Time Base Generator (ECE Department @ LAQSHYA)

Time Base Generator

A time base generator is an electronic circuit which generates voltage or current that varies linearly with time. Ideally the output waveform of time base generator should be a ramp. The most popular and important application of such a ramp voltage is in a cathode ray oscilloscope (CRO), for deflecting the electron beam horizontally across the screen we need sweep voltages or ramp voltages. This sweep signal is either voltage or current.

To generate these signals we go for linear time base generators. It generates a voltage or current which varies linearly with time.

There are two kinds of sweep generators, based on its output

 Voltage time base generator.

 Current time base generator. 

        Time base generator also finds application in radar, television, time modulation and precise time measurements  etc.

Time –base wave forms are generated by the following methods

o    Exponent charging

o    Constant current charging

o    Miller circuit

o    Boot strap sweep circuit

Now we will discuss about the principle and working of Bootstrap sweep circuit

Bootstrap sweep generator

Principle: This constant current flowing through the capacitor develops ramp voltage across it.

Operation:The circuit of a transistor bootstrap ramp generator is shown in figure. The ramp is generated across capacitor C1, which is charged via resistance R1. The discharge transistor Q1 holds the capacitor voltage V1 down to V_{CE (sat)} until a negative input pulse is applied. Transistor Q2 is an emitter follower that provides a low output impedance emitter through Re is connected to a negative supply level, rather than to ground. This is to ensure that Q2 remains conducting when its base voltage V1 is close to ground. Capacitor C3, known as the bootstrapping capacitor, has a much higher capacitance than C1. The function of C3, as will be shown, is to maintain a constant voltage across R1 and thus maintain the charging current constant.

The input Vi is a pulse voltage, when the input signal Vi is positive, the transistor Q1 becomes ON i.e. goes into saturation. Potential of point V_A= V_CE(sat)=0.3v

Output voltage VO=V_A-V_BE (Q2) (in active region)

                              = 0.3-0.6= -0.3

The emitter of Q2 is coupled to the collector of Q1 through the capacitor C3.         

Hence point B becomes negative, hence Diode D readily conducts, with the result that potential at V_B =Vcc

    When the input Vi goes negative, Q1 becomes OFF.C1 startscharging via R1.voltage V1 now increases, and the emitter voltage Vo of Q2 also increases. As Vo increases, the lower terminal of C3 is pulled up. Because C3 has a high capacitance, it retains its charge and as Vo increases, the voltage at the upper terminal of C3 also increases .the result is that the potential of B also rises by the same amount.

Thus V_B rises from Vcc to Vcc +V_A =>    V_B=Vcc +V_A

Let I denote the current through R1

i.e   I =(V_B-V_A)/R1 =Vcc/R1

Since V_B=Vcc +V_A

Since both Vcc and R1 are of fixed magnitude, the ratio (Vcc/R1) is constant. Hence current I is of constant magnitude.

Since the collector current of Q1 is (Ic1=0) zero, I=I₁+I_B2

I_B2    is the base current of Q2.since Q2 is an emitter follower, its input impedance is very, very high and hence I_B2 is practically zero therefore I1=I, a constant current As the current flows through the capacitor C1,a ramp voltage develops across it.

For an emitter follows, voltage gain is almost unity. Therefore, the output Voltage Vo is also a ramp voltage. Thus the bootstrap circuit generates a ramp voltage.
...........................................................................................................................................

Article Presented By 

--------------------------

A.Ravishankar
Assistant Professor
ECE Department
LAQSHYA Institute of Technology & Sciences