Application Note: A Comparison Of Various Bipolar Transistor Biasing Circuits

The bipolar junction transistor (BJT) is quite often used as a low noise amplifier in cellular, PCS, and pager applications due to its low cost. With a minimal number of external matching networks, the BJT can quite often produce an LNA with RF performance considerably better than an MMIC. Of equal importance is the DC performance. Although the device's RF performance may be quite closely controlled, the variation in device dc parameters can be quite significant due to normal process variations. It is not unusual to find a 2 or 3 to 1 ratio in device hFE. Variation in hFE from device to device will generally not show up as a difference in RF performance. In other words, two devices with widely different hFE's can have similar RF performance as long as the devices are biased at the same VCE and IC. This is the primary purpose of the bias network, i.e., to keep VCE and IC constant as the dc parameters vary from device to device.

One solution to the biasing dilemma is the use of active biasing. Active biasing often makes use of an IC or even just a PNP transistor and a variety of resistors, which effectively sets VCE and IC regardless of variations in device hFE. Although the technique of active biasing would be the best choice for the control of device to device variability and over temperature variations, the cost associated with such an arrangement is usually prohibitive.