Before hooking up the feedback connections, let's perform an open
loop analysis of the circuit, which is shown without feedback in Fig. 5.6,
starting with the DC bias
conditions.
Figure 5.6:
Open Loop Configuration of Op-Amp-Like Circuit
The DC analysis is almost the same as before, except now
we have to determine the current going through transistor Q4,
which gets its bias from VCC through RC2 and VC2.
Recall from Section 2, that we have established
the DC voltages at the collectors of Q1 and Q2, by determining
the current provided by the current source Q3, and the determining
the drops across RC1 and RC2, where
VC2 = VCC-IC2RC2
(119)
Now that we have added Q4, it would appear as though the DC voltage
at VC2 would have to be changed to accommodate the current
IB4. This would give
VC2 = VB4=VCC-(IC2+IB4)RC2
(120)
However, since is usually greater than 100, the effective
resistance to ground seen by IB4 at VC2 is .
Since in most
cases is more than 20 times greater than RC2,
IB4 is negligible compared with IC2, so we can say that
even with Q4, VC2 is still given by