Bipolar transistors used in a common emitter configuration to make a high frequency amplifier are sometimes defined for their small-signal properties under the assumption that the gain transistor's collector-to-emitter voltage (Vce) and collector current (Ic) are of particular values. You are expected to design your circuit to yield both of those conditions simultaneously and with fairly good precision but that can be rather a challenge using a conventional bias circuit.


Figure 1
Conventional bias circuit

In a conventional circuit, to put the transistor's Vce and Ic values reliably where we want them to be, we need a resistance in series with the emitter going to ground and then for signal gain reasons, we may need to put a bypass capacitor across that resistor as well.

With or without that bypass capacitor, the emitter may not be really at signal ground because those two components exhibit impedances. There can be self-resonance(s) at some frequency or frequencies which may lie within our intended range of operating frequencies. RF transistors, which are characterized for their small signal performance with the emitter truly at ground, may exhibit quite different small signal properties depending on the behavior of the emitter's "mischief maker" as seen above.

One remedy for this problem is to use active bias control as in the following sketch.


Figure 2
Active bias circuit

With active bias, the Vce of the gain transistor equals the rail voltage minus one base-to-emitter voltage (Vbe) and the Ic of the gain transistor is very nearly that same Vbe divided by the ohms of a shunting resistance, R1, as in the above sketch.

The emitter of the gain transistor can now be right smack dab at ground as called for in the small signal parameter definitions while the bias conditions are predictable and stable.

A SPICE simulation of an active bias arrangement is shown below.


Figure 3
Spice simulation of active bias

Yeah, the PN2222 isn't really a super high frequency transistor, but its model is available in the simulator I have, and it illustrates the point. If your SPICE simulator has a model for your really speedy RF transistor, it should work very nicely as well.


Figure 4
Grounded emitter with active control of bias

John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).

Related articles: