The basic capacitance multiplier circuit is essentially a simple emitter follower with a capacitor on the base and a feed resistor from the input to the base to turn the transistor on. A capacitor from the base to ground provides the smoothing.

The capacitance multiplier circuit operation is quite straightforward. It acts as a simple emitter follower. The resistor R1 provides bias for the base emitter junction, and the capacitor provides smoothing. This considerably reduces the levels on noise on the output, i.e. Vout.

The effect of placing the transistor in the circuit is that it effectively multiplies the capacitance on the base by the current gain of the transistor, i.e. by β

The capacitance multiplier circuit is not a voltage regulator. The output voltage varies directly with the input Vin as there is no voltage reference. Generally the output voltage is about 0.65V less than the base voltage, and around 2 – 3 V less than Vin when a load is applied.

The ripple and noise levels on the output can be reduced to very low levels> Increasing the values of R1 and C1 reduce the output ripple, and increasingly at low frequencies. On the downside large values of R1 and C1 cause the output to rise slowly towards the required value after turn on, because of the large time constant of R1 and C1.

Modified capacitance multiplier

The drawback of the circuit is that in its basic form, there is very little voltage drop across the series pass transistor, and noise reduction is not as high as it may be. To overcome this, some people place a resistor across the capacitor and this provides a potential divider reducing the voltage at the base and increasing the voltage drop across the transistor. This enables it to provide better noise reduction, although it does increase power dissipation and reduces the voltage at Vout.

This version of the capacitance multiplier circuit includes an additional resistor from the base to ground to reduce the base voltage and provide additional voltage drop across the transistor for improved smoothing. This is more important when the levels of ripple are higher.

Typically the voltage through the potential divider should be sufficient to maintain the base voltage sufficiently. A judgement can be made regarding the level of current though the potential divider, but often in these types of circuits it may be ten times the base current. This would ensure that the emitter voltage is maintained over a wide range of output current levels.

Example application for a capacitance multiplier

The power supply shown here provides only smoothing at this stage and no stabilisation or voltage regulation. The input is taken from the mains and rectified by the bridge rectifier. It then passes into a smoothing capacitor, C1, to provide the first smoothing and remove the major ripple. This capacitor should have a large ripple current capability if the supply is to be used for high current levels.

It should be remembered that the capacitance multiplication effect can only be realised if there is a sufficient voltage drop across the series transistor. Typically this should be a minimum of 3 volts at all times.

The capacitor C2 is connected to the base of the transistor TR1. This provides the capacitance for the capacitance multiplication effect.

TR1 is the main pass transistor and must be able to drop the required voltage and at the required current, so power dissipation may need to be calculated.

On the output there is a capacitor to provide a little further decoupling and to ensure that the circuit remains stable. The resistor ensures the output voltage drains away at power removal. The diode D1 ensures that the transistor does not become reverse biased.

Diod Transistors

Transistor as a Zener

Zener diodes connected as shown in Fig. 1 are carefully doped at manufacture such that they break down when a critical reverse bias voltage is applied and allows current to flow. The current is limited by the power ratings of the particular device employed.

Zeners are used to provide a voltage reference and any transistor can be employed to do the same job if connected as shown in Fig. 2.

Different transistors will produce different zener voltages and a series of tests can be done to establish what the typical zener voltage is for that device.

A suitable setup could be employed as shown below.

  A BC548B gives a zener voltage Vz = 8.2V

  A BC549B gives a zener voltage Vz = 10.8V

  A BC547B gives a zener voltage Vz = 9.1V

  A BC184 gives a zener voltage Vz = 8.2V

Using the Configuration in Practice

The above circuit Fig 4 could form the basis of a low power regulated power supply.

V_be Multiplayer

Variable Reference Voltage Circuit

Fig. 5 shows a way of adjusting the voltage Vz to suit by adjusting the resistor values of R1 and R2.

Vz = Vbe((R1 + R2)/R2)

Better regulation can be achieved with a darlington configuration as shown below where it is used in a simple regulated supply, but now the voltage Vz becomes:-

Vz = 2Vbe((R1 + R2)/R2)

and the regulated output voltage becomes:-

Vo = Vz – Vbe

Other more circuits

Calculating ripple voltage and current

To choose the right capacitor for the input filter of a switching regulator, for example, the capacitance needed to achieve a desired voltage ripple can be calculated, if the operating conditions of the regulator are known. When the capacitance is calculated, a candidate component can be identified, and the ripple current determined from the known ESR. This ripple current must be within the capacitor’s ripple current handling capability, if the device is to be suitable for use. This is where selection can become difficult, because both ESR and capacitance are known to vary with temperature, operating frequency, and the applied DC bias.

The capacitance can be calculated using the equation (from TI Application Report SLTA055)

Where CMIN = minimum capacitance required

IOUT = output current

dc = duty cycle (usually calculated as dc=Vout/(Vin*Eff))

fSW = switching frequency

VP(max) = peak-to-peak ripple voltage

Assuming, for example, a regulator with 12v input; 5v output; 2amp output; 85% efficiency; 400kHz switching, and an allowable input ripple voltage of 65mV:

Note that the chosen device must provide this value of capacitance at the regulator operating frequency of 400kHz.

The rms value of the peak-to-peak ripple voltage can be calculated from the equation:

Vrms=Vpp*1/(2*√2)

The ripple current in the capacitor can then be calculated by applying Ohm’s law, if the capacitor’s ESR is known.

profisional bootstrapping circuit