Capacitance multiplier
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A capacitance multiplier mimics a capacitor of different capacitance than the built-in capacitor's.
This is useful in DC power supplies where very low ripple voltage (under load) is of paramount
importance, such as in class-A amplifiers.
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Transistor based
A basic capacitance multiplier.
Here the capacitance of capacitor C1 is multiplied by the transistor's current gain (β).
(原文件名:tr.jpg)
R1 and C1 form a low-pass filter and help smooth any ripple in VS. R1 supplies the charging current
as well as the transistor's (denoted by Q) base current. R2 is the load on the circuit. Without
Q, R2 would be the load on the capacitor and C1 would have to be very large to maintain low ripple.
With Q in place, the loading imposed upon C1 is simply the load current reduced by a factor of β.
Conversely, C1 appears "multiplied" by a factor of β to the load.
Note that this circuit is not a voltage regulator, since the output voltage varies directly with
the input VS. The output voltage is about 0.65V less than the base voltage, which in turn is about
2–3 V less than VS (when loaded). Larger values of R1 (and C1) reduce the output ripple to almost
negligible levels. On the downside this causes the output to rise slowly towards the required value
(especially when the load is connected), due to the larger time constant of R1 and C1.
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Operational Amplifier based
A basic capacitance multiplier.
Here the capacitance of capacitor C1 is multiplied by the quotient of the resistances of R1 and R2.
(原文件名:op.jpg)
