Audio Engineering

Preamplifi ers and Input Signals 205

disuse because it leads to slightly lower quality THD fi gures than are possible with other
circuit arrangements.

All these circuit arrangements suffer from a further disadvantage, from the point of view
of the integrated circuit designer: they employ resistors as part of the circuit design, and
resistors, although possible to fabricate in IC structures, occupy a disproportionately large
area of the chip surface. Also, they are diffi cult to fabricate to precise resistance values
without resorting to subsequent laser trimming, which is expensive and time-consuming.

Q 1

R 1

Ic

Figure 7.35 : Two-terminal constant current source.

R 1

R 2

R 3

C 1

Q 1 Q 2

Eout

V

V

Figure 7.36 : Load impedance increase by boot-strap circuit.