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On Q-Point of a circuit

The top answer says that if you use DC current source, then the Q point is the "solution" of the circuit.

I interpret this as, if you use a DC current source of I amps, then the Q point will tell you what voltage will appear across the nonlinear device (e.g. diode)

But on this page, it says we can have multiple Q points, some are stable, some are unstable.

Can someone confirm my understanding of Q point and elaborate on the meaning of having multiple Q points?

Is quantum tunnel diode the only device having multiple Q points?

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  • \$\begingroup\$ In one word, the result of having multiple stable Q points is hysteresis. \$\endgroup\$ – Ben Voigt Sep 18 '16 at 17:41
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Most devices have a single solution for the Q or operating point. e.g. NPN transistor (graph 1).

However, there are a number of devices that exhibit a region of dynamic negative resistance (dV/dI = -R). In this region you could have three Q points. (Graph 2) Q1 and Q3 would be deemed as 'stable' because in this region they behave as a normal resistance (increase voltage, increase current) . Q2 is unstable because it acts as an negative resistance.

Note that these regions are not actual negative resistance, only the slope (dV/dI) which is negative. This dynamic negative resistance is often used to cancel out the resistance of an LC circuit creating a high Q filter circuit or even an oscillator.

Probably the first device to have this characteristic was the screen grid tetrode valve (tube).

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Circuits, or devices within circuits, have Q points, individual devices do not.

Wikipedia has an article on negative resistance that gives a list of component types that have similar properties to the tunnel diode, which allows them to be used to create circuits with multiple Q-points.

It's also possible to create a negative resistance with an op-amp circuit.

And it's possible to create a multiple Q-point circuit without any negative resistance device, for example a digital flip-flop, or a bistable multivibrator (also sometimes called a flip-flop).

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