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when was this????

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it's interesting to look through old electronics books, especially the kind where they take published circuits from magazines like EDN and Electronics and put them all in a compilation. i have 2 or 3 of these, and more of them can be found at places like a lot of things we associate with much later technology originally appeared in some of these compilations. i don't have a scanner, so i'm not able to pick some of these circuits out and upload the images, but what i will do, is model these circuits in LTSpice and actually show how they work. i will document the circuit as drawn, and anything that had to be done to get it to work right, because i know there are some of these circuits that don't exactly work well as drawn, which may be because a) they were assembled just to test an idea. b) components of the day may be widely different to current components (many of the transistor circuits used germanium transistors, so the bias voltages were different) or c) there may be an "easter egg" in the drawing (not as likely as the other reasons, because the climate of engineering was much different than it is today). remember, these were the days when companies like RCA, GE, etc... published reference designs in their transistor and tube data books. the idea was you buy their active devices, you build their reference design and make changes to it (if you didn't and your product went to market unchanged, you could end up having to pay royalties), and then go into production, obviously using RCA's (or GE's, etc...) active components. these days, most semiconductor companies show a block diagram, and you have to "NDA up" to see the reference design.

i will publish a circuit using modern components. and analyze it. to make it interesting, everybody can take a guess what time frame the circuit originated in. if you don't want to guess, that's ok, we'll just discuss the circuit itself and how it works.

so, to get started... i will pick out a perfect example of what i'm talking about... coming up in installment #1 the earliest known solid state class D amplifier.
this was the earliest example of a Class D amplifier as far as i know. the circuit oscillates at about 43-60khz (the frequency drops a bit at the peaks of the audio input). the input voltage range is very narrow, and if the audio clips the amplifier, it stops oscillating (and fails to restart). there is a slight DC bias required of about 0.046V at the input, and the max audio input is 0.1Vp-p. it's easy to see why this never went into production, the output is very finicky about loading, and the circuit may stop oscillating if the output impedance is too low. i added the MOSFET buffer because of that. if you try this circuit, you will find it's even sensitive to the gate capacitance of the MOSFET, and will stop oscillating with different MOSFETS being driven. the gain of this circuit is around 180-200. the circuit was actually named "Bang-Bang Amplifier" in the book this circuit came from. the term "Class D" had not yet been thought of.

I don't know when that article was published, but the 2N2222 sets a lower limit.

That circuit is similar to the more complex FM modulator in 1960's Ampex VR-1200 broadcast video recorder.

actually there were no transistor types in the schematic, and from the amount of "finicky" in the circuit, i wouldn't be surprised if the originals were germanium. don't go by the part numbers used.
i will publish a circuit using modern components.
i'm doing that so people don't have to go searching the web for germanium models or anything else out of the ordinary. as a matter of fact, i might try some devices with lower beta to see if that corrects some of the "finicky".
this little beauty has the advantage of being super simple. this amp is powered by 2 d-cell batteries. the amp has a voltage gain of 40, and was meant to power a speaker in a portable radio. there's only one real "gotcha" in this circuit, and that's the fact that the idle current is about 120mA. it's a battery eater. the use of a transformer to drive the speaker could be a clue to the "when was this" question. except for the idle current problem, this little circuit could have been the "LM386" of it's day.
on the input side of the amp, are two diodes and a pair of 4.7k resistors. this diode/resistor network has a large effect on the idle current of the output stage. so, let's try to reduce the idle current some...
what i've done is add two more diodes to reduce the voltage on the bases of the transistors. this modification changed the gain to about 7, but it also created a "dead zone" where the output of the amp is zero, while the input waveform is within 0.1V of zero. this is very similar to crossover notch distortion in power amplifiers when the bias circuit fails. the crossover notch is not acceptable, so let's try something different... let's change two of the diodes to schottky diodes
this brought the gain back up, and got rid of the notch. the idle current is 78mA now (change the ac input to 0V, run the sim, and measure the current through R1). 78mA is still a bit high, so let's change the resistors next:

the base bias resistors are now 500 ohms instead of 4.7k. this reduced the idle current to 32mA, which is going to have to be sufficient, because the gain has been reduced to 32. lowering the idle current is now a game of diminishing returns, because it will also reduce the gain drastically. the circuit as it is now offers a fair trade-off, a voltage gain above 30, and idle current of less than 1/3 of what it was originally.
This sounds like a good game. I'm going to take a punt... picking sime numbers out the air:
#1 1967
#2 1959

There's a schematic somethere (maybe on a patent archive?) of a class D amp from, I believe, the 40's - designed with valves, of course. That's by far the oldest reference to the topology that I've ever seen.

Thanks for your effot on this Uncle. I get the impression that the electronics press these days doesn't publish much of this kind of stuff any more, and these circuits can be a valuable learning resource and also a source of useful building blocks and useful ideas for larger projects.
Are you sure that you have the transistors connected correctly in posts 5 & 6? If you had them connected emitter to emitter and the collectors to the power supply, then you'd have a standard complementary symmetry output stage. The diodes would be across the emitter-base junctions to bias the transistor just into conduction and eliminate crossover distortion. I've never seen a circuit like this with the transistors connected backwards.

Bear in mind that if this came out of one of those magazine circuit collections, those circuits were often submitted by people who thought they had some brilliant new idea, but the circuit was often marginal at best. The editorial staff never tested these things. They just published them and hoped for the best. I suppose that if they got a lot of negative reader feedback, they may have left it out of the later circuit collection books.

