Time base generators 1
LINEAR TIME BASE GENERATORS Circuits thatprovide An Output Waveform Which Exhibits Linear Variation Of Voltage or current With Time. Linear variation of Voltage :Voltage time base generators Linear variation of current :Current time base generators. Oscillators By S.M.Mehzabeen 2
Voltage time base generators. The voltage time base circuit finds a major application in CRO. Several methods to achieve Sweep Linearity: Bootstrap circuit Miller circuit Oscillators By S.M.Mehzabeen 3
Miller circuit In Miller,an Operational Integrator is used to convert a step into a Ramp waveform. Miller Integration In this method a constant current is approximated by maintaining nearly constant voltage across a fixed resistor in series with a capacitor. Oscillators By S.M.Mehzabeen 4
Why use Miller integrator? Would the ideal integrator work on a signal with no DC offset? Is there such a thing as a perfect signal in real life? noise will always be present ideal integrator will integrate the noise Oscillators By S.M.Mehzabeen 5
Why use Miller integrator? Therefore, we use the Miller integrator for real circuits. Miller integrators work as integrators at > c where c =1/R f C f Oscillators By S.M.Mehzabeen 6
Application of Ramp Generator Voltage and current linear ramp generator find wide application in instrumentation and communication systems. Linear ramp generators are also known as sweep generators, from basic building blocks of cathode ray oscilloscope and analog to digital converters. Oscillators By S.M.Mehzabeen 7
Application of Ramp Generator Linear current ramp generator are extensively used in television deflection systems. This topic consider the circuits employed in the generation of voltage and current sweeps. Oscillators By S.M.Mehzabeen 8
Ramp Generation Methods Although there are a number of methods of ramp generation, yet the following are important. Exponential Charging In this method a capacitor is charged through a resistor to a voltage which is small in comparison with the supply voltage. Oscillators By S.M.Mehzabeen 9
Ramp Generation Methods Constant Current Charging In this method a capacitor is charged linearly from a constant current source. Oscillators By S.M.Mehzabeen 1 0
Miller Integrator Ramp Generator Oscillators By S.M.Mehzabeen 1 1
Miller Integrator Ramp Generator Q 1 acts as a switch and transistor Q 2 is a common emitter amplifier i.e. a high gain amplifier. Initially, the transistor Q 1 is ON and Q 2 is OFF. At this instant, the voltage across the capacitor and the output voltage is equal to V CC. Oscillators By S.M.Mehzabeen 1 2
Miller Integrator Ramp Generator A pulse of negative polarity as shown in Figure(b) is applied at the base of the transistor Q 1. The emitter-base junction of the transistor Q 1 is reverse biased and it turns OFF. This causes the transistor Q 2 to turn ON. Oscillators By S.M.Mehzabeen 1 3
Miller Integrator Ramp Generator As the transistor Q 2 conducts, the output voltage begins to decrease towards zero. The capacitor C is coupled to the base of transistor Q 2 therefore the rate of decrease of the output voltage is controlled by the rate of discharge of capacitor C. Oscillators By S.M.Mehzabeen 1 4
Miller Integrator Ramp Generator The time constant of the discharge is R B C. Value of time constant is very large, therefore the discharge current remains constant. The rundown of the collector voltage is linear. Oscillators By S.M.Mehzabeen 1 5
Miller Integrator Ramp Generator When the input pulse is removed the transistor Q1 turns ON and Q 2 turns OFF. The transistor Q1 turns OFF, the capacitor C charges quickly through resistor RC to VCC with the time constant equal to RCC. Oscillators By S.M.Mehzabeen 1 6
Miller Integrator Ramp Generator The waveform of the generated ramp or the output voltage is shown in Figure(b). The Miller integrator provide an excellent ramp linearity as compared to the other ramp circuits. Oscillators By S.M.Mehzabeen 1 7
