Vertical Amplifier of Cathode Ray Oscilloscope

We feed the signal to be analysed in the the vertical amplifier. This is to amplify the signal before feeding it to the horizontally placed y – deflecting plates. Hence, the vertical amplifier only amplifies the input signal of the CRO.

Vertical Amplifier

This amplifier normally consists of several stages of amplification. At the output stage of the amplifier there is a push pull amplifier. It is necessary for adjusting the amplitude of the amplified signal. Because a signal should not go out the edge of the screen. In other words, when the input signals are quite large the display of its waveforms may become larger then the size of the screen. In that case we need to reduce the overall output signal of the vertical amplifier to bring the display within the screen. And that is the reason the input signal passes to the vertical amplifier through an attenuator.

Attenuator of Vertical Amplifier

An attenuator circuit is the combination of the capacitances and resistances. It serves to reduce the amplitude of the input signal before it passes to the vertical amplifier or vertical preamplifier.

Because of the attenuator and amplifier circuitry the input impedance of a cathode ray oscilloscope becomes high. And this is the reason we can use a cathode ray oscilloscope to measure and analyse voltages in high impedance circuits.

An attenuator adjusts the vertical size of the the displaying signal on a CRO. Which implies that it controls the sensitivity of the display. Generally an attenuator adjusts the sensitivity of the output signal in the steps of 10, 20, 50, 100 millivolt per centimetre.

Coupling Capacitor

There is a switchable coupling capacitor at the input circuit of the CRO. When the CRO analyses the AC signal, the input signal must pass through the coupling capacitor. The coupling capacitor filters out the DC components of the AC signal. But when we analyse a DC signal we must bypass the coupling capacitor for the purpose.

The value of the coupling capacitor should be such that it can preserve down the frequency response of the CRO to a few Hz.

Gain and Sensitivity

How clearly we can observe a very small signal on the screen of a CRO depends on the gain or the amplification factor of the vertical amplifier. In the electrostatic type oscilloscope the vertical deflection of the electron beam depends on the the applied voltage between the horizontal plates. Again the vertical amplifier feeds this voltage between the plates. So, the gain of the amplifier determines how largely the electron beam deflects vertically on the screen. In conclusion we can say the gain of the vertical amplifier determines the sensitivity of the CRO. If the gain of the amplifier is high the sensitivity of the CRO is also high.

Hence by adjusting the gain of the amplifier we can adjust the vertical sensitivity of the CRO. Normally a CRO consists of a rotary switch for adjustment of the gain of the amplifier that is the vertical sensitivity.

Bandwidth

Each amplifier has its own bandwidth operation. That means the range of frequencies within which the gain of the amplifier is 3db. Within this range of frequencies the vertical amplifier amplifies the input signal properly. Therefore we can observe the proper pattern of the input signal on the CRO screen. Obviously, we can refer to this bandwidth as the bandwidth of the oscilloscope itself. So for obtaining desired output we need to select the frequency of the input signal within the bandwidth of the instrument.

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