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Biomedical Instrumentation Multiple Choice Questions on “Preamplifiers”.
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1. Which amplifier will reject any common mode signal that appears simultaneously at both amplifier input terminal and amplifies only the voltage difference that appears across its input terminals?
a) ac coupled amplifiers
b) differential amplifiers
c) carrier amplifiers
d) dc amplifiers
Answer: b
Clarification: Differential amplifier is one which will reject any common mode signal that appears simultaneously at both amplifier input terminals and amplifies only the voltage difference that appears across its input terminals. Most of the amplifiers used for measuring bioelectric signals are of the differential type. AC amplifiers have a limited frequency response and are, therefore, used only for special medical applications such as electrocardiograph machine.
2. Which amplifier has a limited frequency response?
a) differential amplifier
b) dc amplifiers
c) ac coupled amplifiers
d) carrier amplifiers
Answer: b
Clarification: AC amplifiers have a limited frequency response and are, therefore, used only for special medical applications such as electrocardiograph machine. For electrocardiograms, an ac amplifier with sensitivity, giving 0.5 mV/cm, and a frequency response up to 1 kHz and an input impedance of 2 to 5 MW is used. For such applications as retinography, EEG and EMG, more sensitive ac amplifiers are required, giving a chart sensitivity of say 50 mV/cm with a high input impedance of over 10 MW.
3. ______________ are used with transducers which require an external source of excitation.
a) carrier amplifiers
b) dc amplifiers
c) ac coupled amplifiers
d) differential amplifier
Answer: a
Clarification: Carrier amplifiers are used with transducers which require an external source of excitation. They are characterized by high gain, negligible drift, extremely low noise and the ability to operate with resistive, inductive or capacitive type transducers. They essentially contain a carrier oscillator, a bridge balance and calibration circuit, a high gain ac amplifier, a phase-sensitive detector and a dc output amplifier.
4. DC amplifiers are employed with _______ feedback type.
a) positive
b) negative
c) depends on the application
d) can be any positive or negative does’t matter
Answer: b
Clarification: DC amplifiers are generally of the negative feedback type. They are used for medium gain applications down to about 1 mV signal levels for full scale. They are not practical for very low level applications because of dc drift and poor common-mode rejection capabilities.
5. DC amplifiers are mostly used for very low level applications because they offer very less dc drift and high common mode rejection capabilities.
a) True
b) False
Answer: b
Clarification: DC amplifiers are not practical for very low level applications because of dc drift and poor common-mode rejection capabilities. They are usually employed as pen drive amplifiers in direct writing recorders. They are used for medium gain applications down to about 1 mV signal levels for full scale.
6. Chopper stabilized dc amplifiers are complex amplifiers having ________ amplifiers incorporated in the module.
a) 1
b) 2
c) 3
d) 4
Answer: c
Clarification: Chopper stabilized dc amplifiers are used for low level but preferably wideband applications such as oscilloscopes, tape recorders and light beam oscilloscope recorders. These are complex amplifiers having three amplifiers incorporated in the module. This includes an ac amplifier for signals above about 20 Hz, a dc chopper input amplifier for signals from about 20 Hz down to dc plus wideband feedback stabilized dc amplifier.
7. Which of the following amplifier is employed with resistive transducers which require an external source of excitation?
a) differential amplifier
b) ac coupled amplifier
c) carrier amplifier
d) dc bridge amplifier
Answer: d
Clarification: DC bridge amplifiersare employed with resistive transducers which require an external source of excitation. Essentially, the amplifier comprises of a stable dc excitation source, a bridge balance and calibration unit, a high gain differential dc amplifier and a dc output amplifier. They can be used as conventional dc high gain amplifiers and offer operating simplicity and high frequency response.
8. Chopper input dc amplifiersare preferred for low level inputs to instrumentation systems because of their high sensitivity, negligible drift and excellent common mode rejection capability.
a) True
b) False
Answer: a
Clarification: True. Chopper input dc amplifiersare preferred for low level inputs to instrumentation systems because of their high sensitivity, negligible drift and excellent common mode rejection capability. Their high frequency response is limited to about one half of the input chopper frequency.