Physics. Valve Electronics.

Transcription

1 Physics Valve Electronics

2 Table of Content 1. Do You Know?. Thermionic Emission and Emitters. 3. Vacuum Tubes and Thermionic Valves. 4. Diode Valve. 5. Triode Valve. 1

3 1. Do You Know? (1) Electronics can be divided in two categories (i) Valve electronics (ii) Semiconductor electronics () ree electron in metal exeriences a barrier on surface due to attractive Colombian force. (3) When kinetic energy of electron becomes greater than barrier otential energy (or binding energy E ) then electron can come out of the surface of metal. b (4) ermi energy (E f) Is the maximum ossible energy ossessed by free electron in metal at 0K temerature (i) In this energy level, robability of finding electron is 50%. (ii) This is a reference level and it is different for different metals. (5) Threshold energy (or work function W 0) Is the minimum energy required to take out an electron from the surface of metal. Also W 0 = E b E f Work function for different materials (W 0) Pure tungsten = 4.5 ev (W 0) Throated tungsten =.6 ev (W 0) xide coated tungsten = 1 ev (6) Electron emission our rocess of electron emission from a metal are (i) Thermionic emission (ii) (ii) Photoelectric emission (iii) (iii) ield emission ( (iv) iv) Secondary emission. W0 Eb Ef Ionised energy level ermi energy level Ground energy level W0 = Eb Ef Tem W0

4 . Thermionic Emission and Emitters. (1) Thermionic emission (i) The henomenon of ejection of electrons from a metal surface by the alication of heat is called thermionic emission and emitted electrons are called thermions and current flowing is called thermion current. (ii) Thermions have different velocities. (iii) This was discovered by Edison (iv) Richardson Dushman equation for current density (i.e. electric current emitted er unit area of metal surface) is given as J AT W0 / kt Where A = emission constant = W 0 = work function. e AT e qv kt AT e 11600V T am/ m -K, k = Boltzmann s constant, T = Absolute tem and (v) The number of thermions emitted er second er unit area (J) deends uon following: (a) T W 0 J (b) J e J J / T W1 W loge (J / T ) (W1<W) T T T 3

5 () Thermionic emitters The electron emitters are of two tyes Directly heated emitter Indirectly heated emitter Plate Plate e e Cathode ilament Cathode ilament (i) Cathode is directly heated by assing current. (ii) Thermionic current is less. (iii) Energy consumtion and life is small. (i) Cathode is indirectly heated. (ii) Thermionic current is more. (iii) Energy consumtion and life is more. Note: A good emitter should have low work function, high melting oint, high working temerature, high electrical and mechanical strength. 3. Vacuum Tubes and Thermionic Valves. (1) Those tubes in which electrons flows in vacuum are called vacuum tubes. () These are also called valves because current flow in them is unidirectional. (3) Vacuum in vacuum tubes revents the emission of secondary electrons. (4) Every vacuum tube necessarily contains two electrodes out of which one is always electron emitter (cathode) and another one is electron collector (anode or late). (5) Deending uon the number of electrodes used the vacuum tubes are named as diode, triode, tetrode, entode. resectively, if the number of electrodes used are, 3, 4, 5.. resectively. 4

6 4. Diode Valve. ip P (Anode) P C ma VP Rh + VPP C e ilament Symbol Inventor: leming Princile: Thermionic emission Number of electrodes: Two Working: When late otential ( V ) is ositive, late current ( i ) flows in the circuit (because some emitted electrons reaches to late). If + V increases i also increases and finally becomes maximum (saturation). Note: If V Negative; No current will flow If V Zero; current flows due to very less number of highly energized electrons (1) Sace charge If V is zero or negative, then electrons collect around the late as a cloud which is called sace charge. sace charge decreases the emission of electrons from the cathode. () Characteristic curve of a diode A grah reresents the variation of i with curve. V at a given filament current ( i f ) is known as characteristic The curve is not linear hence diode valve is known as non-ohmic device. Stoing otential A i (ma) B R C E D T T1 TR or SR SCR VP (volt) 5

7 (i) Sace charge limited region (SCR): In this region current is sace charge limited current. Also 3 / V 3 / kv i i ; where k is a constant deending on metal as well as on the shae and area of the cathode. This is called child s law. (ii) inear region (R): i V (iii) Saturated region or temerature limited region: In this art, the current is indeendent of otential difference alied between the cathode and anode. i f( V ) i f (Temerature) The saturation current follows Richardson Dushman equation i.e. i AT / kt e Note: The small increase in i after saturation stage due to field emission is known as Shottkey effect. (iv) Diode resistance V (a) Static late resistance or dc late resistance: R. i (b) Dynamic or ac late resistance : If at constant filament current, a small change V P in the late otential roduces a small change i in the late current, then the ratio V / i is called the dynamic resistance, or the late resistance of the diode r V. i Note: In SCR r R, In TR r R and r. (3) Uses of diode valve (i) As a rectifier (ii) As a detector (iii) As a transmitter (iv) As a modulator 6

8 (4) Diode valve as a rectifier Rectifier is a device which is used to convert ac into dc S. No. Half wave rectifier ull wave rectifier (i) D1 D1 ~ ~ R utut voltage R D (ii) utut voltage utut voltage Vut + V0 D1 D D1 D t t (iii) (iv) (v) (vi) (vii) (viii) I av I 0 Idc and E av E dc V 0 i Rile factor rms r idc i0 irms Value of eak load current V0 r R dc comonent in outut voltage as comared to inut ac voltage less Efficiency r 1 R I I 0 av and r = 0.48 i rms V 0 i r R More r 1 R V E 0 av 7

