Blocking Oscillators and Time Base Generators

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BLOCKING OSCILLATORS AND

TIME BASE GENERATORS

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UNI JUNCTION TRANSISTORS (UJT)•Construction: Originally called “Double-based Diodes.”–“P” Type material doped into the “N” type base

material.–Placement of the Emitter into the Base determines the

voltage level (%) at which the the UJT fires.• This % is called the “Intrinsic Standoff Ratio ( ).”

– Once constructed, the Intrinsic Standoff Ratio cannot be changed.

• The actual voltage value at which the UJT fires is determined by the amount of source voltage applied.

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UJT BLOCK DIAGRAM

Emitter

Base 2

Base 1

P N Emitter

Base 2

Base 1

Equivalent Circuit

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UJT SCHEMATIC SYMBOL

Emitter

Base 2

Base 1

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UJT NO OPERATION

•When VE is less than or equal to the voltage base one to emitter requirement (VE - B1), the UJT will not fire.

Emitter

Base 1

Base 2

P N

++

-

+

No Current Flow

Depletion Region

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UJT OPERATION

Emitter

Base 1

Base 2

P N

++

-

+

UJT Fires

VE > VE-B1

•When VE is more than the voltage base one to emitter requirement (VE - B1), the UJT will fire.

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UJT SAWTOOTH GENERATOR

R1

C1

Q1

E

B1

B2

VBB

SW1VOUT

C1 Discharge

C1 Charge

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Characteristics of UJT

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UJT RELAXATION OSCILLATOR

R1

C1

Q1

VBB

SW1

VOUT1

C1 Discharge

C1 Charge

RB2

RB1

VOUT2

VOUT3

VOUT2

VOUT3

+

+

+

VOUT1

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UJT RELAXATION OSCILLATOR•The output of the Oscillator can be used for sweep generators, gating circuit for SCR’s, as well as timing pulses for counting and timing circuits.

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Advantages of UJT

It is a Low cost device It has excellent characteristics It is a low-power absorbing device under

normal operating conditions

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Pulse Transformer

A pulse transformer is basically a transformer which couples a source of pulses of electrical energy to the load; keeping the shape and other properties of pulses unchanged.

The pulse transformers are designed to handle fast waveforms like train of pulses

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Characteristics of Pulse Transformer

Generally iron cored and small in size The leakage inductance is minimum The interwinding capacitances is low The cores have high permeability They have high magnetizing inductance

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Ideal Pulse Transformer Model

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Practical Equivalent Circuit

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Pulse Response Characteristics

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Applications of Pulse Transformer

Commonly used in pulse generating circuits like blocking oscillators

To change the amplitude and impedance level of a pulse

Used in digital signal transmission