A transistor amplifier with nonlinear tunnel-diode or backward-diode feedback is described for use as a resistor-coupled threshold logic circuit or general purpose switch. The tunnel-diode feedback serves to virtually eliminate saturation and prevent cutoff thus allowing the high gain-bandwidth products of epitaxial switching transistors to be utilized at all times during switching. Switching plus delay times as low as 2 nsec have been observed with a fan in and fan out of three. Ring oscillator experiments show that delays per stage from 2 to 5 nsec are easily obtained with logical gains of two or three. The input impedance of the network is 3 to 12 ohms leading to a very practical threshold network. The particular logic decision is determined only by the bias current. Tolerance calculations are presented that indicate component requirements will not be severe. The peak current is not a critical parameter and can easily vary 100 per cent. Due to the quantizing effect of the tunnel-diode feedback, circuits with fan ins and fan outs of three would operate satisfactorily under worst case conditions with 2 per cent resistors and mild restrictions on the transistor and diode parameters. By adding backward diodes in series with the input resistors the tolerance requirements can be relaxed to 10 per cent resistors and slight restrictions on the diodes or transistor parameters. By using backward diodes the fan in and fan out can be substantially increased with little loss in speed.