincei4252

3 жыл бұрын

@@paulcohen1555 Doh! You're right. I'll correct my comment.

@RexxSchneider

2 жыл бұрын

The capacitance multiplication _is_ the Miller effect, and of course it can only occur when there is voltage gain at the collector. That is why the cascode amplifier, which has current gain but no voltage gain at the lower stage's collector, eliminates the Miller effect. You still have the collector-base capacitance, of course, but it's no longer multiplied. Since the C-B junction is a reverse biased diode, you can decrease its capacitance by increasing the voltage across it.

good explanation of the cascode operation and how it helps with the Miller effect.

Your description of the Miller effect isn't quite right. The Miller effect is a negative feedback effect that depends on voltage gain to create feedback. The capacitance between collector and base is always present (it's a reversed biased diode), so you would get the R-C limitations that you describe even if you were driving an emitter follower which has no gain. The Miller effect effectively multiplies the feedback capacitance by the voltage gain of a common emitter stage. A cascode amplifier is a common emitter stage feeding a common base stage. Because the input impedance of the common base stage is very low and that is the load resistance of the first stage, there is no voltage gain on that first common emitter stage, hence no Miller effect, but does have current gain. The common base stage will, of course, have a load resistor, so you end up with a voltage gain for the cascode amplifier. It is important, though, to ensure that the base of the common base stage is tied firmly to ac ground.

@absurdengineering

Жыл бұрын

The common base stage can be also enhanced by adding an external stage to drive the base with an inverted, amplified AC signal from the emitter. This further lowers the effective emitter AC impedance.

Very cool! I wonder how this circuit performs with an analog signal in its linear region?

Very instructive ! 👍👍👍 Thanks for your time, Eddy.

Great video and explanation as usual. I was experimenting with an LED-based stroboscope where having a very short/sharp strobe pulse is critical to stopping high speed motion. One of the limiting factors is the switching speed of the LEDs, but now I’m wondering if using a cascode configuration would improve the performance, or if the limiting factor is the plain vanilla LEDs I’m using. Either way, this should help when the LED switching speed is not a limiting factor of system performance.

Wow I finally got the Cascode reason - nice job

If you bypass the base of the cascode transistor to ground I think your sweet spot will expand and the performance may improve fruther

Kewl! (The cool way the cool kids say "cool.")

@IMSAIGuy

3 жыл бұрын

Cartman

@t1d100

3 жыл бұрын

@@IMSAIGuy Yeah, I'm too old to know Cartman. Had to google him. I'm sooo out of touch; happily.

@____________________________.x

4 ай бұрын

Bet 🤭

Optocouplers age. I know this because I was at T'dyne when they started getting returns on very fancy boards because the optoisolators they used were going out of spec. Ooops.

@IMSAIGuy

3 жыл бұрын

haven't heard that one. My guess would be the LED side.

@plainedgedsaw1694

3 жыл бұрын

@@IMSAIGuy probably. I also noticed datasheets mentioning how much performance derates with aging but I haven't paid much attention to it.

@rogeronslow1498

3 жыл бұрын

That's true. Particularly if operated near max forward diode current. Apparently it has to do with the emitter degrading over time. I'm not sure if it's been sorted out but I try to avoid them in designs if possible.

I remember the Miller effect from the time I learned about vacuum tubes in Electronics.

Liked

Do you have any comments on maintaining isolation? Maybe what I am really doing is suggesting a quick video on the common methods for isolating two circuits. In my own projects these are: transformers, relays and optoisolators - but I am sure there are tons of options. Great video - can't wait to try this out.

@IMSAIGuy

3 жыл бұрын

I've added it to my list.

The questions for volume production 1. Is there wide tolerance in the PC817 between devices for bias setting of the cascode? 2. Does the bias setting vary with temperature?

