Presentation
On
“
Function Generators
”
By:
Niket Yadav 130020204017
A function generator is a signal source that has the capability of producing different types of waveforms as its output signal.
What is a function generator??
Types of function generator:
1. Analog function generator: This type of function generator was the first type to be developed. First models appeared in the early 1950s when digital technology was not widely used.
2. Digital function generator: As the name indicates, digital function generators utilize digital technology to generate the waveforms.
Comparison of analog and digital function generator
:
• Cost effective: Analog function generators are very cost effective, being at the lower end of the function generator price range whereas Digital function generator are costly.
• Accuracy: Digital function generators are able to offer high levels of accuracy and stability because the clock for the system is crystal controlled.
• Simple to use: Analog function generators provide an effective test instrument that is able to meet most user needs, while remaining simple and easy to use. Whereas digital function generators are complicated.
MULTIVIBRATORS:
A Multivibrator is basically two stage R-C coupled amplifier with positive feedback from the output of one amplifier to the input of the other.
Depending upon the type of coupling network used, these can be classified into the following categories:
•Astable or free-running Multivibrator •Monostable or single-shot Multivibrator •Bistable or flip-flop Multivibrator
Astable or free-running Multivibrator
These are used for generation of square waves or pulses. The Astable Multivibrator is another type of cross-coupled transistor switching circuit that has NO stable output states as it changes from one state to the other all the time.
WORKING:
• Q1 and Q2 are two transistors connected back to back and are not identical.
• So, when we start the supply, either Q1 or Q2 will start conducting. We assume that Q1 starts conducting first, that means it will go to saturation state and Vc (collector potential of Q1) will decrease.
• This low voltage will be passed to the base of Q2 through capacitor C1 and it will go to cutoff state and the collector potential at Q2 will increase. This increased potential will go to base of Q1 and it saturates.
• This cycle will keep on continuing and after sometime Q1 will be completely saturated and Q2 will be in complete cutoff state.
• As we know the output is taken from the collector C of Q1. And at saturation Vc (collector potential of Q1) will be very less (nearly equal to zero). So we get the first state of output waveform as zero (0).
• This increased potential will then be applied to base of Q2 and it goes to saturation state and Vc (collector potential of Q2) will decrease and as we know this decreased potential will be applied to the base of Q1 which will result into increased potential at collector of Q1 and we get a high output as shown below and the cycle continues.
• Now +Vbb starts acting. Here a current I1 will be developed and move across R1 but this current cannot enter Q2 because it is in cutoff state (open circuit). So it will go through C1 and charges it and that charge is stored, making one of its side negative and one positive as in figure.
Monostable Multivibrators have only ONE stable state and produce a single output pulse when it is triggered externally. Monostable Multivibrators only return back to their first original and stable state after a period of time determined by the time constant of the RC coupled circuit.
Monostable Multivibrators
WORKING:
• If we consider zero as stable state then it will continuously give zero but when we trigger it will come to 1 then again goes to zero after a short duration and it will come to 1 only when we trigger again.
• So, when we switch “ON” Vcc supply and give –Vbb then due to this –Vbb the transistor Q1 will go to cutoff state (low voltage, logic 0) and hence, potential across collector of Q1 (Vc) will increase and this voltage will be applied to Q2 passing through C1.
• Due to this high voltage, Q2 goes to saturation state, its collector potential Vc, decreases.
• After sometime, Q1 will be completely in cutoff state and Q2 completely saturates. Therefore, that means output of Q2 will be continuously zero.
• Now we will give input pulse (of short duration and high magnitude).
• This high magnitude pulse will pass through C2 and drive Q1 to saturation state which was earlier in cutoff state and this decreases its collector potential Vc.
• This low potential is applied to base of Q2. Due to which Q2 goes into cutoff region and its Vc increases and we get high output (1). As this pulse is of high magnitude but for only a short duration, the output will come back to 0 and after that –Vbb comes into act again and this drives Q1 to cutoff region and Q2 to saturation region.
Bistable Multivibrator
The bistable multivibrator can be switched over from one stable state to the other by the application of an external trigger pulse thus; it requires two external trigger pulses before it returns back to its original state.
WORKING:
• The Bistable Multivibrator circuit above is stable in both states, either with one transistor “OFF” and the other “ON” or with the first transistor “ON” and the second “OFF”. Let us suppose that the switch is in the left position, position “A”.
• The base of transistor TR1 will be grounded and in its cut-off region producing an output at Q. That would mean that transistor TR2 is “ON” as its base is connected to Vcc through the series combination of resistors R1 and R2. As transistor TR2 is “ON” there will be zero output at Q, the opposite or inverse of Q.
• If the switch is now move to the right, position “B”, transistor TR2 will switch “OFF” and transistor TR1will switch “ON” through the combination of resistors R3 and R4 resulting in an output at Q and zero output at Q the reverse of above.
• Then we can say that one stable state exists when transistor TR1 is “ON” and TR2 is “OFF”, switch position “A”, and another stable state exists when transistor TR1is “OFF” and TR2 is “ON”, switch position “B”.
• The bistable multivibrators output is dependent upon the application of two individual trigger pulses, switch position “A” or position “B”.
Operational amplifier:
An operational amplifier or OP-AMP is a DC-coupled voltage amplifier with a very high voltage gain. Op-amp is basically a multistage amplifier in which a number of amplifier stages are interconnected to each other in a very complicated manner. Its internal circuit consists of many transistors, FETs and resistors.
1. The two states of the circuit between which it switches are those in which the amplifier output is at positive and negative saturation.
2. R1 an R2 provide a fixed level of positive feedback and R and C provide a frequency dependent level of negative feedback.
3. At high frequencies the negative feedback is reduced and the circuit becomes unstable. The circuit cannot hang up in either output stage and is self-starting.
4. As the terminal B is positive with respect to terminal A and its potential is decreasing as C charges down through R.
5. When the potential difference between the two input terminals approached zero the amplifier comes out of saturation.
6. The positive feedback from the output to terminal A causes a regenerative switching which drives the amplifier to positive saturation.
7. The voltage across a capacitor in series with a resistor cannot change instantaneously, the potential at the terminal B remain substantially constant during this rapid transition.
8. Capacitor C now charges up through R and the potential at C rises exponentially, when it reaches (sat) the circuit switches back to the state in which the amplifier is in negative saturation.
