Radio Frequency Integrated Circuit Design - Webs

LNA Design
With any of these mirrors, a voltage at the collector of N � Q 1 must be
maintained above a minimum level or else the transistor will go into saturation.
Saturation will lead to bad matching and nonlinearity.
6.7.1 Temperature Effects
For transistor current given by
189
VBE
iC ≈ IS e vT (6.103)
the temperature affects parameters such as IS and V BE. Also, current gain � is
affected by temperature. IS doubles for every 10°C rise in temperature, while
the relationship for V BE and � with temperature is shown in (6.104) and
(6.105).
�V BE
�T | ≈−2 mV/°C (6.104)
iC = constant
��
�T ≈+0.5%/°C (6.105)
A typical temperature range for an integrated circuit might be 0 to 85°C.
Thus, for a constant voltage bias, if the current is 1 mA at 20°C, then it will
change to 0.2 mA at 0°C and to 1.65 mA at 85°C. Thus, the current changes
by more than eight times over this temperature range. This illustrates why
constant current biasing (for example, with the current mirrors discussed in
Section 6.7) is used. If both transistors in the current mirror are at the same
temperature, then output current is roughly independent of temperature.
6.8 Broadband LNA Design Example
As a final major design example, we will design a broadband LNA to work
from 50 to 900 MHz and with input matched to 75� with an S11 better than
−10 dB over this range. The gain must be more than 12 dB, noise figure less
than 5 dB, and the IIP3 must be greater than 6 dBm. The circuit must operate
with a 3.3V supply and consume no more than 8 mA of current. Assume that
there is a suitable 50-GHz process available for this design. Assume that the
LNA will drive an on-chip mixer with an input impedance of 5 k�.
This is going to be a high-linearity part and it needs to be broadband.
Therefore, the matching and the load cannot make use of reactive components.