7.8 DC Bias Networks 227through resistor R1, which is a PTAT current and the current flowing through R2. The voltage across R2 is equal to VBE1; thus, the current Iref is given by: -=+121ref12BEBEBEVVVIRR (7.150)Since Iref is made up of one component that is proportional to temperature and one that is inversely proportional to temperature, by adjusting the size of these two resistors, the temerature dependence of these two current components can be made to cancel out and sum to a value that is constant over temperature. Using this circuit, it is possible to make a current that will be almost perfectly flat over termperature. 7.8.3 Constant GM Biasing for CMOSIn CMOS processes sometimes it is desirable to have a constant gm bias generator. These circuits do not use pn junctions, but rather actual CMOS devices to develop a voltage that is proportional to the difference between two gate source voltages. A circuit that could be used to generate a constant gm bias is shown in Figure 7.43. In this circuit the current generated is given by: -=11ref1gsgsVVIR (7.151)A common choice for N in this case is 4. The NMOS transistors should have similar parameters to the active RF devices that they are being used to bias. Often this will mean that the device is a short channel device and will suffer from extreme process variation which can be problematic. Also note that in practice, a constant gm bias will not necessarily mean that a practical amplifier will have a constant gain Figure 7.43 A constant gm current generating reference circuit (start-up circuit not shown).