Wideband DC-coupled amp/schmidt

Abstract

AN ALL MONOLITHIC, dc to > I GI*, amplifier/Schmidt trigger with 8 mV rms input sensitivity has been designed using a junction-isolated 5 GHz fT process in spite of the inherent high capacitances. Included on a 1.39 x 1.59 mm chip are electronic gain control, 180’ phase switching, ECL line driver output, all biasing circuits and a one-shot LED driver. A functional block diagram is shown in Figure 1. The amplifier is composed of a cascade of three identical modified Gilbert gain cells’. The current gain of this type of circuit is set by the ratio of device input impedances which in turn are a function of their quiescent currents. Because the currents are added at the output of each stage, a cascade requires a geometric increase in the power dissipation for each additional stage. N t h even the first stage current constrained to be fairly high to obtain wide bandwidth, a cascade would be unreasonable. Modifying the gain cell by the addition of emitter resistors allows the input impedance and thus the current gain, to become independent of dc bias. Furthermore, this modification generally results in a substantial reduction in the mean squared output noise current (in2) since the expression is changed from: