Let’s build an oscillator, now that we can generate linear ramps. But to get there, we’ll need a second op-amp circuit (1/2 Vcc is the middle of two 10 kΩ R’s between Vcc and GND):
In contrast to yesterday’s setup, there is no negative feedback here, but a resistor between “OUT” and “+”. So in this case, when the output rises, it’ll cause “+” to rise even more, instead of bringing it closer to the “-” input.
This circuit doesn’t work towards an equilibrium, it’s unstable. Let’s start with the output being VCC, i.e. +5V. What will the input need to be to make it change?
- the desired change can only happen, when the “+” input drops below 2.5V
- with OUT at +5, that’s 2.5V over 20 kΩ, i.e. 125 µA
- to pull “+” under 2.5V, we need to draw at least 125 µA out through the 10 kΩ resistor
- that’s 1.25V under 2.5V, i.e. with the IN level under 1.25V
So when IN drops under 1.25V, the op-amp has a change of heart so to speak, and starts bringing OUT down in an attempt to bring “+ and “-” back to the same level. The silly thing is that it can’t, because lowering OUT is never going to raise the “+” back up to 2.5V (still with me?). So the output just keeps keeps dropping all the way to its minimum, which will be more or less equal to 0V.
Let’s review what happened: we started dropping the input level, and once it reached 1.25V, the output violently flipped from +5V to 0V.
Now the situation is reversed: how far do we need to raise the input to make the output go up again? Well, that’ll be 3.75V – using the same reasoning as before, but now based on 2.5V (the “-” pin) + 1.25V (the voltage over the 10 kΩ resistor.
So what this circuit does is flip between 0 and 5V, at trigger points 1.25V and 3.75V.
Now the magic part: we tie the input of this circuit to the output of yesterday’s circuit, and vice versa, creating a control loop. If you look back at yesterday’s scope image, you’ll see that the triangular wave flips at… 1.23V and 3.72V. What a coincidence, eh? Nah… it’s all by design. The comparator drives the integrator by feeding it 0 and 5V levels, and it switches when the integrator output reaches 1.25V and 3.75V. Since the capacitor requires a little time to charge, this ends up being a nicely controlled oscillator. Perfect!
Here’s my test circuit (for a complete schematic, check this website, for example):
Tomorrow, I’ll change the signal slightly and I’ll examine how linear those ramps are.