Diode and Triode valves

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DIODE AND TRIODE VALVES

▢ Thermionic Emission:

❖ Definition: Emission of electrons from metal surface when heated suitably

❖ Also Called: Edison effect

❖ Discovered By: Edison

❖ Emitted Electrons: Thermions / thermal electrons

Table 1: Thermionic Emission Formulae

Quantity	Formula / Meaning
Richardson-Dushman equation	\(I=A_0ST^2e^{-W_0/kT}\)
Current density	\(J=\frac{I}{S}=A_0T^2e^{-W_0/kT}\)
Temperature relation	\(J\propto T^2\)
Work function relation

❖ Symbols:

•
\(I\) = thermionic current
•
\(S\) = surface area of metal
•
\(T\) = absolute temperature in Kelvin
•
•
\(A_0\) = emission constant
•
\(W_0\) = work function

❖ Electron Emitter Requirements:

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High melting point
•
Low work function
•
Good conductor of heat and electricity

❖ Important Points:

•
Thermionic emission occurs at different temperatures for different materials
•
Thermions may have different kinetic energies and velocities
•
Thermionic process does not depend on melting point alone

▢ Types of Electron Emitters:

Table 1: Directly vs Indirectly Heated Emitter

Feature	Directly heated emitter	Indirectly heated emitter
Structure	Tungsten filament coated with barium oxide acts as cathode	Metal cathode sleeve surrounds filament; sleeve coated with barium oxide
Heating	Current passes directly through cathode filament	Filament heats cathode indirectly
Thermionic current	Less	More
Life	Small	More
Energy consumption	Small	More
Noise	Noise may be produced in thin emitter	Noise not created

▢ Diode Valve:

❖ Definition: Simplest thermionic tube containing two electrodes

❖ Invented By: Sir Fleming

❖ Based On:

•
Thermionic emission
•
Edison effect

Table 1: Diode Valve Parts

Part	Function / Material
Cathode	Emits electrons
Plate / Anode	Hollow metallic structure surrounding cathode; collects electrons
Anode material	Nickel / molybdenum / tungsten
Envelope	Highly evacuated glass tube

❖ Working:

Current from low tension battery heats filament/cathode
Cathode emits electrons by thermionic emission
Number of emitted electrons per second is controlled by filament current
If plate is positive w.r.t. cathode, electrons reach plate and plate current flows
If plate is negative w.r.t. cathode, electrons do not reach plate and plate current becomes zero
If plate potential is zero, plate current is generally zero/very weak

❖ Current Direction:

•
Conventional current in diode battery: plate → cathode
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Electron current: cathode → plate/anode

❖ Uses:

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Rectifier
•
Modulator
•
Detector

▢ Space Charge:

❖ Definition: Negative electron cloud formed near cathode when plate potential is insufficient to attract all emitted electrons

❖ Effect: Space charge repels electrons emitted from cathode

❖ Condition: Occurs at low plate potential

▢ Diode Characteristic Curve:

❖ Definition: Graph between plate current \((I_p)\) and plate potential \((V_p)\) for constant filament current

Table 1: Regions of Diode Characteristic Curve

Region	Name	Main Feature	Law / Formula
OA	Space-charge limited region		\(I_p=KV_p^{3/2}\)
A→B / A→C / A→D	Temperature-limited region		Richardson equation applicable
Saturation region	Plate current becomes constant	\(r_p=\infty\)

❖ Important Points:

•
Diode valve is non-linear / non-ohmic device
•
Before saturation, diode follows Child's three-half power law
•
After saturation, plate current can be increased only by increasing filament current
•
On increasing plate voltage, plate current first increases and then becomes constant

▢ Child's Three-Half Power Law:

❖ Statement: In space-charge limited region, plate current varies as three-half power of plate voltage

❖ Formula: \(I_p\propto V_p^{3/2}\)

❖ Also: \(I_p=KV_p^{3/2}\)

▢ Triode Valve:

❖ Discovered By: Lee De Forest

❖ Definition: Thermionic valve having three electrodes: cathode, grid and plate

Table 1: Triode Parts

Electrode	Symbol	Function
Cathode	\(K\)	Emits electrons
Grid	\(G\)	Controls electron flow from cathode to plate
Plate / Anode	\(P\)	Collects electrons

❖ Grid:

