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Old 22nd January 2003, 06:49 AM   #1
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Default oscilating in capacitive load

I would like to know, why amplifiers oscilate, when they have capaciteve load?
Is it because their gain is dropping with frequency and so it acts like inductor?

Thank you in advance,

borisov57
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Old 22nd January 2003, 07:26 AM   #2
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To keep it simple, the main reason is that the amp has feedback and what makes it worse is that the amp consists of more than one amplifying element.

An amp with no feedback at all is hard (normally) to get oscillating.
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Old 24th January 2003, 12:14 PM   #3
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Some background. There a a couple of reasons why capacitors cause oscillation.

Resonance
A capacitor is an energy storage device: the energy stored is proportional to the voltage across the capacitor. An inductor is also an energy storage device: the energy stored is proportional to the current flowing through the inductor. Inductors and capacitors are opposites to one another and are analogous to springs and masses in mechanical systems. If a system contains both then they will tend to exchange energy - and this exchange is oscillation. The oscillation will continue until the energy is dispersed, usually by some resistance or friction. There will be a frequency where the energy exchange is greatest and this is known as the resonant frequency of the circuit.

In cars the suspension consists of srpings. If it were not for shock absorbers (resistors) a car would bounce up and down everytime a small bump was encountered. I know, I drove a car with broken shocks once and it was very unpleasant. A trampoline has relatively low resistance thus allowing repeated bouncing - and if you try to bounce at the resonant frequency you'll end up hitting your head on the ceiling.

Amplifiers (even without global feedback) have some output inductance. This will resonate with any output capacitance you apply and there will be a frequency where oscillations are maximized. To reduce this effect you must reduce the amps output inductance and add resistance to absorb the energy. The resistance should be greater then or equal to the reactance of either L or C at resonance in a series circuit or less than or equal to in a parallel circuit.


Phase shifts in control systems
This is what Peranders is refering to. An amplifier is literally pumping a current into and out of the load, related to the difference in voltage between input and output. When a current is driven into a capacitor the voltage across the capacitor will be shifted in phase. The upshot is that the output voltage of the amp tends to be more and more out of phase with the input voltage as frquency increases. There will be a frequency when the phase is inverted - and this means that the negative feedback becomes positive feedback and adds to the output signal. This can cause severe oscillation if care is not taken in the design. Not all capacitances will cause oscillation, some may just cause a little ringing. The way to test a circuit is to apply a square wave input and then load the amp with a range of capacitances until you find the one that causes the circuit to get excited. Beware that some amps will expire under this test!

Why does it matter?
Oscillation matters if it causes the amp to mistrack the audio signal. A loudspeaker and cable is a complex and dynamic load. It will show variable inductance and capacitance. There may be brief periods where the amp becomes oscilliatory and loses control of the audio signal. Many amps are unstable into some capacitive loads and it doesn't matter too much. In general this is a consideration of great importance.
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Old 24th January 2003, 12:29 PM   #4
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Thank you very much for reply.
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