TYPES OF TIME BASE GENERATORS1)UJT as RELAXATION

OSCILLATOR.2)MILLER SWEEP GENERATOR.3)BOOTSTRAP RAMP

GENERATOR.4)CURRENT TIME BASE

GENERATOR.

UJT as RELAXATION OSCILLATOR

Equivalent circuit.Discharging of capacitor through UJT

generates saw-tooth waveform.When VBB is connected, it will start charging

the capacitor through R1.

The capacitor keeps on charging until the voltage across it becomes equal to 0.7V .

We take output across the capacitor ‘C’.During the charging process, the voltage

across capacitor increases until it reaches its peak value.

At the peak value the UJT is switched ON and it starts conducting.

When the UJT starts conducting , the capacitor starts discharging between emitter and B1.

When the capacitor voltage become zero, the capacitor again starts charging.

The frequency of the o/p saw-tooth waveform can be varied by changing the value of R1 resistor, since this controls the time constant(R1C) of the capacitor charging circuit.

FORMULAt=2.3R1Clog10(1/1-ŋ) t=R1Cloge(1/1-ŋ)VC=VBB[1-e^(-t/R1C)]

MILLER SWEEP GENERATOR

Q1 acts as a switch and Q2 acts as a CE configuration high gain amplifier.

When Q1 is ON, Q2 is OFF.At this instance o/p voltage across

capacitor is equal to VCC.

If a pulse of negative polarity is applied, then Q1 is reverse biased and it is switched OFF. Q2 then is switched ON.

Since Q2 conducts, o/p voltage begins to decrease towards zero.

When the i/p pulse is removed the capacitor again begins to charge.

FORMULA

tS=CRC(β+1) where tS=sweep time(rising

time).

BOOTSTRAP RAMP GENERATOR

The transistor Q1 acts as a switch and Q2 as an unity gain amplifier.

Suppose the transistor Q1 is ON and Q2 is OFF.

The capacitor C1 begins to charge to VCC through the diode forward resistance RE.

At this instant, the output voltage Vo is zero.When negative pulse is applied to the base of

transistor Q1, it turns OFF.

 The output voltage (Vo) is the same as the base voltage of transistor Q2.

Diode is reversed biased. Capacitor ‘C’ starts

charging.Output voltage begins to increase from zero.

Since the value of capacitor C1 is much larger than that of capacitor C, therefore the voltage across capacitor C1 practically remains constant.

 Thus the voltage drop across the resistor R also remains constant because of this, the current iR through the resistor also remains constant.

This causes the voltage across the capacitor C (and hence the output voltage) to increase linearly with time.

CURRENT TIME BASE GENERATOR

There are 3 important blocks-constant current source, capacitor and a switch.

During the sweep interval, the capacitor ‘C’ is charged by the constant current.

When the sweep waveform reaches to its maximum value VS the switch opens and does not allow the current to flow further.

The capacitor ‘C’ starts discharging.The o/p voltage waveform is measured across

the capacitor.

THANK YOU !!