There are two groups of classic circuits: those which have served us well in the past, but have been superseded by better designs, and those which are still in use, perhaps with updated components.
In the former group, for example, goes the superregenerative receiver invented by E.H. Armstrong. While it is still used in specialized, niche designs, it has pretty much been made obsolete by the superheterodyne architecture (also developed by Armstrong -- who also invented FM radio!). And who knows? The superhet itself may be made obsolete by software defined radio (SDR) and zero-IF designs.
But some simple, basic circuits are still going strong after many decades. Consider the analog peak detector shown in Figure 1 in simplified form. It's sometimes called a peak-hold circuit (and sometimes called a full-wave rectifier, although I think designation is misleading, for reasons too long to explain here). The peak detector does one thing: it monitors a voltage of interest and retains its peak value as its output.
The basic peak-detector circuit requires just a few high-quality analog components to capture and hold the signal's maximum value.
How does the circuit operate? It's simple: the input signal charges the hold capacitor, and the diode prevents the capacitor from discharging. The input op amp, in conjunction with the capacitor, presents that held value as the output via the driver op amp. As the input voltage increases further, the capacitor is charged to the higher voltage; if the input voltage decreases below the previous value, the voltage on the capacitor stays at the previous peak value.
By adding a simple comparator to the output to compares the present input value to the already held value, it can also indicate that a peak has been reached when the present input value is less than the held peak by some desired amount, Figure 2. This transforms the circuit from providing a peak-hold function to implementing the peak-detect function, with comparator hysteresis to establish a valid-peak threshold.
By adjusting the hysteresis of an output comparator, the designer can set the threshold for the "valid peak detected" value.
It may seem that such a circuit itself is nearly obsolete. After all, wouldnít it make more sense to use an ADC on the signal of interest, with a basic software loop checking the newest converted value against the previous ones?
Reality is that it would not make more sense, for both hardware and software reasons, except for fairly slow to moderate-speed analog signals. On the hardware side, the cost of the ADC and its input signal conditioning could be significant (the ADC has to sample at much higher than the Nyquist rate, in practice); and the power consumption and footprint would be higher. On the software side, the checking loop would need some filtering algorithm to make sure a little noise on the signal did not cause a false output (some sort of moving average, perhaps?), and soon the "basic" software loop could easily grow to consume a inordinate amount of the CPU resource.
In contrast, the analog circuit requires no software, little power or real estate, and is inherently self-filtering (up to a point). The main component constraints are that the designer's bill of materials (BOM) calls out low-leakage, high-quality components, which are a little more costly than commodity parts.
More important, this peak detector can operate at frequencies much higher than those for which there are ADCs, since its only semiconductors are the op amps and diode, both of which are available into the tens of GHz. This can be for a relative assessment where the voltage alone can stand as a proxy for actual power levels, or for absolute measurement when used in conjunction with the known impedance across which the voltage is measured. Due to its virtues, this design is still used as the core of the RF peak-power measurement function, either using discrete components, as a single-purpose IC/module, or as part of a larger IC.
The peak detector was such a common configuration that some analog vendors offered it as a standard application design. For example, the data sheet for the OPA111 op amp from Burr-Brown (now part of Texas Instruments) showed a possible circuit (see Figure 18), and Analog Devices offered the PDK01 IC, which I believe (I'm not sure on this) was a monolithic version of an older, multichip Burr-Brown device.
A relative of the peak detector is the crystal (diode) radio, which uses a diode to demodulate (detect) a basic amplitude-modulated broadcast signal; see Figure 3.
The venerable crystal radio circuit is a self-powered envelope detector, and a precursor of the peak detector.
This circuit is an envelope detector that tracks the overall modulated signal shape (a.k.a. envelope) -- the information-bearing part of the signal -- while ignoring the much higher-frequency carrier waveform. (If you don't understand how the crystal radio works, please review your analog basics.)
Note that the early crystal radios were entirely self-powered, as enough energy could be captured via a long-wire antenna to drive high-impedance headphones (usually around 20kΩ). To anyone who insists that energy harvesting is a new phenomenon, or that RF energy harvesting is a recent development, just point to the classic crystal radio of the early 20th century as a counterexample. [See Nothing new about energy harvesting and Remember: voltage is not power (but we still need it).]
Are there any similar long-lived "classic" circuits that are still in use that you know of? Are there any that you have used?