Lecture 7

pn Junctions

pn junctions 1-1 Sunday 8/10/2016

Outline Voltage regulator (Zener diodes) Clipping and clamping circuits Voltage Multiplier Circuits

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Voltage Regualtor

Function: to maintain a constant voltage across the load when the source voltage or the load current varies

Zener Diode A special type of diode that is supposed to be reversed

biased

Zener diode works in zener region where the diode start to breakdown at breakdown avalanche voltage (VZ), and the current is avalanche current (IZ)

It limits a voltage to a certain point to pass through the Zener diode

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Example (1)

Determine the Q-point of Zener diode

Solution: Using graphic analysis

KVL: Choose 2 points (0V, -4mA) and (-5V, -3mA) to draw the load line.

It intersects with i-v characteristic at Q-point (-2.9 mA,-5.2 V)

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Zener Diode Analysis The analysis of Zener diodes can be divided into three

categories: Fixed source voltage (Vs) and load resistance (RL)

Fixed Vs, variable RL

Variable Vs, fixed RL

Examples will be given to explain Zener diode analysis pn junctions 1-6

Example (2) Determine VL, VR and IZ

• To check whether VZ is in

the “on” or “off” mode, the

value of VL must be determined first

• To do that, take out the Zener diode from the diode

• By using voltage divider rule :

• VL is smaller than VZ , so the Zener diode is in the “off” mode, Which will result in:

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V 73.8

2.2

2.1*16

L

L

V

k

kV

A 0ZI V 27.773.816VVVV RLRi

Zener Diode Analysis (cont’d) • If R is too large, the Zener diode cannot conduct because

the available amount of current is less than the minimum current rating, IZK. The minimum current is given by:

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ZKRL I II min

• The maximum value of resistance is:

min

max L

ZL

I

VR

• If R is too small, the Zener current exceeds the maximum current rating, IZM . The maximum current for the circuit is given by:

R

VV

R

V

R

V I Zi

L

Z

L

LL

min

max

• The minimum value of resistance is: Zi

ZL

VV

RVR

min

Example (3) Determine the range of RL and IL that will result in VL

being maintained at 10 V • To maintain VL at 10 V, the Zener diode

must be in the “on” mode

• For IZM = 32 mA, the current at

load would be:

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mA 8

mA32k1

1050

III ZMRL

k 25.1mA8

V10

I

VR

L

LmaxL

For IZ(min), the Zener diode are assume “off” but the voltage VZ are maintained at 10 V, The load current would be:

mA 40

1

1050

k

II RL

250mA40

V10

I

VR

L

LminL

Example (4) Determine the range of Vi that will maintain the Zener

diode in the “on” mode

Solution To maintain Zener diode in “on” mode,

VZ must equal to VL:

Taking the maximum current of the Zener diode, input current becomes:

The input voltage will become:

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V 20 LZ VV

mA 67.76k2.1

20mA60III LZMR

V 87.36V

)220)(mA67.76(RI20V

maxi

Rmaxi

Example (4) cont’d

For IZ(min), the Zener diode is

assumed “off” but the voltage VZ

is maintained at 20 V

- Using nodal analysis at node VL:

- Retrieve back all the values of Vi:

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V 67.23V

k2.1

20

220

20V

mini

mini

,max ,min36.87 V, 23.67 Vi iV V

Clippers Clipping circuits (also known as limiters) are used to remove

the part of a signal that is above or below some defined reference level

The motivation of having limiting circuits is to keep the signal below a threshold so it will not saturate the entire circuitry

When a receiver is close to a base station, signals are large and limiting circuits may be required

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Clippers (examples)

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Example Sketch Vo (ideal diode)

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ON is diode the-5Vor 05 for a) iiD vvV

5 io vv

b) for 5 0 or v -5V the diode is OFFD i iV v

0 ov

Example (cont’d)

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Clampers

Clamping circuits (also known as clamped capacitors) shift an input signal by an amount defined by an independent voltage source

Construct of a diode, a resistor and a capacitor

The capacitor and resistor ( ) must be large to ensure that the capacitance doesn’t discharge during the interval that the diode is non-conducting

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RC

Clampers Steps for clampers analysis:

1. Start the analysis with the condition where the diode is in forward bias

2. The capacitor will charge up instantaneously during the interval of +ve or –ve input supply where the diode is in forward-bias condition

3. The capacitor will discharge during the next interval of +ve or –ve input supply where the diode is in reverse-bias condition

4. Check that the total swing of the output is the same with the input

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Example Sketch vo

check whether the capacitor is appropriate for clamper’s configuration

For the input given:

This shows that the capacitor is capable of charging and discharging according to the clamper’s configuration requirement

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ms 10)1.0)(100( kRC

ms 11000

1

f

1T

2

T

0.1 μF

Example (cont’d)

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To start the analysis with the diode in forward-bias mode, the negative input cycle has to be inserted first into the circuit - The circuit: - The output waveform:

The capacitor will charge up to = 20+5 = 25V Cv

Example (cont’d)

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For the next half input cycle that is the +ve cycle: - The circuit: - The output waveform:

The capacitor will discharge the voltage of 25V

Example (cont’d)

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The whole output waveform will become:

The total swing of the output is the same with the input that is 30 V

Voltage Multipliers Using clamping circuits (increase peak of

rectified circuits)

Used in high voltage low current applications such as cathode-ray tubes (CRTs) and particle accelerators

Circuits: Voltage doublers

Voltage triplers

Voltage quadruplers

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Voltage Doubler

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Voltage Doubler

Voltage Tripler & Quadrupler

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Voltage Tripler

Voltage

Quadrupler

Take Home Problem

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Sketch the output voltage observed across RL in the clamping circuit shown in the figure. (Assume 0.7 V drop diode model)

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Lecture Summary

Covered material Voltage regulator (Zener diodes) Clipping and clamping circuits Voltage Multiplier Circuits

Material to be covered next lecture

Introduction to Bipolar Junction Transistor (BJT) BJT principles Configurations and operating regions DC analysis