Capacitor charging

[Kevin Weddle]

When using a small value capacitor, a nonlinear situation exists. The signal quickly charges the capacitor. This creates DC charging for each value of the signal and makes it follow the capacitor charging function. Does anybody follow? Ideally, you want all of the signal without the nonlinear function of the charging capacitor.

 

[audioguru2]

Kevin,
Connect a resistor in series with a capacitor, so that the resistor can charge and discharge the capacitor. Connect a signal to the resistor. You now have a linear low-pass filter, with its output across the capacitor.
It doesn't matter that the capacitor charging is exponential instead of linear, unless you need a linear triangle wave or sawtooth.
If you feed the low-pass filter with a pure sine wave, then the output will be just as pure (very linear) as the input, except at high frequencies when its output level is reduced and is purer (less distortion and even more linear).

 

[Kevin Weddle]

You will notice that even a triangle wave is based on the charging curve.

 

[Guest]

Are you asking how does a coupling capacitor work between two stages of an amplifier?
How is it able to pass the alternating wave through it ?

 

[ante1]

audioguru,

Excuse me, can a triangle wave be linear?

Ante :

 

[audioguru2]

Ante,
A triangle wave can be extremely linear if you use an integrator that is made from an opamp that has a voltage-gain of 100,000 or more, and use a low frequency.
Of course the linearity drops at higher frequencies.
Gross linearity error can be seen as a departure from a straight line on a scope.

 

[Kevin Weddle]

The triangle wave rely's soley on the DC charging of the capacitor. I would tend to use the capacitor and a transistor driven by a square wave to generate a triangle wave. The use of an opamp is not necessary.

 

[audioguru2]

Kevin,
A transistor can produce a very linear ramp if cofigured as a constant current source. But to make a triangle wave you need 2 transistors (1 ramping up, the other ramping down) and a complicated circuit to switch them on and off. An opamp has all those things built-in.
You could also use a transistor as an integrator, but linearity would not be as good as an opamp because its gain is no where near as high as an opamp.

 

[Kevin Weddle]

A square wave applied to a transistor with a capacitor to ground on the collector will produce a charge ramp as well as a discharge ramp.