Audio Engineering

210 Chapter 7

of the even more exotic later developments such as the NE5534 or the OP27, as the basic
gain blocks, around which the passive circuitry can be arranged, or whether, as some
designers believe, it is preferable to construct such gain blocks entirely from discrete
components.

Some years ago, there was a valid technical justifi cation for this reluctance to use
op-amp ICs in high-quality audio circuitry, as the method of construction of such ICs
was as shown, schematically, in Figure 7.41 , in which all the structural components
were formed on the surface of a heavily ‘ P ’ doped silicon substrate, and relied for their
isolation from one another or from the common substrate on the reverse-biased diodes
formed between these elements.

This led to a relatively high residual background noise level, in comparison with discrete
component circuitry, due to the effects of the multiplicity of reverse diode leakage
currents associated with every component on the chip. Additionally, there were quality
constraints in respect to the components formed on the chip surface—more severe for
some component types than for others—that also impaired the circuit performance.

A particular instance of this problem arose in the case of PNP transistors used in normal
ICs, where the circuit layout did not allow these to be formed with the substrate acting
as the collector junction. In this case, it was necessary to employ the type of construction
known as a “ lateral PNP, ” in which all the junctions are diffused in, from the exposed
chip surface, side by side.

In this type of device the width of the ‘ N ’ type base region, which must be very small
for optimum results, depends mainly on the precision with which the various diffusion

BE BECBEC

Isolation Isolation Isolation Isolation Isolation

N P

P

C

P N PPN N

P

P

PNP transistor Resistor NPN transistor

‘Lateral’

PNP transistor Dielectric capacitor

SiO 2 surface I

Epitaxial layer

Substrate

Sub

P P P NP P

N N N

Mounting pad

Figure 7.41 : Method of fabrication of components in a silicon-integrated circuit.