A charge amplifier circuit comprising an operational amplifier unit whose signal output is back to a signal input of the amplifier unit via a capacitative feedback element, the signal output of the operational amplifier unit also being coupled to an input of a following, further transmission unit that has a proportional, integrating transfer characteristic with a break-over frequency that is at least approximately equal to the break-over frequency of the operational amplifier unit, this being at least in the proximity of the break-over frequency defined by the feedback element of the operational amplifier unit. The feedback element of the operational amplifier unit can be optimized independently of the operating frequency range, this, allowing for a low resistance value for the feedback element given a transfer behavior that remains linear, this despite the low capacitor value that increases the charge sensitivity.

An infra-red receiver front-end includes a trans-impedance amplifier connected to receive an input current and provided with frequency selective transadmittance feedback between its output and input, the transadmittance feedback having a DC path to ground for low frequency noise components in the input current, caused by background illumination.

A limiter amplifier includes amplifiers connected in cascade each having differential outputs and differential inputs, and a lowpass filter connected between adjacent amplifiers. The filter includes resistors having different values connected between the differential outputs and the differential inputs of adjacent amplifiers, and a capacitor connected between the differential inputs of one of the adjacent amplifiers, the filter thereby limiting a high frequency characteristic of the limiter amplifier.

A circuit useful as a voltage tunable resistive element comprises four matched MOS transistors in which each of the first and second have their drains interconnected to a first input terminal and each of the third and fourth have their drains interconnected to a second input terminal, the first and third have their sources interconnected to a first output terminal and the second and fourth have their sources interconnected to a second output terminal, and the first and fourth have their gates interconnected to a first control terminal and the second and third have their gates interconnected to a second control terminal. A continuous-time integrator is described which includes such a resistive element.

An amplifier is provided for audio or instrumentation applications which exhibits radically improved performance over existing amplifiers of comparable cost. In a preferred embodiment, the amplifier comprises two operational amplifiers (A.sub.1, A.sub.2) connected in cascade, where feedback elements (Cf.sub.2, Rf.sub.2) are connected across the second op amp (A.sub.2) to cause an intermediate voltage signal (Z) to be substantially advanced in phase. In this embodiment, a capacitor (Cf.sub.1) is also connected across the first op amp (A.sub.1), and an outer DC loop is provided with damping circuitry (Cf.sub.3) to reduce the closed loop gain at high frequencies, stabilizing the combination. The composite amplifier has performance far exceeding a conventional stand-along op amp and is very cheap to construct.