An op-amp made from 555 chips

Posted on Feb 5, 2014

Is it possible to make an op-amp out of nothing but 555 chips and passive components? Not a terribly practical question, given the existence of very inexpensive and capable op-amps covering every corner of op-amp performance space; but it has some aesthetic appeal. If you find yourself on a desert island with nothing but a pile of 555s and a need for an op-amp, by all means read on. The 555 has two comparators, but offers direct access to neither. The `trigger` comparator could conceivably work with feedback from the control pin, but the existence of a 2:1 resistive divider in the feedback path is very awkward. And while the `threshold` comparator sees both the input pin and the control pin directly, unfortunately its output can't flow through the digital portion of the 555: the threshold comparator can only reset the RS latch, not set it. Even if we could use the threshold or trigger comparators directly, they have the disadvantage that one of their inputs is tied to a resistive bias network, resulting in a very low input impedance on that input (in the neighborhood of 30k ohm for the CMOS flavors of the 555; 3k ohm for bipolar). Potentially tolerable for something like a unity-gain buffer, where a low-impedance output drives the input pin, but not good for a general-purpose op-amp.

An op-amp made from 555 chips
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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...

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