lna bias

I am designing a LNA with SiGe Bipolar.
Since the collect current of the HBT npn is very sensitive to the Voltage VBE, a accurate image current(PTAT) is needed to provide a stable base current, which should be influenced by the variation of Supply Voltage or temperature as little as possible.
Can anyone give me some advice? Is there any document referring the practical schematic of PTAT. Thanks