Operational Amplifiers, Electronic Circuits Flashcards
The desirable features of op-amps include which one of the following:
A. Low gain to reduce distortion
B. Low input impedance so as not to disturb the input signal
C. Differential inputs to allow all signal frequencies to be amplified
D. A high common mode rejection ratio so as not to amplify signals common to both inputs
E. Non-linear amplification across a range of input magnitudes
D. A high common mode rejection ratio is a measure of the ability of the op-amp to reject signals common to both inputs.
A. Incorrect. The primary purpose of an op-amp is to amplify the input signal as much as possible without distortion.
B. Incorrect. The input impedance must be high in order not to draw current from the input signal.
C. Incorrect. The purpose of differential inputs (one +, the other -) ensures that any unwanted signals common to both inputs are not amplified, but cancelled out, or rejected.
E. Incorrect. Although it is hard to achieve in practical terms, a linear amplification profile is preferred.
In converting an analogue signal to a digital one, the analogue signal is ‘sampled’ at intervals and digitized. Regarding the sampling rate:
A. The sampling rate is not important
B. The sampling rate must exactly match the analogue frequency
C. A sampling rate which is too low will inadequately represent the analogue signal
D. A sampling rate which is too high will also inadequately represent the analogue signal
E. An excessive sampling rate will distort the signal
C. In digitizing an analogue signal, the sampling rate must not be too low or the resulting signal pattern will be an inadequate digital representation of the original analogue one. The sampling rate does not have to match any frequency relationship of the input signal, and a sampling rate which is high merely produces an unnecessarily accurately reproduced digital form of the original analogue signal.
Op-amps have a high input impedance because (true or false):
To reduce the input voltage
False
Op-amps have a high input impedance because (true or false):
To reduce signal distortion
False
Op-amps have a high input impedance because (true or false):
To reduce the current drawn by the amplifier
True. By keeping input impedance high, less current will be drawn by the op-amp itself, thus reducing changes to the original circuit current.
Op-amps have a high input impedance because (true or false):
To match its impedance to the rest of the circuit
False. This is not linked with impedance matching.
Op-amps have a high input impedance because (true or false):
In order not to degrade the input signal
True. By not reducing the input current, the current to be amplified is preserved.
Op-amps have a high input impedance because (true or false):
To allow connection of other circuit elements
False.
The following are true of common mode rejection ratio (CMRR) (true or false):
It is the rejection of signals to a single input amplifier
False. It is the rejection of signals common to both inputs of a differential input amplifier.
The following are true of common mode rejection ratio (CMRR) (true or false):
It is the rejection of DC inputs
False. It is not related to the type of input signal.
The following are true of common mode rejection ratio (CMRR) (true or false):
It is the acceptance of signals common to both inputs of a differential input amplifier
False
The following are true of common mode rejection ratio (CMRR) (true or false):
It is the rejection of signals common to both inputs of a differential input amplifier
True. This is the definition of CMRR.
The following are true of common mode rejection ratio (CMRR) (true or false):
A CMRR of 10⁴ means a common mode signal of 10 000 mV is needed before 1 mV enters the op-amp for amplification
True
The following are true of common mode rejection ratio (CMRR) (true or false):
A high CMRR is desirable
True. The ability to reject unwanted common mode signals is important for enhancing differential mode signals.
The purpose of a differential input on an op-amp is (true or false):
To mathematically differentiate the input signal
False. A differential an op-amp has two inputs, one which maintains the sign of the input signal and one which switches the sign (inverts) the input signal. In this way, unwanted signals which are common to both inputs are added together and cancel each other out at the amplifier input. Therefore only signals which have a differential signal at the input are processed (amplified) by the op-amp.
