Electrical – Biasing a cascode in a BJT differential pair


How do I bias the cascode in a differential pair?
For some reason when I set it to a constant bias voltage it either does not work at all or the cascode BJTs turn on for only a fraction of the input cycle. At first I thought it is probably because the collector of the input differential pair is swinging with the input, but then I remembered that one of the advantages of a cascode is that it keeps the collector of the differential pair at a rather constant potential. Following this logic the cascode should turn if their base is around 0.7V higher than their emitter, but when I set Vbias to 0.7V higher than the quiescent emitter voltage it does not work either. How is it usually done? Also, don't pay attention to the 50mA value for the current source, it was just one of the parameters I varied while trying to understand why the cascode BJTs were not behaving as expected.

cascode differential pair

Best Answer

VBIAS (the shared base voltage of the PNP cascade) should be at least 0.7 volts LOWER (more negative) than the most negative base of the diffpair.