Alternative bias control

Article By : John Dunn

Bring a transistor's emitter straight to ground using a simple circuit.

The use of active bias control to bring a transistor’s emitter truly to ground was discussed here. However, there is yet another way to get the emitter straight to ground which uses a simpler circuit, but a circuit which does have a gain issue that might or might not be a problem in your design.

The alternate bias control, shown side-by-side with the active bias control, looks like the right-hand sketch below. Do note that the bias equations as shown are simplified, but usable.

Figure 1 Bias control comparison

A comparison of the small signal properties of these two circuits is shown as follows:

Figure 2 Small signal response comparison

These two arrangements have similar gains and frequency responses in their intended passbands from approximately 100 kHz and up, but the alternative bias arrangement has a low-frequency re-entry property at frequencies below a notch frequency as highlighted in Figure 2.

The low frequency re-entry comes to us courtesy of the relocated R2 which brings about an unintended signal path. The collector of Q2 in the earlier circuit blocks that signal path.

We can suppress the low frequency re-entry to some degree in the new circuit if we increase the value of bypass capacitor C3, but there are practical limits to how large a C3 capacitance value can be afforded.

If you don’t care about the low frequency re-entry trait, the alternative bias circuit might adequately serve your needs. If you do care though, then I think that the original bias control circuit would be your better choice.

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

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