1. ELECTRONIC DEVICES AND CIRCUITS
Faculty:
Mr. M SRINIVAS REDDY

2. UNIT-I
ELECTRON DYNAMICS AND CRO: Motion of charged particles in electric and magnetic fields. Simple problems involving electric and magnetic fields only. Electrostatic and magnetic focusing. Principles of CRT, deflection sensitivity (Electrostatic and magnetic deflection), parallel and perpendicular electric and magnetic fields

3. Deflection of Electrons in a Uniform Electric Field
Consider an electron beam directed between two oppositely charged parallel plates as shown below.
With a constant potential difference between the two deflecting plates, the trace is curved towards the positive plate. + - d

4. Deflection of Electrons in a Uniform Electric Field
The force acting on each electron in the field is given by
where E = electric field strength,
V = p.d. between plates,
d = plate spacing. p

5. Deflection of Electrons in a Uniform Electric Field
The vertical displacement y is given by
This is the equation for a parabola.

6. Deflection of Electrons in a Uniform Magnetic Field
The force F acting on an electron in a uniform magnetic field is given by
Since the magnetic force F is at right angles to
the velocity direction, the electron moves round
a circular path.

7. Deflection of Electrons in a Uniform Magnetic Field
The centripetal acceleration of the electrons is
Hence
which gives

8. Cathode Ray Oscilloscope (CRO)
The structure of the cathode ray tube

9. Cathode Ray Oscilloscope Controls
Y-Gain
Time Base

10. Y-Gain amplifies the Y-deflection
small input voltages are amplified by built-in amplifiers before applying to the Y-plates.
Y- Gain = 0.5 V/div
0.5 volt will cause a vertical deflection of 1 division

11. Time Base
is a saw-tooth voltage applied internally across the X-plates.
volts
time

12. Time Base
controls the speed at which the spot sweeps across the screen horizontally from left to right.
volts
time
Fly back
spot on right side
of screen
spot at centre
of screen
spot on left side
of screen
Time taken
for spot to
move across
the screen and
back

13. volts Screen Fly back time spot on right side of screen spot at centre
Screen
Fly back
spot on right side
of screen
spot at centre
of screen
spot on left side
of screen

14. Time Base
it helps to display the actual waveform of any a.c. applied across the Y-plates
normally calibrated in
s/cm
ms/cm
s/cm
gives the time required for the spot to sweep 1 cm horizontally across the screen.

15. Time Base: How It Works B A C volts Fly back time spot on right side
of screen B
spot at centre
of screen
time
spot on left side
of screen A C
Time taken for spot to move across the screen and back

16. Uses of c.r.o. Measure potential difference
d.c.
a.c.
Display waveforms of alternating p.d.
Measure short intervals of time, and
Compare frequencies

17. Measuring d.c. Potential Difference
switch off the time-base
a spot will be seen on the c.r.o. screen
d.c. to be measured is applied to the Y-plates
spot will either deflected upwards or downwards
deflection of the spot is proportional to the d.c. voltage applied

18. Measuring d.c. Potential Difference
Y-input
If the Y-gain control is set at 2 volts/division y
And the vertical deflection, y, is 1.5
Then d.c. voltage
= 1.5 x 2
= 3.0 V

19. Measuring a.c. voltage switch off the time-base
a spot will be seen on the c.r.o. screen
a.c. to be measured is applied to the Y-plates
spot will move up and down along the vertical axis at the same frequency as the alternating voltage
spot moves to the top when the voltage increases to its maximum (positive)
spot moves to the bottom when the voltage decreases to its lowest (negative)

20. Measuring a.c. voltage When the frequency is high
the spot will move so fast that a vertical line is seen on the screen
Length of the vertical line gives the peak-to-peak voltage (Vpp) applied to the Y-plate
The peak voltage (Vp) is
= Vpp/2

21. Measuring a.c. voltage
Y-input
Vpp

22. Measuring a.c. voltage
Vpp Vp
Vp = Vpp/2

23. C.R.O. as a Voltmeter
it has nearly infinite resistance (between the X- and Y-plates), therefore draws very little current;
it can be used to measure both d.c. and a.c. voltages; and
it has an immediate response.

24. Displaying Waveforms Set the time-base to a suitable frequency,
Apply the input to the Y-plate
a steady waveform of the input will be displayed on the c.r.o.

25. Displaying Waveforms
Y-input

26. Displaying Waveforms
When input voltage frequency is the same as the time-base frequency
c.r.o. screen
Input Voltage

27. Displaying Waveforms
When input voltage frequency is the twice the time-base frequency
c.r.o. screen
Input Voltage

28. Measuring Short Time Intervals
Set time-base to its lowest frequency range
Connect microphone to the Y-input
Blow two short whistles into the microphone
two short pulses, at short interval apart will be displayed on the c.r.o. screen

29. Measuring Short Time Intervals
c.r.o. screen
If the time-base is 10 ms/division
t divisions
and
if the separation between pulses
is t divisions
then
time interval is 10t ms

30. Measuring Short Time Intervals
Y-input

31. Lissajous’ Figures
Lissajous’ figure can be displayed by applying two a.c. signals simultaneously to the X-plates and Y-plates of an oscilloscope.
As the frequency, amplitude and phase difference are altered, different patterns are seen on the screen of the CRO.

32. Lissajous’ Figures
Same amplitude but different frequencies