Every so often a design calls for an inverting amplifier of gain that is variable in discrete steps related by a constant factor, say two. That is, an inverting amplifier that might have, perhaps eight, nine or ten different gains, with each successive one a factor of two different from its predecessor. The lowest of these gains might be actual amplification, unity gain, or even attenuation. That is, the lowest gain setting might be a gain of 1/8, a gain of unity, or a gain of two. Each higher gain would be a gain of twice as much. Of course, the use of two as the ratio between consecutive steps is merely illustrative. In principle the ratio can be any practical (in the ordinary sense of the word) value, as can be the number of steps of gain.
There are at least two prior art circuits which have been used in this sort of service, and each has at least one disadvantage. For example, the circuit shown in FIG. 1A requires very tight tolerances on the resistors if the gains are to vary by a wide margin, say 2.sup.10. The circuit can only produce gain, and not attenuation. Another disadvantage of this circuit is that it is not naturally inverting, and an additional stage is needed to accomplish inversion. The circuit of FIG. 1B in an inverting circuit, but imposes stringent requirements on the switching network. Field Effect Transistors (FET's) are usually used (arranged as form C switches), and circuit operation is affected by variations in ON resistance and by leakage.
There is yet another prior solution, which is to simply precede a high gain amplifier with an attenuator. This is a very brute force solution that has the disadvantage that the gain of the amplifier must be stable and precise, the amplifier must always operate at maximum gain (which is probably also maximum power dissipation), and there needs to be precision high ratios of resistance values in the attenuator.
The difficulties associated with the issues of resistor tolerance, and of maintaining precise ratios and temperature coefficients for resistors of values over a wide range, become more understandable when it is remembered that it is often necessary to fabricate the resistors using either thin or thick film processes as part of a hybrid integrated circuit process. This type of implementation imposes these constraints today in a way that simply was not as painful back when the designer would (probably could only) use a Teflon rotary switch and hand selected individual precision resistors.
It would be desirable if there were an inverting variable gain amplifier circuit having discrete gains with a constant ratio between adjacent gains, but that did not require high tolerances of the resistances nor heroic performance from the FET switches. Such a circuit would lend itself to economical and reliable production using the available standard fabrication techniques for hybrid integrated circuits.