BJT Circuit Analysis: The CASCODE Amplifier (Pt 2) - DESIGN (066g2)

In the last video, I explained the WHY behind the CASCODE configuration and the HOW it works behind its operation. Here is a LINK to that video if you missed it:
kzbin.info/www/bejne/oGTVYWBpq76ehZY
In this video I will be showing you how to design one. I will walk through the entire design process including all of the assumptions, rules of thumb and engineering estimates needed to make this work.
I will also demonstrate the benefits of this configuration both in the simulation world and with a bench experiment.
LINK to the "Go-Along-With-The-Video" formula sheet:
drive.google.com/file/d/1uzO3a4mXY2uli8bmabFcdhMsybLnG88g/view?usp=drive_link
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Time Markers for Your Convenience
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00:05 Introductory Comments
01:18 Establish Design Goals
03:00 Calculating Transistor Currents
05:31 Calculate Re
06:46 Calculate Q2's Base Voltage
07:55 Calculate R1, R2 (Rb3), Rth, Vth (Thevenin Stuff)
07:55 Introduction to the Thevenin Stuff (a reminder)
08:45 Calculate Rth
09:31 Calculate V(Rth)
10:21 Calculate Vth
11:36 Calculating R1 & R2
11:43 Calculate R1
12:20 Calculate R2
13:14 Calculating the Currents in the Bias Resistor Chain
13:21 Calculating I(Rb3)
14:12 Calculating I(Rb2)
15:41 Calculating I(Rb1)
17:21 Unstucking the Stuck with Engineering Knowledge (V(Rb2))
19:16 Calculating the Rest of the Bias String (Rb2 & Rb1)
19:16 Calculating Rb2
19:58 Calculating V(Rb1) so we can find Rb1
21:16 The Value of Rb1 by Ohm's Law
22:03 The Conundrum ... Solved
24:05 Calculating Vcq so we can find Rc
26:44 Calculating V(Rc)
27:11 Calculating Rc
28:19 Calculating the Anticipated Gain
30:32 Performance Evaluation
30:33 Using LTSpice Simulation
33:18 The Bench - Breadboard Results
35:17 Final Comments and Toodle-Oots

[@michaeldavenport6556]

Bob Pease and Jim Williams are smiling with approval from the great test lab in the sky.

[@eie_for_you]

I am humbled to be put in the same room with these guys. Thank you! 🙂

[@davidluther3955]

SO WOULD BOB WIDLAR!

[@davidluther3955]

SUPERB PRESENTATION!WHAT RULE OF THUMB AND,OR HOW DID YOU CHOOSE 3V Vcb?

[@eie_for_you]

@@davidluther3955 Thank you so much! How did I pick 3V? Knowing that I needed at least 1.7 volts, I just picked a number that felt good to me that was greater than that. Call it a "gut call." Nothing special beyond that. 🙂

[@eie_for_you]

@@davidluther3955 Putting me in his camp is a tremendous compliment! Thank you. 🙂

[@W1RMD]

That's a very well packed episode! Thanks and have a great day!

[@eie_for_you]

You are welcome! This was really fun to put together! 🙂

[@haraldlonn898]

Great video. Thanks.

[@eie_for_you]

Thanks, man! 🙂

[@RensePosthumus]

This was fun and I've learned a lot. I particular liked the whiteboard scene and your effective use of color for the electric quantities and shirts. That must have taken a lot of preparation. Also thanks for granting access to the "Go-along". Just for curiosity: what happens if we are in the non-linear region?

[@eie_for_you]

Thanks! I wanted to do something different. Did you notice my wardrobe issue? My collar was up on the orange shirt! LOL. I didn't notice until I got into editing. I *need* a mirror down here! As far as the non-linear region is concerned ... all bets are off. 🙂

[@RensePosthumus]

@@eie_for_you I did notice that, but thought it was intentional in order to make a potential dull part somewhat more interesting.

[@eie_for_you]

@@RensePosthumus LOL! No ... it was a genuine oopsie! 😅

[@byronwatkins2565]

The gain of 2N3904 might also be 300; typically it is about 230. It will also vary substantially with temperature and quiescent state. Even well-designed transistor circuits are 5% propositions at best. There is no point in keeping more than 3 significant figures and you might as well neglect the base current's contribution to the emitter current.

