Electronics Principles V11

Clive W. Humphris

BIPOLAR TRANSISTOR: Signal Clipping.  

The 'emitter follower' transistor configuration has no output phase reversal. C1 providing DC isolation from the previous stage.

To ensure the output signal not distorted the transistor has to be correctly DC biased or set-up. Its importance is demonstrated here. R1 and R2 determine the quiescent (no signal) transistor base voltage. The emitter will always be 0.6V less than the base voltage.

To faithfully reproduce (distortion free) the input signal at the output without transistor clipping requires the emitter to be biased to about 5.4V, therefore the base voltage needs to be set to 6V by choosing equal values for R1 and R2.

The incoming AC signal will add to or cancel this standing DC bias condition. When a test meter is applied it will show the average of the excursions, which is the same as the DC voltage. An oscilloscope on the other hand will follow the voltage changes and display the AC waveform. To observe the effect of clipping or saturation, reverse the default values of R1 and R2.

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