@RexxSchneider

2 жыл бұрын

There isn't an issue with the bias setting of the cascode stage. The collector current of the phototransistor is limited in its linear region by the input current, not by its collector voltage. The dc bias on the base of the upper transistor sets its emitter bias point and that becomes the collector bias point of the lower phototransistor. You need work with currents at sensible levels to ensure you don't exceed dissipation. There seems to have been a change in production around 1977 when the maximum Vce was increased from 35V to 80V, so be careful of older parts. The collector current is 50mA max and power dissipation is 200mW. I'd recommend setting the phototransistor collector to about 5V and aim for an input current around 1mA to 5mA, which means the maximum collector current will be between 2.5mA and 30mA, depending on sample. You can buy a gain selected part with the A rank giving 4mA to 8 mA, the B rank giving 6.5mA to 13mA, the C rank giving 10mA to 20mA and the D rank giving 15mA to 30mA from an input of 5mA. That would allow a dc bias point for the collector of the upper transistor (i.e. the amplifier output) to be set to give a usable output swing despite a potential 2:1 variation in collector current.

Good lab project. BTW, was it called "Miller" effect because Glen had a really tight vibrato?

@IMSAIGuy

3 жыл бұрын

I thought it was named after Miller Time!

I take offense to this "Miller Effect" !....LOL

Would a base resistor be well advised on the top transistor?

@IMSAIGuy

3 жыл бұрын

No, it the same biasing as any common emitter amplifier. No base resistor.

@RexxSchneider

2 жыл бұрын

@@IMSAIGuy You mean "common base amplifier". No base resistor because you want it tied firmly to ac ground.

I came across your video on how dangerous drilling plastic is, and I hope you can help me! I got an office chair that has this lumbar support pad thing that slides up and down in these side rails on the back of the chair, and the pad constantly slides down the rails on its own cuz there's nothing that keeps it there, making the chair really bad to sit in. I fit thick cardboard strips inside those rails and it kinda works, but it doesn't seem like a long term fix, so now I'm thinking if I could drive a screw through each of those 2 side rails with the lumbar pad in the highest position, to keep it there permanently. I hope you could gimme the best solution around this, thanks in advance!

@IMSAIGuy

3 жыл бұрын

screws, pins, hose clamps

@jdmccorful

3 жыл бұрын

@@IMSAIGuy just be careful you don't void the warranty.

what is voltage attached to the 1k ? What is the base voltage of the the 2n2222a ?

@IMSAIGuy

3 жыл бұрын

12V but not critical. I don't remember the base voltage you will have to experiment. It is probably dependent on what optoisolator you use.

Why can’t you just bias through the resistor and possibly a diode? What role does the added transistor play? I thought in the beginning it was a voltage regulator, but nope.

@IMSAIGuy

3 жыл бұрын

I suggest you watch the video again and not skip.

@RexxSchneider

2 жыл бұрын

The added transistor is a common base amplifier. That presents a low impedance input and a high impedance output, and has a current gain of very close to 1. That means the voltage at its emitter hardly changes as the current fed into its emitter changes (which eliminates the Miller effect from the phototransistor), but any current fed into the emitter can generate a significantly larger voltage change through a load resistor at its collector.

Was the application note from Vishay instead of ON Semiconductor? Can you please provide the AN?

@IMSAIGuy

3 жыл бұрын

www.onsemi.com/pub/Collateral/AND8273-D.PDF

@pbaemedan

3 жыл бұрын

Thank you for the application note.

miller killer. Does anyone know the origin of the term "cascode" ? I've tried to find out but I couldn't find a satisfying explanation. Seems like some wag riffed off of the term "cascade", but I need to know more.

@IMSAIGuy

3 жыл бұрын

"casc(aded triode amplifier having characteristics similar to, but less noisy than, a single pent)ode"

@nickcaruso

3 жыл бұрын

@@IMSAIGuy WOW where'd you find that? In Terman?

@nickcaruso

3 жыл бұрын

no I just searched the text in the internet archive -- no hits for cascode.

@IMSAIGuy

3 жыл бұрын

en.wikipedia.org/wiki/Cascode

@IMSAIGuy

3 жыл бұрын

under History

bro how to mobille phone reire spectrum analyzer.plzzzzzzzz

@IMSAIGuy

3 жыл бұрын

Sorry, the English does not make sense.

feedback: the drawings are too small,, shown with a too sharp angle,

@IMSAIGuy

Жыл бұрын

get a bigger screen

@ytrew9717

Жыл бұрын

@@IMSAIGuy well, I won't waste more time writing feedback. I thought you could handle them.

Tired of your EE TA? follow circuit solver on the playstore!

@IMSAIGuy

3 жыл бұрын

I recommend a true spice program. you can get Microcap for free now: www.spectrum-soft.com/download/download.shtm