TIME BASE GENERATORS Circuits which is used to generate a linear variation of voltage with time are called voltage time base generators. Circuits which produce a current which linearly increases with time are called current time base generators. Since these circuits provide an output waveform which exhibits a linear variation of voltage with time are called linear time base generators. Important application is in CRO which is used to sweep the electron beam horizontally across the screen is called sweep circuits.

General features of a time base signal Time base or sweep waveform The time required for the return to the initial value is called the restoration time or return time or flyback time(Tr). The period during which voltage increases linearly is called sweep time (Ts).

Sawtooth or ramp waveform Usually the restoration time is very less than the sweep time.(Tr<<Ts). when restoration time tends to zero we get the above waveform. Such a waveform is called sawtooth waveform or ramp waveform.

Errors of generation of sweep waveform There are three most commonly used measures of sweep voltage: Sweep speed error Displacement error Transmission error These are called sweep parameters.

Sweep Speed Error (es): Sweep generator to keep sweep speed (rate of change of sweep voltage with time)constant. Any change in sweep speed deviates sweep voltage from maintaining linear slope. The error due to sweep speed is called sweep speed error or slope error. es=Difference in slope at beginning & end of sweep --------------------------------------------------------------- Initial value of slope

Displacement Error(ed): It is defined as the maximum Difference between the actual sweep voltage & linear sweep which passes through the beginning & end points of the actual sweep ed=(Vs-Vs’)max/Vs

Transmission Error(et): When a ramp voltage is transmitted through a high pass rc circuit its output falls away from the input as shown. The transmission error is defined as the difference b/w the input &o/p divided by i/p. et=vs’-vs/vs’

Exponential sweep circuit At t=0,the switch S is opened & the sweep voltage vs is If the switch is closed after time interval Ts,when the sweep value has attained the value Vs we get the sweep waveform.

Expression for sweep speed error(es) Sweep speed error is es=diff in slope at beginning & end of sweep/initial -(1) value of slope

Expression for displacement error(ed):

Expression for transmission error (et):

Relation b/w es,ed,et

Expressions for various sweep parameters

Methods of generating time base generators Exponential charging: A capacitor is charged through a resistor to a voltage which is small in comparison with the supply voltage. Constant current charging : A capacitor is charged with a const. current source. As it is charged with const. current, it is charged linearly. Miller circuit: Integrator is used to convert a step waveform into ramp. Phantastron circuit: It is modified miller circuit which requires only pulse i/p to get the ramp waveform.

Boot strap circuit: A const Boot strap circuit: A const. current is obtained by maintaining nearly const. voltage across a fixed resistor in series with a capacitor. Compensating network: A compensating circuit is added to improve the linearity of the bootstrap & the miller time base generators. Inductor circuit: Linear capacitor charging is achieved by introducing RLC series circuit.

UJT SAWTOOTH GENERATORS Sweep circuit using UJT

Condition for TURN ON & TURN OFF

Miller Voltage Generators Fig (a) is the basic sweep circuit in which switch S opens to form the sweep. In fig (b) we introduce an auxiliary variable generator v & if v is always kept equal to the voltage drop across c, the charging current will be kept constant at i=V/R & perfect linearity can be achieved.

© Point Z is grounded Let us consider the circuit shown in fig(b) with its Z terminal grounded as shown in fig©. With this circuit, linear sweep will appear between terminals Y & ground (Z terminal) & it will increase in the negative direction. Let us now replace the auxiliary variable generator by an amplifier with o/p terminals YZ & i/p terminals XZ, as shown in fig(d).

Basic Miller circuit Since we have assumed that the magnitude of voltage v equals the voltage Vc across the capacitor at every instant of time, then the input vi to the amplifier is zero. We can say that point X behaves as a virtual ground. With this situation if we want to obtain finite o/p, the amplifier gain A should be ideally be infinite.

Such a need of amplifier can be satisfied by using operational amplifier & circuit is recognized as the operational integrator amplifier. It is referred as Miller integrator or Miller sweep. (e) Miller circuit with amplifier equivalent circuit In Fig(e) Ri = input impedance of the amplifier, A =open circuit voltage gain , Ro = o/p resistance. Here, switch is added at the closing of which the time base waveform will start.

Fig(f) Miller circuit with i/p circuit is replaced by a Thevenin’s equivalent The input circuit V,R,Ri are replaced by V’,R’ as follows.

Slope Error for Miller Circuit

Sweep Speed of Miller Circuit Substituting values of I from equation (11) we get Sweep speed= V’/R’C=V/RC Thus the sweep speed for miller circuits is same as in the case where the capacitor charges through R directly from the source V.

Transistorized Miller Sweep Generator

It consists of transistorized switch(Q4),RC timing circuit & Amplifier of 3 stages. First stage emitter follower(Q1) which provides high i/p impedance, second stage (Q2) voltage amplification, last stage (Q3) emitter follower again because Its low o/p impedance allows it to drive a load such as horizontal amplifier(in CRO) the hold-off circuit etc. Its high i/p impedance does not load the collector circuit of second stage(Q2) It provides large overall amplifier gain. The timing capacitor is connected between the base of Q1 & the emitter of Q3.

In the quiescent condition the base of Q4 is held negative In the quiescent condition the base of Q4 is held negative. Because of this current flows through R1 & diode D conducts. As transistor Q4 & diode D both are conducting capacitor C is prevented from charging. The charging of capacitor is started by charging the state of schmitt gate generator. this changed state makes base of transistor Q4 positive, turning it OFF. This results +ve voltage across R1 which causes diode D to become reverse biased. As transistor Q4 & diode D both are off capacitor C is allowed to charge through R. the charging sweep speed will depend on the values of R&C. It also depends on the voltage VBB.

Practical bootstrap sweep generator transistorized bootstrap circuit the sum of V’ & the small initial voltage Vo across Re when S is closed. disadvantage is that neither side of the supply V’ is grounded. this difficulty is solved by replacing V’ with a charged capacitor C1. This capacitor is charged through R1 in fig (b). Voltage across C1 should not change during the sweep time. For this reason value of C1 is kept very large. When voltage across C1 is const. & voltage gain of emitter follower is precisely unity.

practical bootstrap sweep circuit Point A in fig(b) will exactly follow point B. This bootstrap action results vol drop across pt A &B const. hence iR is const.since this const current is used to charge C increases linearly with time