Heres another discrete OPAMP
Lets remove the offensive Q6 and replace it with a jumper. Probably this transistor is the bane of IC chips
Lets remove the offensive Q6 and replace it with a jumper. Probably this transistor is the bane of IC chips
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The OPAMPs are predominantly 2nd harmonic. Its an old technique that was mastered by OPAMP IC engineers back in the 70s through 90s
In the JFET configuration, they are completely useless (and I might argue that detrimentary). In the one with the two BJTs, the first BJT is an emmiter follower, it should provide higher input impedance.
The people who designed the uA741 opamp integrated circuit in 1969, had a similar 3 x NPN current mirror to post #1's Q6-Q11-Q18 .
But on the uA741 IC, emitter follower transistor "Q6" is provided with an resistor from emitter to negative rail. Unfortunately the schematics attached to post #1 of this thread, do not include any resistor from Q6 emitter to bottom supply rail. It's not a match.
I would like to ask, why is this? What mistake did they make in the uA741, which this circuit repairs? What is the mathematically optimum resistor from Q6 to negative rail? Infinity? 2E7? 2E6? 2E5? some other number?
Thank you
But on the uA741 IC, emitter follower transistor "Q6" is provided with an resistor from emitter to negative rail. Unfortunately the schematics attached to post #1 of this thread, do not include any resistor from Q6 emitter to bottom supply rail. It's not a match.
I would like to ask, why is this? What mistake did they make in the uA741, which this circuit repairs? What is the mathematically optimum resistor from Q6 to negative rail? Infinity? 2E7? 2E6? 2E5? some other number?
Thank you
Attachments
Thanks, it was probably for stability or linearization. A lot of work was done in that golden age, they also had to use exact tolerances to have nano and pico DC offset values
Probably that but also there were a few undocumented trade secrets on how to do it right. Although many of those engineers are now in the cloud, they left many clues behind. For example the Red Diamond Amplifier is just badass and will smoke any and all amplifiers.
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Any slight improvement in fabrication unleashes another wave of criticism about "that old junk".
Excellent gear has been done with '741.
Maybe its first "crisis" was a low slew rate and high midband output levels just as Disco snare-drums got popular. An over-driven '741 may do bass and vocal OK but schmear high-level snares/cymbals like applesauce from a firehose. And it was not immediately clear "why" (well, not to everybody).
It also has input hiss voltage twice as high as it could be, you can not ignore the hiss voltage (but to our shame, we did), bad samples had crossover distortion because it's only an opamp, who cares?
'301 was a fix for slew-rate IF you did math. But why did we love opamps? No technical calculations!
Several of '741s descendants with different numbers are still well-regarded in good (not boutique) systems, notably the NJR opamps in CD players of 2002. Natural evolution gave just enough more slew and less hiss to be fault-free.
I think I have used every njm op amp, been a fan for them for a long time from my side, but can't rememver any to show an input cascode in its principle schematic in the datasheet, on the other hand almost all njm opamps had the same schematic the datasheet 🙂
5 years ago i saw mouser suddenly starting to sell a batch of njm2043 and at the time nobody, absolutely nobody was talking about it as it was not found on the market since the 90's, but I knew Nakamichi used it in every REC section of its decks so it must have been smth specisl.I bought 25 of them.In the mean time it became obsolete again , but what I found about my batch was that it had terrible offset, like 2V or smth in a normal circuit where I'd expect 10...50mV.That set me back a bit... Then another hit came with new njm4556DD having higher noise than ones recovered from old pcb's , then getting 20x njm4562 from mouser that all proved overly sensitive to my smartphone talk, then just stopped buying op amps...cause I had more than 100 new op amps from 10 different species and probably 300 recovered from various dead electronics...just enough for my next 3 lifes 🙂...
5 years ago i saw mouser suddenly starting to sell a batch of njm2043 and at the time nobody, absolutely nobody was talking about it as it was not found on the market since the 90's, but I knew Nakamichi used it in every REC section of its decks so it must have been smth specisl.I bought 25 of them.In the mean time it became obsolete again , but what I found about my batch was that it had terrible offset, like 2V or smth in a normal circuit where I'd expect 10...50mV.That set me back a bit... Then another hit came with new njm4556DD having higher noise than ones recovered from old pcb's , then getting 20x njm4562 from mouser that all proved overly sensitive to my smartphone talk, then just stopped buying op amps...cause I had more than 100 new op amps from 10 different species and probably 300 recovered from various dead electronics...just enough for my next 3 lifes 🙂...
In the end some OPAMPs schematics were just approximations or equivalent representations
Hi Mark am sorry I took so long to answer your question. There's a way to calculate a suitable resistance. In this case 22k to 30k should work just fine. For OPAMPs there may be stray capacitances on the die,
Any two conductors that are separated by a dielectric form a capacitor. It doesn’t matter if these conductors happen to be placed on a semiconductor wafer or if they are traces and planes on a PCB. If you look at certain components (such as coils) or the surface of a typical PCB, it’s easy to spot some pairs of conductors that will have some capacitance.
Another consequence of adding the emitter follower buffer is, in general, a loss in the frequency response of the mirror. Transistor Q3 is potentially operating at a very small current of 2IB. If there were to be a significant capacitance to ground at the base connection common to Q1 and Q2 the current available to discharge this current will also be small equal to 2IB. But the current available to charge this node is potentially equal to ßQ3IIN which is very much larger than 2IB. This asymmetry in the charging vs. discharging current available for this node in the current mirror can lead to very undesirable response to fast changes to IIN.
A Wilson current mirror would require all transistors have the same current gain ß.
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Just because the uA 741 designer chose to perfectly match his R3 and his R12 {uA 741 schematic in post 12}, and then to make both of them 50K, doesn't automatically mean that 50K is the best value for your input stage with your bias currents.
So what IS the best resistor value for your discrete opamp? Assuming you'll build it using thru hole components, on a standard PCBoard made of standard FR4 expoy fiberglass.
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So what IS the best resistor value for your discrete opamp? Assuming you'll build it using thru hole components, on a standard PCBoard made of standard FR4 expoy fiberglass.
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