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My attempts at a design of a 3 stage amplifier
My attempts at a design of a 3 stage amplifier
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Old 8th August 2018, 11:16 AM   #1
edbarx is offline edbarx  Malta
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Default My attempts at a design of a 3 stage amplifier

First, I will admit to myself that these attempts may prove useless.

After so much head scratching, I inputted the input stage of what I think the input stage should look like. I am using the CAD hog named KiCad. It is a huge piece of software that probably requires one to buy a book to even start using it. As you can see, I haven't figured out how I can neatly display the various texual descriptions displayed next to components.

The attachment is the first stage as I am imagining it. I will use a cascode to isolate the input from the high voltage of the power supply. I am aiming to first make the input stage, test it to make sure it meets its required standard and then proceed to the VAS. The power stage is already built as it was part of a large public address amplifier that failed. I will usethe PA amplifier's chassis, power supply and power stage and redesign the pre-driver stages.

You may deem me mad, but sometimes being mad may be the only adventurous and successful route.

I am posting both for your comments (criticism) and hints about how I can use KiCad to get a better looking schematic and for improvements that I can make to the input stage.

Thanks to all those who made this website a reality and to those who will contritbute to this thread.
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File Type: pdf input_stage.pdf (25.0 KB, 357 views)
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Old 8th August 2018, 11:31 AM   #2
Mooly is offline Mooly  United Kingdom
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My attempts at a design of a 3 stage amplifier
I can't answer your KiCad question, but would say that if your are interested in both drawing circuits on screen (and if you wish simulating them as well) then LTspice might be worth looking at.

As to your circuit, well the most obvious problem at the moment is you have all NPN transistors as drawn whereas the upper two should be PNP's while the lower NPN appears to be the wrong way around. Not quite sure what you are intending the lower one to do actually
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Old 8th August 2018, 12:21 PM   #3
wintermute is offline wintermute  Australia
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My attempts at a design of a 3 stage amplifier
How are you producing your kicad schematic pdf?

Attached is one I created by opening the schematic, printing (and not selecting the black and white tick box, which I think makes it a lot nicer) using a printer driver called cutepdf CutePDF - Convert to PDF for free, Free PDF Utilities, Save PDF Forms, Edit PDF easily.

edit: I just tried using the built in plotter as well plotting to pdf and colour with a similar result.

Tony.
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File Type: pdf kikad_schematic.pdf (47.0 KB, 150 views)
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Last edited by wintermute; 8th August 2018 at 12:25 PM.
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Old 8th August 2018, 03:42 PM   #4
spladski is offline spladski  United Kingdom
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My attempts at a design of a 3 stage amplifier
I am seconding Mooly's reply with a stronger recommendation that you use LTspice simulator. The circuit as drawn cannot work so there is no point in a prototype.

LTspice will prove the concept functionally. Although simulation cannot emulate with 100% accuracy, it will save you a heck of a lot of time. It is also standard procedure for this site, as it makes collaboration with others more productive.

LTspice is also free. Your bank/mattress account is safe.
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Old 8th August 2018, 05:04 PM   #5
edbarx is offline edbarx  Malta
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I am on Linux, Devuan ASCII. Installing more software means more time dedicated to setup software that I have never used in my life. Using KiCad my concentration is more focused on the new commands that I am learning to use rather than the components. Yes, as mentioned to me, there were 3 transistors that should be PNP: the two comprising the current mirror and the transistor at the bottom negative power rail. Software teething problems are distracting me from focusing on circuit design and on doing circuit calculations to make sure everything works within planned currents and voltages.

The lower part of the circuit consists of a series pass voltage regulator which powers a current source that is used to supply a constant current to the differential pair. The voltage of the differential pair is truncated using a cascode so that they work with a voltage of around 6.8V - 0.7V = 5.9V. According to Douglas Self, I should try to use a constant current of 6mA but I find this a little too high. I know there are amplifiers that use less about 2.5mA. I am still hesitant about using a current mirror as the interface for the differential pair. I think, using a complementary pair of small signal transistors with their bases connected to both heads of the differential pair, can also work and ensure the differential pair always conduct nearly equal currents.
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File Type: jpg input-stage-v3.jpg (227.1 KB, 792 views)

Last edited by edbarx; 8th August 2018 at 05:10 PM. Reason: Removal of useless phrase at the end of the top paragraph
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Old 8th August 2018, 10:10 PM   #6
suzyj is offline suzyj  Australia
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Hover over the thing you want to move and press M. Or R to rotate. KiCAD is pretty friendly when you get used to it.

But yes, LTspice first to get a working simulation, then KiCAD for schematic and layout.
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Old 8th August 2018, 10:22 PM   #7
jaycee is offline jaycee  United Kingdom
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KiCAD is quite frankly vile. Typical open source dreadful user interface.

Someone really needs to clone EAGLE
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Old 8th August 2018, 10:39 PM   #8
suzyj is offline suzyj  Australia
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I disagree. The most obnoxious CAD tools I’ve used have all been closed source (Cadence comes to mind as being particularly awful to use). I took a couple of days to bring myself up to speed on KiCAD from a background in Altium/Protel and Cadence, and now prefer KiCAD strongly from an ease of use perspective.

KiCAD is reasonably straightforward, plus very powerful. And being open is huge. I am so over the bloated crap and enforced updates from, for example, Altium.
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Last edited by suzyj; 8th August 2018 at 10:42 PM.
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Old 8th August 2018, 10:53 PM   #9
stocktrader200 is offline stocktrader200  Canada
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ExpressPCB has excellent tools and PCB's , but poor (expensive) board prices and sky high shipping outside of USA
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Old 8th August 2018, 11:34 PM   #10
rco3 is online now rco3  United States
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I'm with suzyj.

Altium is more powerful than KiCAD, but I like using KiCAD better. EAGLE is a distant third behind both, and DxDesigner/PADS has similar or more capability than Altium but a far more frustrating UI.

Unless I'm routing something like a 64-bit 800MHz DDR3 data bus, KiCAD is my favorite. Altium irritates me multiple times a day.
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