An op-amp made from 555 chips

So instead we can adopt the following approach: use two 555s, one for each op-amp input, using the high-z threshold pin on each 555 compare the inputs to a common ramp generated by an auxiliary 555, converting voltage to time use postprocessing logic (implemented with "555 gates") to compare the PWM signals, generate error pulses, and integrate them The schematic below shows one implementation. I used the TS555 from ST Microelectronics; it is a CMOS 555 with improved specs over the bipolar original. First, a conventional astable oscillator generates a sawtooth waveform ranging between 1/3 Vcc and 2/3 Vcc. This is fed to the control inputs of two additional 555 chips serving as analog comparators; their outputs encode the op-amp input voltages as PWM waveforms. The digital gates (implemented with 555 chips, of course, as shown at the bottom of the schematic) compare the edges of the PWM waveforms, generating pulses if waveform A is ahead of waveform B or vice-versa. These pulses are integrated with a capacitor, using diodes to isolate the two totem-pole outputs (I suppose one could dispense with the lower diode and use the open-drain 555 pin instead). Ten 555 chips are used in all. This is all quite similar to a charge-pump PLL. In fact I built the circuit first without the inverter pairs and wound up with some flaky behavior: hysteresis, distorted waveforms, etc. This is because at equilibrium the edges are...