Capacitance multiplier
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A capacitance multiplier is a transistor configured such that it acts to multiply the value of the capacitor connected to its base by an amount equal to the transistor's current gain (β). This circuit is usually found in DC power supplies where very low ripple voltage (under load) is of paramount importance, such as in class-A amplifiers.
[edit] Operation
R1 and C1 form a low-pass filter and help smoothen 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.
