AC Coupling using an Emitter Follower


Input signals often have a DC component that is not of interest and inconvenient to work with.  The DC component might be unknown or of a value that doesn’t match the circuit the input is driving.  Under standard linear circuit  assumptions, the response to the time-varying component of the input is independent of the DC component, so we can strip away the DC component and replace it with a more convenient value.  A blocking capacitor generally provides the “AC Coupling” of the input signal.

A problem with blocking capacitors arises when I try to select one for my audio circuits.  To the signal source, my circuit appears to have some input impedance, R, and the equivalent circuit becomes a single-pole high-pass filter:


The commonly-defined “cut-off” frequency for such a circuit is f_{c}=\frac{1}{2\pi RC}, and I often try to have a cut-off frequency one decade below my lowest frequency of interest.  To accomplish this I need a very large RC product, but the input impedance of my circuit is generally too low.

Here’s a recent example:

 Simple_Coupler  Simple_Graph


The input impedance of this circuit is R\approx 8k\Omega .  For ideal low-end response (down to 20 Hz or so), I would need a coupling capacitor of at least 10µF.  With a more practical coupling capacitor of 0.1µF as shown, I end up with f_{c} \approx 200 Hz , which may be too high for some applications.


Because my biasing resistors, R1 and R2, largely determine the input impedance, I want  to replace them.  Simply using bigger values in the same proportion will not work (Q1 base current will begin to load the divider significantly).  What if I use the bigger resistors in conjunction with a transistor in emitter follower configuration?

Here’s the result:

 EF_Bias  EF_Graph

Design discussion coming next…