Radio Frequency Integrated Circuit Design - Webs

188 Radio Frequency Integrated Circuit Design
Figure 6.36 Various current mirrors: (a) simple mirror; (b) mirror with improved noise performance;
(c) mirror with improved current matching; and (d) mirror with transistor
doing double duty as current source and driver.
current through Q 1. Scaling the second transistor allows the current to be
multiplied up and used to bias working transistors. One major drawback to
this circuit is that it can inject a lot of noise at the output due primarily to the
high g m of the transistor N � Q 1 (larger than Q 1 by a factor of N ), which acts
like an amplifier for noise. A capacitor can be used to clean up the noise, and
degeneration can be put into the circuit to reduce the gain of the transistor, as
shown in Figure 6.36(b). If Q 1 is going to drive many current stages, then base
current can affect the matching, so an additional transistor can be added to
provide the base current without affecting I bias, as shown in Figure 6.36(c).
Another useful technique for an LNA design is to make the N � Q 1
transistor function both as a mirror transistor and as the LNA driver transistor,
as shown in Figure 6.36(d). In this case, resistors have to be added in the base
to isolate the input from the low impedance of Q 1. Provided that R B is big
compared to the input impedance of the transistor N � Q 1, little noise is injected
here.