Question

Q) Explain the cause of the harmonics in the single stage CE amplifier when doing simulation on the LTSPICE???

Answer 1

When a signal passes through a non-ideal, non-linear device, additional content is added at the harmonics of the original frequencies, a factor of distortion is a measurement of the extent of that distortion.

In a BJT working as a common emitter an amplifier, the primary attention needs to be devoted and centred on the nonlinear characteristics of the input point at the emitter/base junction where the value of the base current plotted against the value of the forward biased emitter/base voltage( a diode) is not so linear. Studying this forward diode relation, it is found that initially, if a little voltage is applied, the current starts to be in a linear relation, but with increasing voltage across the emitter base diode, the base current does not remain linear and in fact the rate of change of base current for a constant rate of emitter /base voltage . For a silicon BJT it seems that the current shoots up as soon as the emitter base voltage reaches about 0.7 volts. This difference in rate of change of base current for a constant rate of change of emitter/ base voltage reflects itself in the Collector current for a variation of current in the base. If one inspects the Ic/ Vc characteristics of a BJT, it is seen that the distance between the collector current characteristics, is not the same for a constant rate of change of signal at the base. For equal changes of signal at the base, it appears clearly that the collector current characteristics are wider when one considers the higher collector currents.

This non uniformity of rate of changes between the base voltage and the collector current means that for a given DC bias at the base, a sinusoidal signal would have its negative peaks rounded off and the positive peaks sharpened up which means that there was a harmonic distortion in the process through the BJT . If one finds a mathematical approximation to the base voltage/ base current relationship one would find that there is a proportional plus squared plus cubed function and this means that any single frequency input signal would show at the output with many other harmonics. In the early laboratory experiments at any school where many students design transistors common emitter amplifiers, they hardly ever notice the sharpening and rounding off of the peaks when a perfect sinusoidal signal is applied at the base. Changing the bias of a BJT one can shift the clipping of from the upper to the lower peaks, but even if the biasing is where it should be, increasing the input signal to make the BJT act as a large signal amplifier, this sharpening and rounding off of the peaks at the output is clearly seen. The harmonic distortion is quite substantial if the bias point is not at the optimum position , but in any BJT even if the bias point is at the optimum position, if the simple BJT common emitter amplifier is used as a large signal amplifier, this harmonic distortion in the form of sharpening and rounding off of peaks in a sinusoidal signal is clearly seen. There are methods of feedback to minimise this harmonic distortion, but that could be discussed elsewhere in feedback application.

It is to be noted that the base/ emitter capacitance varies with the magnitude of the voltage of the input signal and this is another modulating effect. Normally one does not consider this type of harmonic distortion and the sharpening and rounding off of peaks due to the different separation in “horizontal lines” of the Ic/ Vc characteristic of a BJT is always given more attention