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Third electrode placed between plate and cathode
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Thin conducting cylindrical mesh surrounding cathode
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Placed nearer to cathode than anode
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Usually kept slightly negative w.r.t. cathode
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If grid is positive, electrons may hit grid instead of plate

❖ Plate: Always kept positive w.r.t. cathode

❖ Important Points:

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If grid is at zero potential, triode behaves like diode
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Current in plate circuit is controlled by grid
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Plate current direction is plate → cathode
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Triode can be used as amplifier, oscillator and modulator

▢ Triode Characteristic Curves:

Table 1: Static Characteristics

Type	Graph	Condition
Plate characteristics	\(I_p\) vs \(V_p\)	\(V_g\) constant, no load
Mutual characteristics	\(I_p\) vs \(V_g\)	\(V_p\) constant, no load

Table 2: Dynamic Characteristics

Type	Graph	Condition
Plate characteristics	\(I_p\) vs \(V_p\)	\(V_g\) constant, with load
Mutual characteristics	\(I_p\) vs \(V_g\)	\(V_p\) constant, with load

❖ Important Point: While using triode as amplifier, grid is not made positive because mutual characteristic is not straight

▢ Triode Constants:

Table 1: Amplification Factor, Plate Resistance and Mutual Conductance

Constant	Symbol	Definition
Amplification factor	\(\mu\)	Ratio of change in plate potential to change in grid potential for same change in plate current
Plate resistance	\(r_p\)	Ratio of change in plate potential to change in plate current at constant grid voltage
Mutual conductance	\(g_m\)	Ratio of change in plate current to change in grid voltage at constant plate potential

❖ Relation: \(\mu=r_pg_m\)

❖ Amplification Factor:

•
\(\mu>1\)
•
\(\mu\) is unitless and dimensionless
•
•
At constant plate potential, if grid is moved closer to plate, amplification factor decreases

❖ Plate Resistance:

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Depends on plate voltage
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Depends on grid voltage
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Depends on relative separation and size of plate, grid and cathode
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❖ Mutual Conductance:

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Equal to slope of mutual characteristic curve
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Depends on separation between grid and cathode
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Smaller separation → larger \(g_m\)
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▢ Triode as Amplifier:

❖ Use: Converts small input signal into large output signal

Table 1: Amplifier Points

Fact	Answer
Voltage gain in triode	Depends on plate resistance
Cascaded amplifier gain
Positive grid avoided	Mutual characteristic is not straight
Grid function	Controls plate current

▢ Filter Circuit:

Table 1: Filter Components

Component	Function
Inductance	Smooths current
Capacitor	Smooths voltage

▢ Getter:

❖ Definition: Gas-absorbing material used in triode/valve

❖ Function: Maintains high vacuum by absorbing residual gases

▢ Read and Digest:

Table 1: Important Points

Fact	Answer
Thermionic emission	Emission of electrons from heated metal surface
Thermionic emission discovered by	Thomas Edison
Thermions	Thermal electrons
Richardson-Dushman equation	\(I=A_0ST^2e^{-W_0/kT}\)
Good emitter	High melting point, low work function, good conductor
Diode valve	Two-electrode thermionic tube
Diode invented by	Sir Fleming
Diode use	Rectifier, modulator, detector
Space charge	Electron cloud near cathode
Child's law	\(I_p\propto V_p^{3/2}\)
Diode characteristic	Non-linear / non-ohmic
Temperature-limited region	\(r_p=\infty\)
Triode discovered by	Lee De Forest
Triode electrodes	Cathode, grid, plate
Triode grid	Controls plate current
Triode plate	Positive w.r.t. cathode
If grid voltage is zero	Triode behaves like diode
Plate current direction	Plate to cathode
Amplification factor
Plate resistance
Mutual conductance
Triode constant relation	\(\mu=r_pg_m\)
Cascaded amplifier gain	\(A_v=A_1A_2...A_n\)

▢ Objective Answer Key:

Table 1: Diode and Triode Valves MCQ Answers

Q	Ans
1	a
2	a
3	b
4	b
5	c
6	b
7	b
8	c
9	d
10	c

▢ High-Yield Recall:

