Hello, I have a question I hope someone can shed some light on. The circuit below is meant to be a sample and hold circuit. It will send a voltage to the non-inverting input while a button is pressed. This causes the output to follow the input. Then the button is released and the output voltage simultaneously feeds both inputs in a forever sample and hold. This doesnt work. But why? Output feedback is certainly capable of affecting the input, so why doesnt this work?
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Hello there,
In theory with ideal components it works, so the answer is because of the non idealities and parasitics of the op amp.
If you have a perfect buffer with gain of 1 and apply an input of exactly 2 volts from an ideal voltage source you get an output of exactly 2 volts. The ideal op amp output acts like an ideal voltage source so you can tie the output to the input and no current will flow between sources.
You can then disconnect the original voltage source and the output and input will stay at 2 volts.
The problems come in though, because of all the non idealities of the op amp which include:
imperfect gain
input offset
input and output impedances (reduced to resistances for this case)
Taking the first above, if we input 2v we dont get 2v output. If we get 2.001v output then when we connect it to the input and remove the first source the output ramps up to infinity or until it saturates. If we get 1.999v output then when we connect the output back to the input and remove the first voltage source the output ramps down to minus infinity or until it saturates negatively. Since the gain is not exactly 1, the input voltage is always changed by the output because the output responds to the input by either increasing it or decreasing it, and once it increases it or decreases it the input also increases or decreases again and so the repetition of this condition will cause the voltages to either fall completely or rise until they hit some limit like the level of the power supply.
So just the imperfect gain alone will prevent this from working well in real life.
The next best thing for comparison is an SLC memory cell that acts like a very very very long time period sample and hold. The basic property is that the voltage level is held by a small capacitance, and for extremely long holding time part of the output is fed back to the input, almost just like you had suggested but not exactly like that.
We can also compare this to an oscillator that feeds back part of the output on purpose too. The thing here though is that there is always some non linear gain adjustment that acts to keep the voltage levels within a certain range. So in theory you could in fact wire up an oscillator and energize it when you want it to store a logical '1' and de-energize it when you want to store a logical '0', and the presence or absence of an output wave would indicate the present state of the memory cell. Unfortunately, it may be hard to control the output level at anything other than on or off.