There are many electric feedback circuits having a feedback branch between a signal output and a signal input of a signal processing circuit. When the signal processing circuit acts as an amplifier, undesirable oscillations of the feedback circuit may occur due to the feedback. Such oscillations can appear when the overall amplification of the feedback circuit is larger than 1 and when the signal portion fed back from the signal output of the signal processing circuit via the feedback branch to its signal input, has approximately the same phase as the input signal fed from a signal source to the signal input of the signal processing circuit (regenerative feedback). For avoiding undesirable oscillation of the feedback circuit, the overall amplification or gain of the feedback circuit must be kept lower than 1 and/or the phase position of the output signal, which is fed back via the feedback branch to the signal input of the signal processing circuit, must be kept in a sufficient phase distance from the case of phase-identical regenerative feedback.
For realizing the second-mentioned measure it is possible to use in conventional manner a frequency response compensation circuit by means of which the frequency response of the feedback circuit is changed in the frequency range in which an oscillation tendency exists, in such a manner that said oscillation tendency is eliminated. To this end, one can use conventionally a frequency response compensation capacitor connected between the signal output of the feedback circuit and a ground terminal. This frequency response compensation capacitor has the effect that the frequency response of the feedback circuit is lowered in the upper frequency range. This means that the frequency spectrum of the signal processed with the feedback circuit is curtailed in the upper frequency range.
Such a frequency response decrease results in a reduction of the gain-bandwidth product of the frequency response compensated circuit. When the feedback circuit serves for processing or producing square wave pulses, the frequency response compensation effective in the upper frequency range results in a steepness reduction of the pulse edges of such a square wave signal. Both is undesirable, the latter in particular for fast digital circuits.
Examples for electric feedback circuit arrangements comprising a feedback branch which is switchable between two different feedback states in the manner indicated at the outset, are on the one hand an amplifier that is fed back in SC technology (switched capacitor technology), or a comparator which, via a switch connected between the comparator input and the comparator output, is switchable between a feedback-free state and a state with high feedback.
SC technology is used in the field of monolithically integrated circuits. In this respect, ohmic resistors, for example in the feedback branch connected to an amplifier, are not formed as genuine resistors in the integrated circuit since these require, on the one hand, much chip area of the integrated circuit and, on the other hand, are difficult to produce with close tolerances of their resistance value, but they are simulated by switched capacitors which need less chip area and can be produced with closer tolerances. In case of the already known SC technology, a capacitor replacing the ohmic resistor is periodically reversed in terms of charge by means of two switches disposed on both sides of the capacitor. In case of an amplifier with an SC feedback, a first SC switch is provided between an SC capacitor and the amplifier input and a second SC switch is provided between the SC capacitor and the amplifier output. In a second switching state of the two SC switches, the amplifier output is fed back via the SC capacitor to the amplifier input. In a first switching state of the two SC switches, the two sides of the SC capacitor are each connected to a ground terminal. With this amplifier circuit the two SC switches thus cause switching over of the feedback branch such that in the second switching state a feedback exists via the SC capacitor, whereas in the first switching state the feedback branch is open and thus no feedback occurs.
The already mentioned comparator having a switch in the feedback branch is utilized for an analog-signal to square-wave-signal reshaping circuit with offset compensation as it is described in more detail in the same applicant's patent application submitted simultaneously with the present application and entitled "Analog-signal to square-wave-signal reshaping device with offset compensation" (U.S. patent application Ser. No. 08/944,073, filed Dec. 19, 1997). With this reshaping circuit, a controllable switch is connected between the comparator output and the comparator input of an offset-inflicted comparator, with said switch being open during normal comparator operation, so that no feedback takes place between comparator output and comparator input, and said switch, for the purpose of offset measurement and offset storage in an offset storage capacitor, being temporarily closed and then effecting a strong feedback between comparator output and comparator input. While in case of the amplifier with the SC feedback, feedback exists via the SC capacitor and in case of the comparator with offset compensation, feedback exists via the feedback switch controlled to the conducting state, there may be instability with respect to an oscillation tendency. This can be eliminated in the manner mentioned hereinbefore by means of the frequency response compensation circuit, in particular in the form of the already mentioned frequency response compensation capacitor, however with the also already mentioned loss in gain-bandwidth product and pulse edge steepness, respectively.