Bootstrap circuit In Bootstrap,a constant current is approximated by maintaining nearly constant Voltage across a fixed Resistor in series with a capacitor. Oscillators By S.M.Mehzabeen 1 8
Bootstrap circuit Oscillators By S.M.Mehzabeen 1 9
Bootstrap The transistor Q1 acts as a switch and Q2 as an emitter follower i.e. a unity gain amplifier. The transistor Q 1 is ON and Q 2 is OFF The capacitor C1 is charged to VCCthrough the diode forward resistance RE. Oscillators By S.M.Mehzabeen 2 0
Bootstrap The output voltage V o is zero. Negative pulse is applied to the base of transistor Q 2 it turns OFF. Oscillators By S.M.Mehzabeen 2 1
Bootstrapping circuit Transistor Q 2 is an emitter follower, the output voltage (V o ) is the same as the base voltage of transistor Q 2. The transistor Q1 turns OFF, the capacitor C1 starts charging this capacitor C through resistor R. The base voltage of Q2 and the output voltage begins to increase from zero. Oscillators By S.M.Mehzabeen 2 2
Bootstrapping circuit The output voltage increases, the diode D becomes reverse biased The output voltage is coupled capacitor C1 to the diode. through the The current ir through the resistor also remains constant The voltage across the capacitor C to increase linearly with time Oscillators By S.M.Mehzabeen 2 3
Sweep speed error Displacement error Transmission error SWEEP PARAMETERS Oscillators By S.M.Mehzabeen 2 4
UJT Relaxation Oscillator Oscillators By S.M.Mehzabeen 2 5
UJT Relaxation Oscillator Generates a voltage waveform V B1, which can be applied as a triggering pulse to an SCR gate to turn on the SCR. When switch S is first closed, applying power to the circuit, capacitor C starts charging exponentially through R to the applied volatage V. Oscillators By S.M.Mehzabeen 2 6
UJT Relaxation Oscillator The voltage across its the volatge V E applied to the emitter of UJT. When C has charged to the peak point voltage V P of the UJT, the UJT is turned on, decreasing greatly the effective resistance R B1 between the emitter and base1. Oscillators By S.M.Mehzabeen 2 7
UJT Relaxation Oscillator A sharp pulse of current I E (limited only be R 1 ) flows from base 1 into the emitter, discharging C When the voltage across C has dropped to approximately 2V, the UJT turns off and the cycle is repeated. Oscillators By S.M.Mehzabeen 2 8
UJT Relaxation Oscillator The waveforms in figure 3 shows the saw-tooth voltage V E, generated by the charging of C and the output pulse V B1 developed across R 1, V B1 is the pulse which will be applied to the gate of an SCR to trigger the SCR. Oscillators By S.M.Mehzabeen 2 9
Pulse Transformer The coefficient of coupling between primary and secondary is K. Its relation with transformer inductances is given by K L M p L s Let transformation ratio be n given as n N N s p
For an ideal transformer various ratios can be obtained as p s p s B p i L L N N i i n V V 0 Equivalent circuit of pulse transformer the resistance reflected to primary side can be written as 2 ' 2 2 n R R R L
Pulse Transformer Response
As per the transformer ratio V 2 nv1 nv nv CC i B nv R nv R CC i n 2 V R CC i c n 2 V R CC i B nv R CC
Equivalent Circuit
Base Potential increases as collector potential increases When loop gain is greater than 1, regenerative action takes place and transistor gets driven in to saturation from its off state For an ideal transformer V V 2 1 n i i B In saturation region i ni B i ni B 0 V V 1 V CC
Monostable Blocking oscillator Using Base Timing
Collector Characteristics
Current Waveforms
Monostable Blocking oscillator using Emitter Timing
Equivalent circuit
Applying KVL V CC nv V 0 V VCC n 1 nv ( I I ) R B C 0 I B I C I E nv R nv CC ( n 1) R Sum of the ampere turns in ideal transformer is always zero n ni B n 1I1 0
Current Waveform
L CC B R n L t R n n V i 2 1 2 1) ( L CC C R n L t R n n V i 2 1 2 2 1) (
Voltage Waveform
Astable blocking oscillator
Equivalent Circuit The discharge equation of a capacitor is V C V i e t RC
V V V BB C V i V 1 f i t t 1 1 1 ln V V V R C t BB f V V V R C t BB f 1 1 1 ln f p t t T L RC t p R n e R n L t p 2 1 1