The Vertical Amplifier : The vertical amplifier has to amplify the signal it receives from attenuator. Its requirements are critical. The following are its requirements :
1. It must be designed to have sufficient gain to drive the deflecting plates.
2. Its gain must be constant for any frequency of the input signal with in the limits of the frequency range specified for the oscilloscope. In other words its band width must be large.
3. Keeping in view of the required stability and band width, it must be an amplifier with fixed gain. The signal handling ability can be improved to the requirements by proper design of the attenuator.
4. The vertical amplifier should not introduce any phase shift, in the signal it is handling.
5. It should not produce distortion of the signal under study.
6. It must be free from spurious responses.
Vertical amplifier consists of the following stages.
1. Pre-amplifier.
2. Phase inverter.
3. Driver amplifier.
4. Output amplifier.
With all the above stages, its fixed overall sensitivity or gain is expressed in terms of the deflection factor, V/div. Fixed gain amplifier will only be used as the design is easier to offer the required standard of stability.
When the attenuator is kept in the most sensitive position, the gain of the vertical amplifier corresponds to the lowest reading of the VOLTS/DIV scale of the attenuator. The preamplifier and the phase invertor can be of the plug in type, to cater to the multiplex modes in which a CRO can be operated. The driver amplifier and the output amplifier will be provided in the main frame of the cathode ray oscilloscope. Oscilloscopes with different plug in type units designed for specific measurements are available. They extend the scope of the instrument.
The functional block diagram of a vertical amplifier is shown in Figure.
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| Vertical Amplifier Block Diagram |
The preamplifier uses its first element a FET. The attenuator is isolated from the input of the amplifier by the high input resistance of the FET. An impedance transformer action is done by the FET acting as the first element in the source follower form. Its output impedance is moderate. In some cases to match the source follower to the input of the paraphase amplifier (phase invertor) a BJT may be used as a emitter follower. This again acts as an impedance transformer, to couple the output of the source follower to the input of the paraphase amplifier. A paraphase amplifier is required to drive the output stage which mostly is a push pull stage. Such push pull amplifiers require .equal amplitudes of out of phase voltages as inputs to the two base circuits. To accomplish the. required, signal voltage at the input of the output amplifier the output of the paraphase amplifier must be sufficiently amplified. This is done by the driver amplifier which by virtue of its gain provides the required input voltage to the output amplifier.
The push-pull output stage supplies the deflecting voltage to the vertical deflecting plates. The reason for using the push-pull stage is in that it aids the linearity of the deflection of the CR Tube. The vertical amplifier section may use vacuum tubes or transistors. Vacuum tube versions of CROs are out dated now. Excepting the cathode ray tube, now a days the cathode ray oscilloscopes use either transistor version or integrated circuit version. A complete CRO may consist of both transistorised sub-systems as well as ICs. With the introduction of the semi_ conducting devices, the size, weight of the oscilloscope have been reduced considerably. Portable oscilloscopes are now available with smaller screen diameters.
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Cathode Ray Oscilloscope