This invention relates to amplifiers for producing an accurate amplified version of a low voltage wave form. Specifically, the amplifier of the invention comprises independently fed first and second resistors connected to a common output which feeds a third resistor via a voltage to current converter, the other end of the third resistor being driven from the same point as the first resistor, the ratio of the resistances of the first and second resistors being made equal to the amplification required so that when the low voltage wave form is fed to the second resistor and a wave form approximating to the required output and of slightly higher voltage is fed to the first and third resistors, the voltage at the common output is proportional to the error in the approximated wave form and the voltage thereby developed across the third resistor by the voltage current converter is approximately equal to the error.

A circuit (10) having multiple poles within an active frequency range employs a movable zero (66) to maintain stability in the circuit (10) under variable load conditions. A pole (62) created by a frequency compensation element (14) maintains a fixed frequency within the active frequency range of the circuit (10). In addition, a variable load impedance (36) coupled to the circuit (10) generates a load pole (64) within the active frequency range of the circuit (10) that changes frequency over time. As the load pole (64) changes frequency, the frequency of the movable zero (66) is adjusted to achieve an enhanced stability condition within the circuit (10). In one embodiment, the frequency of the movable zero (66) tracks the frequency of the load pole (64) as the load impedance (36) changes.

A current controlled variable transconductance amplifier having a differential input stage and a current source output stage is employed with negative feedback, either through a buffer amplifier or directly, to provide various all-pass, low-pass or high-pass filter configurations, each with controlled bias current to the differential input stage for current control of the variable transconductance amplifier to provide dynamic control of phase shift and filter cutoff frequency.

A low distortion filter circuit implementing variable gain amplification (VGA). An aspect of the present invention increases the degrees of freedom (number of components which can be independently programmed/changed to corresponding desired values) to achieve a desired combination of D.C. gain and filter characteristics (e.g., corner frequency, Q-factor, notch frequency, etc.). Such additional degrees of freedom are attained by including additional components in either on input block or a feedback block (implemented with reference to an operational amplifier), and by redesigning the other block using principles such as admittance concellation to remove the effects of such additional components. The blocks are designed such that a terminal of the programmable components is connected to a fixed/constant voltage (e.g., ground). Embodiments implementing bi-quad single amplifier with and without notch are disclosed.

A double integration type filter circuit, consisting of a pair of integrators connected in series with each other, has a changeover switch for commutating the filter characteristics to a filter configuration which emphasizes portions proper to the characteristics of the filter circuit to facilitate filter adjustment to reduce filter adjustment time and to realize improved accuracy while providing for automatic adjustment of the filter characteristics.