Progress I'm happy with the routing except for the connection pins but it's what we have to have to stay true to the DIP layout. All transistors in blocks, input differential, current mirrors, gain stage and output. 3 transistors not blocked are for biasing the push pull output. I'll keep tweaking it but it's looks acceptable now.

 

Do you want to keep the LxW ratio the same as a DIP-8? Apparently not, but might look neat. The pins on a DIP-8 are on a 0.6" x 0.6" square.

the nom aspect ratio for DIP8 is .68
mine is .66 which is perfect for SOP8

I believe you are talking about the pin spacing. I plan on 3D printing a stand (chip body) with single male header pins mounted up from the board and have been playing with the idea of soldering scale pin legs to the board (using longer header pin on the thru side). The scale is approx 6.5/1 and needs about 1mm added to width to match chip aspect ratio. The pin spacing between the extremes should be about 30mm to be scale. Material should be easily soldered... so no aluminum... preferred material is tin plated copper. The pin material thickness is 0.008” and scaled up is a heavy 14 gauge (0.052”) With copper it would be pretty malleable. Brass would be better choice Offering more rigidity. Best practice would be a die cut and would need considerable stamping pressure more than my press could do... die is expensive would need to be done on EDM or laser using tool steel For the 2 shapes if ends can be reversed. Plus to finish would require tumbling, forming and electroplating. I will need to find a cost effective solution...


the body would scale up to about 32mm thick. Could be hollow underneath to give access to soldering legs if we can go that direction.

 

The enclosure will be printed so I started a model in Solidworks. The board should snap in into this from the bottom... it may take some work to get it how I want it. There will be a bottom half and perhaps a lid on top ... not sure about the scale pins yet. I had another thought that would be interesting if we made board smaller and positioned it like the die, you could place pins and solder wires just like a miniature chip. It's an interesting idea but again more helpful having the layout resemble the schematics. My scale is now off again so we will have to recalculate the ratios. All values in mm.

 

Hello,

In the old days we had the CA3096 transistor array, to play with:

View attachment 190075
The CA3096 suggests the HFA3096 as current replacement.
The voltages and currents are limited, but the frequency characteristics are outstanding.

Bertus

The CA3086 as well; still have some of them in a drawer.

I recall buying a bunch when quiting vessels, almost 30 years ago, signing off for the last time in Singapore.

Went to the Sim Lim Towers, a paradise for electronic components; several tens of small shops in each of the so many floors.

 

the nom aspect ratio for DIP8 is .68
mine is .66 which is perfect for SOP8

I was referring to the pad pattern of the DIP8, not the epoxy chip size. One recognized example is in Eagle:



It may not look square, but it is.

 

Oh yeah sure, I don’t think I’ve ever put a DIP8 on a PCB. I keep thru hole components for breadboarding only. It’s a bit of a tradeoff because DIP with sockets are so convenient to exchange, program and test. I have SOP test sockets but it’s not the same.

 

that should do it... lots of undercuts and stuff in this model, hopefully it'll print ok... I only need one that works so may be some adjustments... the top was most complicated, Once I figure out pins I can do bottom...

 

There are a lot written about the 741 some better than others. I found is this from 1974, written by James E Solomon IEEE, which applies to all solid state opamps. This treads a nice balance between simple and the actual physics behind the operational characteristics. Topics on slew rate, bandwidth, calculated gain and thermal feedback are interesting.

Great section about reducing slew rate which can be achieved by decreasing gm; transconductance on the first stage has an inverse relationship with bandwidth and of course cm. Its such a balancing act with current, phase relationship, slew rate and other factors involved in opamp design. This 1974 document Is pretty good I wonder where we are today.

http://hifisonix.com/wordpress/wp-content/uploads/2010/10/James-E-Solomon-Opamp-Tutorial.pdf

 

@OBW0549, it looks like I will have to match transistors as you suggested. Here’s another thought, using the diode test in the multimeter we should be able to get fairly consistent Vbe measurement since the meter provides a constant current source around 1mA. As long as we use the same meter we should be able to match to about +/-1 mV, error of less than 1%. This looks like the easy button without buying or building more complicated devices.

I will have to match some of the resistors as for the best ratios.

