In this tutorial, we provide several examples of signal amplification and analyze the amplifier response in time and frequency domains.

Example 1: In this example, a 200 Hz sinusoid with an amplitude of 1 V has been applied to the amplifier input port. The signal is displayed on channel 1 of the oscilloscope. The amplifier is powered by a single 10 V supply and the voltage gain is set to 5. The amplitude of the output signal can be found by multiplying the input signal amplitude with the voltage gain: Vamp(out) = Av x Vamp(in) = 5 x 1 = 5 V.

The peak-to-peak value of the output signal is equal to twice its amplitude or 10 V. Since this value is equal to the power supply voltage, the output port can support the signal without clipping. Indeed, channel 2 displays an undistorted sinusoid with an amplitude of 5 V.

The transfer characteristic shows that when Vin = 0, the output voltage is equal to 5 V, which means that the output signal has to be equal to 5 V at every zero crossing of the input signal. This implies that the output signal will be raised to a DC offset of 5V. Since DC coupling is used for channel 2, the DC offset can be measured off the oscilloscope display. Alternatively, the DC value of the output signal can be measured using the channel 2 multimeter.

The spectrum analyzer is set to display the amplitude spectrum of the output signal and it displays a single peak of 5 V at the frequency of the signal.

Example 2: In this example, we increase the amplitude of the input sinusoid from 1 V to 1.5 V while we keep everything else the same. Since the amplifier has a voltage gain of 5, the amplitude of the output signal has to be Vamp(out) = Vamp(in) x Av = 1.5V x 5 = 7.5 corresponding to a peak-to-peak value of 15 V. Since this voltage range is greater than the power supply range of 10 V, the output signal is clipped, which can be seen on channel 2.

More interestingly, since the output signal is no longer a pure sinusoid, it must have harmonics at multiples of the fundamental frequency, which has to be the main frequency of the sinusoid or 200 Hz. Indeed the spectrum analyzer shows a series of harmonics occuring at odd multiples of 200 Hz, i.e., 600, 1000, 1400 etc. Hz. Note that in order to be able to see all these harmonics, the power spectrum is displayed instead of the amplitude spectrum. We also note the peak at 0 Hz corresponding to the DC valur of the signal. Since the clipping is symmetric, the DC value of the output signal is still 5 V.

Example 3: In this example, a 400 Hz random periodic signal with a peak-to-peak value of 2.4 V has been applied to the amplifier. A dual power supply of +/-10V is used and the voltage gain is set to - 5 V.

Looking at the oscilloscope screen, we can see that the output signal is a mirror image of the input signal. This is normal because the voltage gain is negative. The two signals appear to have the same peak-to-peak value, however, note that the vertical scales of the two channels are not equal. Indeed, we can measure the peak-to-peak value of the output signal as 12 V, which can be found from Vpp(out) = Vpp(in) x Av = 2.4 x 5 = 12. Since a dual power supply has been used, the output signal does not have a DC offset.

The output spectrum consists of the fundamental at 400 Hz and harmonics at multiples of 400 Hz. which can be seen on the output spectrum as well. Since the DC value of the signal is zero, the output spectrum does not have a peak at 0 Hz.