414  Equalisation

Capacitance Multiplication for the Biquad Equaliser

Since this equaliser is commonly used at the LF end of the spectrum and must work at low frequencies, if the resistors are to be kept low to minimise noise, then the capacitors can become inconveniently large in value, physical size, and cost. This is particularly true if polypropylene capacitors are used to prevent capacitor distortion. The use of a capacitance-multiplier architecture in an LF biquad equaliser means that the capacitors can be kept to a reasonable size, and any desired capacitance value can be obtained without paralleling components, simply by varying the multiplication factor.

The capacitance multiplication is very straightforward because the capacitors in question are grounded at one end in the basic circuit.Applying it to a floating capacitor would be much more difficult. The basic plan is to connect the normally grounded end of the capacitor to an opamp output, which is then driven so that as the voltage at the top of the capacitor (pointAin Figure 14.28) rises, then the voltage at the bottom of the capacitor falls, increasing the current through the capacitor for a given voltage at pointA. If the fall is equal in voltage to the rise, then as far as the rest of the circuit is concerned, the value of the capacitor has doubled. Varying the amount by which the bottom of the capacitor is driven allows the multiplication factor to be set to any desired value by varying a single resistor.

This technique is demonstrated in Figure 14.28, which implements the circuit of Figure 14.25 without the need to find and pay for a 635 nF capacitor.A3 is a shunt-feedback inverting stage that drives the bottom of capacitor C1 in anti-phase to the voltage at it top end. R4 and R5 are kept low in value to minimise current noise and Johnson noise, and the resulting low input impedance of theA3 stage is therefore buffered from the rest of the circuit by the voltage-follower A2. The multiplication factor, determined by R5/R4, is 1.924 times. so the 330 nF capacitor will appear to the rest of the equaliser circuit as 635 nF.

Figure 14.28: Capacitance multiplication applied to the biquad equaliser of Figure 14.25. Here a 330 nF capacitor acts like a 635 nF capacitor, as its lower terminal is driven in anti-phase.