Improving HF properties of limiters with modified bipolar transistors

Very often the collector-to-substrate capacitance of bipolar transistors is the largest parasitic capacitor. This implies that in circuits in which the collector can not be short-circuited, like in resistor loaded differential pairs, the HF-behavior can deteriorate a lot. However, in the case of resistor loaded transistors, the influence of the capacitor can be reduced by placing a shielding implant between the isolation diffusion of the transistor and its active collector region. Several limiter circuits have been built with the special transistors and they have been compared to limiters built with standard transistors. The improvement of the HF-behavior has been verified by measurement. For the shielded version 3 dB roll off is at about 1 GHz, for the standard version it is at about 720 MHz, at equal bias conditions.