9 (ix) orm factor = (x) Rile frequency equal to the frequency of inut ac Double the frequency of inut ac (5) ilter circuit ilter circuits smooth out the fluctuations in amlitude of ac rile of the outut voltage obtained from a rectifier. (i) ilter circuit consists of caacitors or/ and choke coils. (ii) A caacitor offers a high resistance to low frequency ac rile (infinite resistance to dc) and a low resistance to high frequency ac rile. Therefore, it is always used as a shunt to the load. (iii) A choke coil offers high resistance to high frequency ac, and almost zero resistance to dc. It is used in series. (iv) ilter is best for rile control. (v) or voltage regulation choke inut filter (-filter) is best. 5. Triode Valve. ip G P ma R P + K VP VPP G K Symbol P G C Inventor: Dr. ee De orest Princile: Thermionic emission Number of electrodes: Three Grid: Is a third electrode, also known as control grid, which controls the electrons going from cathode to late. It is ket near the cathode with low negative otential. 8

10 Working: Plate of triode valve is always ket at ositive otential w.r.t. cathode. The otential of late is more V than that of grid. The variation of late otential affects the late current as follows i k VG ; where = Amlification factor of triode valve, k = Constant of triode valve. When grid is given ositive otential then late current increases but in this case triode cannot be used for amlifier and therefore grid is normally not given ositive otential. When grid is given negative otential then late current decreases but in this case grid controls late current most effectively. (1) Cut off grid voltage: The valve of V G for which the late current becomes zero is known as the cut off V voltage. or a given V, it is given by VG. 3 / () Characteristic of triode: These are of two tyes Static characteristic Grahical reresentation of V or V g and i without any load Dynamic characteristic Grahical reresentation of V or V g and i with load Note: Both static and dynamic characteristics are again of two tyes-late characteristics and mutual characteristic Static late (or anode) characteristic Grahical reresentation of i and V P at constant V g. i Vg = 0 V 4V Static mutual (or trans) characteristics Grahical reresentation of i P and V G when V P is ket constant i VP=10V VP=100V VP=80V VP +Vg 9

11 oad line (a) It is a straight line joining the oints ( V, 0) on late voltage axis and 0, V / R ) on late current axis of late characteristics of triode. (b) In grah, AB is a load line and the equation of load line is: V i R V or i P 1 R V P V R PP ( i Vg=0 V 4V B (c) The sloe of load line di AB dv 1 R A (VPP, 0) VP (d) In grah, axis. A V intercet of load line on V P axis and B V / R intercet of load line on i (3) Constant of triode valve (i) Plate or dynamic resistance (r P): The sloe of late characteristic curve is equal to it is the ratio of small change in late voltage to the change in late current roduced by it, the grid voltage remaining constant. That is, V r, VG constant. i ip 1 late resistance Vg=0 A V or It is exressed in kilo ohms (K). Tyically, it ranges from about 8 K to 40 K. The r can be determined from late characteristics. It reresents the recirocal of the sloe of the late characteristic curve. If the distance between late and cathode is increased the r increases. The value of r is infinity in the state of cut off bias or saturation state. C VP B VP 10

12 (ii) Mutual conductance (or trans conductance) (g m) (a) It is defined as the ratio of small change in late current i ) to the corresonding small change in grid otential V ) when late otential V is ket constant i.e. g m i V g ( g V is constant (b) The value of gm is equal to the sloe of mutual characteristics of triode. (c) The value of gm deends uon the searation between grid and cathode. ( The smaller is this searation, the larger is the value of gm and vice versa. (d) In the saturation state, the value of i 0, g 0 m B A ig i C VP =100 V i VG (iii) Amlification factor () : It is defined as the ratio of change in late otential V ) to roduce certain change in late current i ) to the change in grid otential V ) for the same change in late ( ( g current ( i ) i.e. V ; negative sign indicates that V and Vg a constant I ( V g are in oosite hase. (a) Amlification factor deends uon the distance between: Plate and cathode ( d k ) Plate and grid ( d g ) Grid and cathode ( d gk ) Also d g d k 1 d gk (b) The value of is greater than one. (c) Amlification factor is unit less and dimensionless. 11

13 tube arameter Note: The triode constants are not indeendent of each other. They are related by the relation. r g m The r and 1 / 3 r i, g m deends on i in the following manner. 1 / 3 gm i gm r Does not deend on i. The variation of triode arameters with i are shown in figure. Above three constant may be determined from any one set of characteristic curves. r g V I P1 P, PA I V I PB I PA PB m, VG1 VG V V P1 G V V P G1 C A i B ia ib VG i (4) Triode as an Amlifiers Amlifier is a device by which the amlitude of variation of ac signal voltage / current/ ower can be increased (i) Princile and circuit diagram: The amlifying action of the triode is based on the fact that small change in grid voltage roduces the same change in the grid voltage as due to a large change in the late voltage. A circuit for triode as an amlifier R utut Inut ac signal Eb + (ii) Working: irst of all the mutual characteristic curves of a triode to be used as an amlifier are lotted and the grid otential noted. Vg b corresonding to the mid-oint of straight ortion of characteristic curve is This negative grid otential is alied on grid and is known as grid bias. The AC signal to be amlified is connected in series with this grid bias Vg ). et the inut signal be reresented as e sin t. ( b e g 0 1

14 The net inut grid voltage Vg b e 0 sin t, varies between Vg b e 0 and Vg b e 0. The corresonding amlified outut current shown in fig. The outut voltage is taken across load resistance R. If e (or Vg ) is the inut signal voltage and V R i R i ) is the consequent voltage change g across load R, then ( IP outut voltage Voltage gain inut voltage or A 1 R / R V V V g V g R R R c a b f d e g h i utut current + The maximum voltage gain is obviously equal to for R. Vgb e0 Inut current Vgb+e0 13