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As one can see that there are circuits in figure 1 which consists of a AC voltage source with \$V= V_m sin(2\pi f t)\$ ,a diode and a resistor.

Now, if I do \$V_{o1}+V_{o2}\$, I will get same bidirectional AC signal, IE. \$V\$

But if I do \$V_{o1}-V_{o2}\$, I will get a unidirectional AC which is also can be called as DC signal(?).

Now, I want to know that is it really possible to obtain an AC signal ( Voltage/current) from pure DC (IE \$f=0\$) input signal. If yes, what could be the circuitry in the question box of figure2?

enter image description here

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    \$\begingroup\$ Have you heard of oscillators, multivibrators, voltage inverters and similar? \$\endgroup\$
    – Eugene Sh.
    Sep 14 '16 at 17:19
  • \$\begingroup\$ @EugeneSh. Could you please tell me how they convert pure DC signal into AC signal ? If possible with its circuit diagram? \$\endgroup\$ Sep 14 '16 at 17:25
  • \$\begingroup\$ He already told you. Search about sinusoidal oscillators. \$\endgroup\$ Sep 14 '16 at 17:27
  • \$\begingroup\$ Take a look here for one example, though you better start with theory. \$\endgroup\$
    – Eugene Sh.
    Sep 14 '16 at 17:28
  • \$\begingroup\$ In the old days they did this electromechanically: a DC motor driving an AC generator. Now we use electronics. There are many ways to do this. Since you're just learning about diodes, you're not ready for those schematics yet. If it was suddenly impossible to convert DC into AC almost all electronic devicees would stop working instantly !!! \$\endgroup\$ Sep 14 '16 at 17:47
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Yes and no.

In the real world, there's no such thing as "pure DC". Any real voltage source must be built and turned on, which is a transient event (AC). Real circuits also receive noise from background radiation and other sources. These things help keep oscillators from getting stuck in a metastable state.

In mathematical theory, "pure DC" is eternal -- it never begins or ends. A theoretical DC-powered oscillator must also be eternal. But if the DC voltage was always there, and the oscillation was always happening, I'm not sure it makes sense to say that the oscillation comes from the DC source. Can one eternal thing cause another?

As a practical matter, we ignore the philosophical questions. Yes, you can make a time-varying signal from a DC voltage supply. A simple example is a ring oscillator, which connects digital inverters in an unstable way:

Ring oscillator

There will usually be R-C filters on the inverter outputs to slow down and stabilize the oscillation.

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