UNIT III - APPLICATIONS OF OPAMP
1. Mention some areas where PLL is widely used.
Radar synchronization
Satellite communication systems
air borne navigational systems
FM communication systems
Computers.
2. List the basic building blocks of PLL
Phase detector/comparator
Low pass filter
Error amplifier
Voltage controlled oscillator
3. What are the three stages through which PLL operates?
Free running
Capture
Locked/ tracking
4. Define lock-in range of a PLL.
The range of frequencies over which the PLL can maintain lock with the incoming signal is
called the lock-in range or tracking range. It is expressed as a percentage of the VCO free running
frequency.
5. Define capture range of PLL.
The range of frequencies over which the PLL can acquire lock with an input signal is called
the capture range. It is expressed as a percentage of the VCO free running frequency.
6. Define Pull-in time.
The total time taken by the PLL to establish lok is called pull-in time. It depends on the initial
phase and frequency difference between the two signals as well as on the overall loop gain and
loop filter characteristics.
7. For perfect lock, what should be the phase relation between the incoming signal and VCO
output
signal?
The VCO output should be 90 degrees out of phase with respect to the input signal.
8. Give the classification of phase detector:
Analog phase detector
Digital phase detector
9. What is a switch type phase detector?
An electronic switch is opened and closed by signal coming from VCO and the input signal is
chopped at a repetition rate determined by the VCO frequency. This type of phase detector is
called a half wave detector since the phase information for only one half of the input signal is
detected and averaged.
10. What are the problems associated with switch type phase detector?
The output voltage Ve is proportional to the input signal amplitude. This is undesirable
because it makes phase detector gain and loop gain dependent on the input signal amplitude. The
output is proportional to cos making it non linear.
11. What is a voltage controlled oscillator?
Voltage controlled oscillator is a free running multivibrator operating at a set frequency
called the free running frequency. This frequency can be shifted to either side by applying a dc
control voltage and the frequency deviation is proportional to the dc control voltage.
12. On what parameters does the free running frequency of VCO depend on?
External timing resistor, RT
External timing capacitor, CT
The dc control voltage Vc.
13. Give the expression for the VCO free running frequency.
fo = 0.25 / RT CT
14. Define Voltage to Frequency conversion factor.
Voltage to Frequency conversion factor is defined as,
Ky = fo / Vc= 8fo /Vcc
where, Vc is the modulation voltage required to produce the frequency shift fo
15. What is the purpose of having a low pass filter in PLL?
It removes the high frequency components and noise.
Controls the dynamic characteristics of the PLL such as capture range, lock-in range, band-
width and transient response.
The charge on the filter capacitor gives a short- time memory to the PLL.
16. Discuss the effect of having large capture range.
The PLL cannot acquire a signal outside the capture range, but once captured, it will hold on
till the frequency goes beyond the lock-in range. Thus , to increase the ability of lock range, large
capture range is required. But, a large capture range will make the PLL more susceptible to noise
and undesirable signal.
17. Mention some typical applications of PLL:
Frequency multiplication/division
Frequency translation
AM detection
FM demodulation
FSK demodulation.
18. What is a compander IC? Give some examples.
The term companding means compressing and expanding. In a communication system, the
audio signal is compressed in the transmitter and expanded in the receiver. Examples : LM 2704-
LM 2707 ; NE 570/571.
19. What are the merits of companding?
The compression process reduces the dynamic range of the signal before it is transmitted.
Companding preserves the signal to noise ratio of the original signal and avoids non linear
distortion
of the signal when the input amplitude is large.
It also reduces buzz, bias and low level audio tones caused by mild interference.
20. List the applications of OTA
OTA can be used in
programmable gain voltage amplifier
sample and hold circuits
voltage controlled state variable filter
Current controlled relaxation oscillator.
21. List the broad classification of ADCs.
Direct type ADC.
Integrating type ADC.
22. List out the direct type ADCs.
a) Flash (comparator) type converter
b) Counter type converter
c) Tracking or servo converter
d) Successive approximation type converter
23. List out some integrating type converters.
Charge balancing ADC
Dual slope ADC
24. What is integrating type converter?
An ADC converter that perform conversion in an indirect manner by first changing the analog
I/P signal to a linear function of time or frequency and then to a digital code is known as
integrating type A/D converter.
25. Explain in brief the principle of operation of successive Approximation ADC.
The circuit of successive approximation ADC consists of a successive approximation register
(SAR), to find the required value of each bit by trial & error. With the arrival of START
command, SAR sets the MSB bit to 1. The O/P is converted into an analog signal & it is
compared with I/P signal. This O/P is low or High. This process continues until all bits are
checked.
26. What are the main advantages of integrating type ADCs?
The integrating type of ADC’s do not need a sample/Hold circuit at the input.
It is possible to transmit frequency even in noisy environment or in an isolated form.
27. Where are the successive approximation type ADC used?
The Successive approximation ADCs are used in applications such as data loggers &
instrumentation
where conversion speed is important.
28. What is the main drawback of a dual-slop ADC?
The dual slope ADC has long conversion time. This is the main drawback of dual slope ADC.
29. State the advantages of dual slope ADC:
It provides excellent noise rejection of ac signals whose periods are integral multiples
of the integration time T.
