Abstract: An improved transimpedance amplifier includes a common cathode-connected vacuum tube first stage, coupled to a common-drain-connected MOSFET second stage, and an optional common drain-connected MOSFET third stage. Preferably, capacitive feedback is coupled from the output of the second stage to the input of the vacuum tube stage, to control the transimpedance of the amplifier. The circuit takes advantages of the low stray inter-electrode capacitances, low delay and transit times of the vacuum tube and the high transconductance of the MOSFET second and optional third stages, in a way complementary to one another to form a high transimpedance amplifier whose characteristics are controlled almost exclusively by the feedback capacitance or other feedback impedance, rather than by gain and stray capacitance terms of individual devices. The resulting transimpedance amplifier is useful in a number of applications, for example, in audio preamplifiers and audio power amplifiers.

Abstract: Improved forward transimpedance amplifiers are provided using a field effect transistor first amplification stage, direct coupled to a bipolar junction transistor second amplification stage. In some embodiments an optional third bipolar junction transistor amplification stage is provided, direct coupled from the second stage. Negative feedback signals are connected from the output of the bipolar junction transistor stage to the input of the field effect transistor stage. The invention provides simple and low-cost amplifiers with very high forward transimpedance and other performance characteristics approaching those of an ideal amplifier, permitting the realization of a nearly ideal transimpedance amplifier with performance characteristics dominated by its feedback network.