Noise reduction in ECL logic gates
IThe equivalent circuit for the noise source by itself is found by zeroing out all of the other voltage sources (principle of superposition).
Consider how RC2 and Q4 are connected. The collector of Q2 can be ignored because its effective impedance is much larger than RC2. The effective resistance of this combination (hib4) is RC2 divided by the current gain (β) of Q4. For example, if the β is 100, the effective resistance is 2.45 Ω.
β is not a tightly controlled parameter, but if we can assume that its minimum value is 50, then the maximum equivalent resistance will be about 4.9 Ω, giving a minimum attenuation of 10:1 at the output terminal.
Note that the diagram is a bit confusing, because they reversed the polarity of Vn2 between the two circuits.