# 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.

• When assumed balanced and I take half of your $50\:\textrm{mA}$ and multiply it by your mirrors' $470\:\Omega$ I get almost $V_{SS}+12\:\textrm{V}$ at the emitters of $Q_6$ and $Q_5$. Was that intended?
– jonk
Commented Nov 4, 2017 at 5:27
• Don't pay attention to the values, once it first didnt work I started changing them randomly just to see what would happen. Commented Nov 4, 2017 at 11:56
• Well, you have wires going "off sheet" and I pretty much don't know what you are trying to achieve. There seems to be a "stream of consciousness" but for some reason I can't convert it into a mental picture that is concrete enough. Could just be me. I do see $Q_1$ and $Q_2$ as the diff-amp pair, $Q_3$ and $Q_4$ as your cascodes, and $Q_5$ and $Q_6$ as the mirror pair. The wire from collectors of $Q_3$ and $Q_6$, heading off to the right, carries the diff current. Fine. But no real specs, just random hacked values. So I've no idea what to suggest.
– jonk
Commented Nov 4, 2017 at 20:31
• How not to design a cascode amp. Values and ratios are critical. THis example has no gain Commented May 30, 2019 at 15:08