[@eie_for_you]

Yes, I understand where you are coming from. The datasheet for the 2N3904 says that the hFE can be from 100-300. I have often used 200 as my go to value. But here, I used an hFE of 100 t0 try to minimize thermal effects. Regarding rounding of values ... in my **many** years of product design and development, I never rounded off until the final calculations were complete. The reason? Because I have seen *way* too many times when rounding came early and caused issues. When subsequent calculations depend on previously rounded values, the error grows as one proceeds through the design and analysis. I do not discount your advise. I just choose to do my engineering differently. 🙂

[@ornithopterindia]

👍Thank you sir.

[@eie_for_you]

You are welcome! 🙂

[@Blinkertlurer]

Hi Ralph, thank you so much for your lessons, your knowledge is beyond price. I wish I'd had you as a teacher. (That was a long time ago) . I have a question about the Cascode amplifier: Based on your amplifier. What if I replaced the RC resistor with a constant current source or a 1.5mA current mirror? Would the rest of the circuit remain the same? Kind regards and thank you very much for your answer!

[@eie_for_you]

At quiescence I believe it would all be the same. The problem lay in its dynamic response as an amplifier. I took my model, ran it as the original design and then replaced the collector resistor with a 1.5mA current source and ran it again. The dynamic response suffered dramatically. 😞 Here is a link to the ZIP file with the spreadsheet that has the simulation results: drive.google.com/file/d/1e8YPFSiPKWydFh99T81U2NYisKQjeucw/view?usp=sharing

[@Blinkertlurer]

@@eie_for_you Hi Ralph, thanks for the quick reply. I would have expected the exact opposite, you can be so wrong. I would really like to have you in the neighborhood ;) kind regards and have a good time Werner

[@eie_for_you]

@@Blinkertlurer Yeah, it was somewhat of a surprise to me, too. Download the **FREE** LTSpice program and play with the design. If you want me to provide my model, I'd be glad to. 🙂

[@U812-k7j]

Ralph, any plans on doing videos on MOSFETS?

[@eie_for_you]

Yes, I absolutely do plan on working my way up through the whole FET side of things. This is in my queue. 🙂

[@U812-k7j]

Cool can't wait

[@eie_for_you]

@@U812-k7j Me too! 🙂

[@surrealmemes3518]

awesome

[@eie_for_you]

Thanks!! 🙂

[@gregwmanning]

Good morning Ralph I have a couple of questions if I may please When calculating the value of Rb2 value at 18:45 we decided the minimum voltage across it was 1.7V (Vbc2 = 1V and Vbe2 = 0.7V) but we choose to go with 3.0V meaning Vbc2 is assumed to be 2.3V not the minimum 1.0V, whereas when deciding Rc1at 25:55 we stuck using Vbc1 as the minimum 1.0V. Why round up Vbc in Q2 to 2.3V but maintain Q1 Vbc at 1.0V? [I note the use of the rounded value of Rc1 as 2.03k instead of calculated 2.0333K] I read the other comments for this video and you expressed your desire to not round any resistor value till all calculations are finished "I never rounded off until the final calculations were complete". My question is: would choosing the closest actual resistor values as soon as you calculate the theoretical value, ie with the very first calculation, the one that would cascade errors the most is Re2 calculated to be 784.2ohms (I could not read the resistor value used on the breadboard) the nearest actual value would be 7500 or 8200 ohms, would not using this chosen resistance value to calculate the new "actual" value of Ve2 & Vb2 cause less cascading of errors? I hope this all makes sense. Again thanks for the video and I off to watch part3.

[@eie_for_you]

First question...the use of Vcb of 1 volt for Q1 was to establish an absolute minimum to establish the bounds of possible voltages for the collector voltage. The actual operating Vcb is much higher. The choice of Vcb for Q2 was to establish the actual operating point of the transistor. Second question ... I use 1% resistors almost exclusively. The price as compared to 5% is almost indistinguishable these days, so why not. This puts the value of the chosen resistor MUCH closer to the theoretical value. When I chose resistor values for the breadboard, I chose them from the list of 1% resistors. Hope this helps. 🙂

[@davidluther3955]

WHAT WOULD WE DO WITHOUT ALGEBRA.WHERE DID THAT LITTLE MAN GO?I MISS HIM?

[@eie_for_you]

LOL! He reappears in Pt 3 🙂

[@ivolol]

Are these only good for frequencies >1khz?

[@eie_for_you]

The low end frequency response is determines by the values of the Emitter capacitor (Ce2) and the Base capacitor (Cb1). The lower the frequency, the larger the capacitors need to be to maintain gain. 🙂

[@cornbread-KO5RN]

👋🍺🍺

[@eie_for_you]

Thanks! 🙂