As for the evolution of transistor push-pull output stages, they began by emulating tube style push-pull output stages, with transistors replacing the tubes. The first generation used same polarity germanium transistors, usually PNP since those were the easiest to fabricate. These drove a centre tapped output transformer primary, just like the tube version.

The next generation used a quasi-complementary output stage again using same polarity transistors, again for the same reason. However, these eliminated the output transformer, saving space and money. It looked similar to some of those silly OTL (output transformerless) tube circuits. They either used a dual polarity supply, or else a large value electrolytic to block DC from the speaker.

I don't ever recall seeing a true complementary output stage using both NPN and PNP transistors, until germanium was pretty much history, which would have been sometime in the early 1970's.

If I had to guess, I would say that your circuit came from the early to mid-1960's when they were still using output transformers on transistor circuits. The availability of matched PNP/NPN transistor pairs would be uncommon in those days, but whoever designed it may have had access to some factory samples. I'd be interested in finding out what its claimed advantages are.

Edited typo
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circuit #2 is from around 1963. the circuit is drawn correctly with opposed collectors. the whole point of the circuit is the voltage gain of a common emitter amplifier combined with the simplicity of symmetrical push-pull outputs. the original devices in the schematic were actually germanium transistors. the design actually works quite well, even though the idle current in my final modification is still a bit high. unlike the circuit in the earlier post (the class D amp, which turns out to be from about 1958 or so), the amplifier actually has a wide tolerance in output device characteristics and component values. the class D amp in post #2 is on the edge of not working, and changing component values more than a few percent stops it from working at all. the circuit in posts #5 and 6 can actually operate directly to a speaker, but i left the transformer there because it was in the original circuit. since the resistors from the bases to the supply rails set the idle current in the output devices, it's obvious that the 1.5V supply rails are about all these transistors can tolerate.

there have been more recent amp designs using the same opposed collector topology, usually as part of Sziklai pairs (kind of a modified darlington, but instead of using two NPN or two PNP transistors, Sziklai pairs are an NPN and a PNP. or a PNP and an NPN). whoever the author of this opposed collector amp was, it's clear they were thinking "out of the box" because as you mentioned, most transistor amplifiers were mimicking tube amps, with use of interstage and output transformers, and very similar topology to vacuum tube amplifiers.

in the mid 60s an engineer named Lin created the topology that has been in use ever since, with a diff amp, voltage amp and bipolar output stage (although the original Lin design was a quasi-complementary output using a darlington pair for the positive side, and a Sziklai pair for the negative side). various hybrids of Lin's design existed, some used a single ended input stage (like in the Dynaco amps), but eventually the advantages of the diff amp won out, as well as the completely symmetrical bipolar output stages (as soon as PNP power transistors were inexpensive enough, and could be fabricated with characteristics closely matching their NPN complement).

in modern amplifiers using the LIN topology, the amplifier is primarily an "op amp done large" because the amp has an inverting and a noninverting input, voltage amplifier stage and complementary output pair, in almost the same configuration as in an op amp.

btw, in case anybody is interested at looking at a modern version of a quasi-complementary amplifier, Yamaha's RS-202 receiver uses quasi-complementary outputs (drivers are NPN-PNP, and outputs are NPN-NPN). i guess Yamaha found that there's a market for the "vintage sound" of a quasi-comp amplifier in the receiver market.
So, my guess of early to mid-1960's was in the right ballpark.
I second the thanks for your effort, Unclejed.

I also have some circuit collection books.....but these use vacuum tubes!
I second the thanks for your effort, Unclejed.

I also have some circuit collection books.....but these use vacuum tubes!
same here... also, if you look at the old vacuum tube and transistor data books (RCA, GE, etc...), there were reference designs in them that were quite interesting. application notes in data books are other great sources for circuits.
Empty-state device.
some pretty amazing things have been done with "glass & steel". you probably don't know it, but Philo Farnsworth not only invented electronic television with vacuum tubes, but also used a vacuum tube for early nuclear fusion experiments (since i don't recall anybody on ETO writing any articles about the Farnsworth Fusor, i think i'll write one in the near future). as a matter of fact, the Manhattan project used some very large vacuum tubes to separate U-235 from U-238 (the electromagnetic separation method) with a machine that was very similar in principle to a magnetron (instead of curving the path of electrons in a magnetic field, it used magnets to curve the path of positively charged uranium nuclei).
the following circuit is a variable capacitor using 4 transistors in a variable capacitance circuit. the current source is used as a way to measure the capacitive reactance. this circuit is kind of crude, and has a "dead band" within 0.7V of the zero crossing. R1 and R2 are a 10k potentiometer. with the wiper at the ground end, the effective capacitance is the same as C1, and moving the wiper towards the left decreases the capacitance. the advantages of using this circuit are that this circuit can operate at frequencies approaching the Ft of the transistors, as long as C1 is more than about 10x the junction capacitance of the transistors. the disadvantages of this circuit are a) this is a single ended capacitor (i.e. one end of the cap is grounded, and not floating), b) the +/-0.7V dead band can distort signals across the capacitance, and so this device is ineffective for small signals c) because the capacitor is operated in a half-bridge from bipolar supplies, polarized capacitors should not be used

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