We consider known, modified, and new transistor circuits for transconductors and some theory on their application in integrated tunable Gm-C or transconductor-eapacitor filters for high frequencies. The circuits are balanced and mostly complementary. They use bipolar (NPN and VPNP) and/or CMOS transistors. The low delay and low input capacitance of the newer circuits makes them particularly suitable for very high frequencies, in the hundreds of megahertz. The relatively high power efficiency and low noise lead to a low power consumption in combination with a high signal-to-noise ratio. Their simplicity results in a small chip area and hence in low cost. The partly new theory is on noise, sensitivities, quality factors, linearization, power and power efficiencies.
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