Audio Engineering

208 Chapter 7

MOSFETs, where the lower effective mutual conductance values for the devices would
normally result in relatively poor stage gain fi gures.

This has allowed the design of IC operational amplifi ers, such as the RCA CA3140
series or the Texas Instruments TL071 series, which employ, respectively, MOSFET and
junction FET input devices. The circuit layout employed in the TL071 is shown, by way
of example, in Figure 7.40.

Both of these op-amp designs offer input impedances in the million megohm range— in
comparison with the input impedance fi gures of 5–10 kilohm, which were typical of
early bipolar ICs—and the fact that the input impedance is so high allows the use of such
ICs in circuit confi gurations for which earlier op-amp ICs were entirely inappropriate.

(Non-inverting)

Output

VDD

Vcc

Inputs

(Inverting)

Figure 7.40 : Circuit layout of Texas Instruments TL071 op-amp.