Table 1: Diode and Triode One-Liners

Fact	Answer
Thermionic emission	Electron emission from heated metal
Edison effect	Thermionic emission
Thermions	Thermally emitted electrons
Richardson-Dushman	\(I=A_0ST^2e^{-W_0/kT}\)
Current density	\(J=A_0T^2e^{-W_0/kT}\)
Emitter requirement	High melting point, low work function
Diode valve	Cathode + plate
Diode inventor	Sir Fleming
Plate positive	Current flows
Plate negative	No plate current
Space charge	Electron cloud near cathode
Child law	\(I_p\propto V_p^{3/2}\)
Diode nature	Non-ohmic
Diode use	Rectifier
Triode inventor	Lee De Forest
Triode electrodes	Cathode, grid, plate
Grid	Controls plate current
Plate	Always positive w.r.t. cathode
Amplification factor	\(\mu=\frac{\Delta V_p}{\Delta V_g}\)
Plate resistance	\(r_p=\frac{\Delta V_p}{\Delta I_p}\)
Mutual conductance	\(g_m=\frac{\Delta I_p}{\Delta V_g}\)
Triode relation	\(\mu=r_pg_m\)
Triode uses	Amplifier, oscillator, modulator
Getter	Gas absorber
Filter inductor	Smooths current
Filter capacitor	Smooths voltage

Q1.

During full-wave rectification, current in each diode flows until

📅CEE MBBS 2025

Q2.

Minimum number of junction diodes needed to make a full-wave rectifier is:

📅CEE MBBS 2023

Q3.

In common-base mode of a transistor, collector current is 5.488 mA and emitter current is 5.6 mA. The value of current amplification factor \(\beta\) is:

📅CEE MBBS 2023

Q4.

In NPN transistor electron moves from

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Q5.

In NPN transistor the collector current is 18 mA. If: electrons emitted reach the collector, then base curr be

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Q6.

In a transistor, emitter base is forward biased, then the collector base is:

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Q7.

If npn transistor electron move from:

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Q8.

According to Child–Langmuir's law, how does the space-charge-limited plate current in a vacuum diode depend on the plate voltage?

📅BP 2011

Q9.

In a triode with fixed electrode dimensions otherwise, the control grid is moved closer to the plate and farther from the cathode. What happens to the amplification factor?

📅IE

Q10.

What is the approximate dynamic plate resistance of a vacuum diode in its ideal saturation region?

Q11.

Before saturation, a vacuum diode has plate currents \(I_1\) and \(I_2\) at plate voltages \(400\,\mathrm V\) and \(200\,\mathrm V\), respectively. Assuming Child–Langmuir's law applies, find \(I_1/I_2\).

Q12.

A triode has dynamic plate resistance \(r_p=3\times10^3\,\Omega\) and transconductance \(g_m=1.5\times10^{-3}\,\mathrm S\). What is its amplification factor?

Q13.

A triode has transconductance \(g_m=2\times10^{-3}\,\mathrm S\) and amplification factor \(\mu=50\). Its anode is connected through a \(25\,\mathrm{k}\Omega\) load resistor to a \(250\,\mathrm V\) supply. Assuming an ideal bypassed cathode and no additional load, what is the magnitude of its small-signal voltage gain?

Q14.

A triode has amplification factor \(\mu=20\). If the grid potential decreases by \(1\,\mathrm V\), how much must the plate potential increase to keep the plate current unchanged?

Q15.

Which electrode controls the plate current in a triode vacuum valve?

Q16.

A triode's control grid is at \(-2\,\mathrm V\) relative to its cathode. An electron leaves the cathode with kinetic energy \(5\,\mathrm{eV}\). Neglecting collisions, what kinetic energy does it have on reaching the grid?

Q17.

In a triode at constant grid voltage, the anode voltage increases from \(200\,\mathrm V\) to \(240\,\mathrm V\), while the anode current rises from \(5.0\,\mathrm{mA}\) to \(9.6\,\mathrm{mA}\). What is the approximate AC plate resistance?

53Diode and Triode valves

Table 1: Thermionic Emission Formulae

Table 1: Directly vs Indirectly Heated Emitter

Table 1: Diode Valve Parts

Table 1: Regions of Diode Characteristic Curve

Table 1: Triode Parts

Table 1: Static Characteristics

Table 2: Dynamic Characteristics

Table 1: Amplification Factor, Plate Resistance and Mutual Conductance

Table 1: Amplifier Points

Table 1: Filter Components

Table 1: Important Points

Table 1: Diode and Triode Valves MCQ Answers

Table 1: Diode and Triode One-Liners

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