blocking oscillators and time base generators
TRANSCRIPT
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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