 

Hello,

Perhaps the following article might interest you on matching transistors:
http://musicfromouterspace.com/analogsynth_new/TRANSISTORMATCHER/TRANSISTORMATCHER.html
Bertus

 

Thanks @bertus yeah I skimmed that one and looked at the Moog circuit. It’s a go/no go which would speed things up if you were in production but I have a handful I have to match and don’t mind probing and sorting.

 

@OBW0549, it looks like I will have to match transistors as you suggested. Here’s another thought, using the diode test in the multimeter we should be able to get fairly consistent Vbe measurement since the meter provides a constant current source around 1mA. As long as we use the same meter we should be able to match to about +/-1 mV, error of less than 1%. This looks like the easy button without buying or building more complicated devices.

I will have to match some of the resistors as for the best ratios.

Hola @Wolframore

Some time ago, I spent few days matching transistors. For that I built a jig able to hold 8 of them at a time plus a simple but effective 1-mA constant current source. The DMM measured the Vbe in quick sequence.

A caveat: it is rather easy to change their temperature (even just by breathing on them) or having some fan blowing over the bench. I used to insert them and wait few minutes to measure the lot quickly without even touching them.

I separated around 50 in lots thanks to this procedure.

Ah, any lamp you use for this, keep it far from the jig, and always in the same position.

 

I was just about to mention the thermal issues. I have to use tweezers for SOT-23. I’m too clumsy to use my fingers for those. So it should go faster. Good point about lighting and fan to make them room temp sounds like a good idea.

 

It will be interesting to do frequency response curves at different gain levels. We should be able to discover if we have added Cm value from stray capacitance.

I see this as an invaluable learning experience ... the opamp's internal circuitry itself is quite beautiful. Also, I'd like to see the implications of building it using discreet components. .

@cmartinez you‘re absolutely right, it is a beautiful circuit. So well thought out and a great foundation. We will test the discrete opamp against an actual chip and datasheet.

I’m tempted do a die correct PCB version, I believe it offers some thermal and routing advantages. Then add some transformer wire for bonding to the DIP8 model... I love the visual. Maybe with exposed copper traces with ENIG finish... ugh it would be a challenge to solder without solder masks. Hmmm tbd...

 

OK to explore all options I've made another PCB layout, this time it follows the die more closely except when it doesn't make sense for routing...

The first PCB is 40x60mm = 2400 mm^2

This one is 23.75 x 39 mm = 926 mm^2 - It's narrower than the height of a quarter. This sort of design would be best for modular discrete op-amps done on 20x20mm stacked boards for the standard DIP8 module.

Due to the high density using discrete components the markings are difficult to decipher. Placement and soldering will be a challenge without a stencil. The dual transistor package might be a good idea (sometimes they can make things harder) if going denser...

Here's the side by side comparison:

 

If you're worried about being able to solder that dense PCB of yours, then allow me to show off a little. I had to patch a circuit I built last year in which I used four dual magnetic-coupled isolators that did not work as expected. Mainly because of their susceptibility to electrical noise of industrial levels. So in the end I was forced to use single-channel 6N137 optoisolators because they were the only ones that complied with speed and power restrictions. And on top of that, I had to add a few SMT transistors and resistors to make the circuit work as intended. The through-hole resistors seen protruding between the chips are there because they also work as jumpers.

Being single-channel, I had to use eight, instead of four of them. And I had to make a PCB with pins protruding from under it that were compatible with the equivalent pinout of the four chips that it was replacing. The result was an extremely compact PCB that was directly plugged on top of the original board... worked like a charm.

​

How's that for dense? ...

 

I love seeing project photos, thanks... I will order some of the "die" design as well but not looking forward to manually putting solder paste and figuring out what components go where... nevermind probing it... lol.

 

I love seeing project photos, thanks... I will order some of the "die" design as well but not looking forward to manually putting solder paste and figuring out what components go where... nevermind probing it... lol.

I seldom use paste, btw. I always tin the solder pads a little bit using very thin diameter solder, and then I position and align the component using tweezers while pressing the iron's tip on the component's pin until the solder under it melts and adhere's to the part.

I only use paste when the soldering temperature is critical, such as in ADC chips.

 

Here's an example of what I've just said:

​

 

Yeah that would take all day with 24 transistors and the passives... I paste and hot air or oven... they come out looking amazing... plus I have a feeling I want to make more than 1. I want to give one of them to the professor.

I only use iron for headers and thru hole.