Intermodulation Noise in Translinear Filters

We examine the noise properties of translinear filters operated in class A and class AB, discussing how the output noise power depends on the circuit topology. We use the simplest first order low pass filter built with four BJTs. We show that for in band useful signal the output noise does not depend on the relative position of the transistors in the filter. For class A operation we show that, with a proper choice of the circuit topology, an out of band interfering signal does not add any noise in the average to the output. For class B operation the output noise is due to intermodulation between internal noise sources and signal; the noise power grows with the signal power. For class B operation and out of band interfering signal we calculate that, depending on the circuit topology, the output noise power is reduced by 1.5 or 3.9 dB as compared with the output noise due to an in band signal of the same power.