ELECTRONIC DEVICES AND CIRCUITS

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. + d - 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.

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

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

12. volts Fly back spot on right side of screen spot at centre of screen 0 time spot on left side of screen Time taken for spot to move across the screen and back Time Base • controls the speed at which the spot sweeps across the screen horizontally from left to right.

13. volts Fly back spot on right side of screen spot at centre of screen 0 time spot on left side of screen 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. volts Fly back spot on right side of screen Time taken for spot to move across the screen and back spot at centre of screen time spot on left side of screen Time Base: How It Works B 0 A C

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. y Y-input Measuring d.c. Potential Difference If the Y-gain control is set at 2 volts/division 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. Vpp Y-input Measuring a.c. voltage

22. Vp Vpp Measuring a.c. voltage 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. Y-input Displaying Waveforms

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

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

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

30. Y-input Measuring Short Time Intervals

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