About the Vertical amplifier and Horizontal deflecting system.

Vertical amplifier and Horizontal deflecting system.

• Vertical Amplifier:

The sensitivity (gain) and frequency bandwidth (B.W) response characteristics of the

oscilloscope are mainly determine by the vertical amplifier .Since the gain-B W. product is

constant, to obtain a greater sensitivity the B.W. is narrowed, or vice-versa.

Some oscilloscopes give two alternatives, switching to a wide bandwidth position, and switching

to a high sensitivity position.

Block Diagram of a Vertical Amplifier

The block diagram of a vertical amplifier is a shown fig 

4.1, 

The vertical amplifier consists of several stages, with fixed overall sensitivity gain expressed in

V/div. The advantage of fixed gain is that the amplifier can be more easily designed to meet the

requirements of stability and B.W. The vertical amplifier is kept within its signal handling

capability by proper selection the input attenuator switch. The first element of the pre-amplifier

is the input stage, often consisting of a FET source follower whose high input impedance isolates

the amplifier from the attenuator.

it  FET input stage is followed by a BJT emitter follower, to match the medium impedance of

FET output with the low impedance input of the phase inverter.

This phase inverter provides two anti phase output signals which are required operate the pushpull

output amplifier. The push-pull output stage delivers equal signal voltages of opposite

polarity to the vertical plates of the CRT. The advantages of push-pull operation in CRO are

similar to those obtained from push-pull operation in other applications; better voltage

cancellation ran the source or power supply (i.e. dc), even harmonic suppression, especially large

2nd harmonic is cancelled out, and greater power output per tube as a suit of even harmonic

cancellation. In addition, a number of defocusing and non linear effects are reduced, because

neither plate is at ground potential.

• HORIZONTAL DEFLETING SYSTEM: 

The horizontal deflecting system consist of a time,

base Generator and an output amplifier.

Sweep or Time Base Generator:

A continuous sweep CRO using a UJT as a time base generator , The UJT is

used to produce the sweep. When the power is first applied, the UJT is off and the CT charges

exponentially through RT. The UJT emitter voltage VE rises towards VBB and when VE reaches

the peak voltage VP, as shown in Fig. 4.3, the emitter to base '1' (B1) diode becomes forward

biased and the UJT triggers ON. This provides a low resistance discharge path and the capacitor

discharges rapidly. The emitter voltage VE reaches the minimum value rapidly and the UJT goes

OFF. The capacitor recharges and the cycle repeats.

To improve sweep linearity, two separate voltage supplies are used, a low voltage supply for

UJT and a high voltage supply for the RTCT circuit.

RT is used for continuous control of frequency within a range and CT is varied or changed in

steps for range changing. They are sometimes called as timing resistor and timing capacitor

respectively.

The sync pulse enables the sweep frequency to be exactly equal to the input signal frequency, so

that the signal is locked on the screen and does not drift.