Chapter 4: Operational Amplifier Flashcards
How is the voltage gain of an amplifier defined as?
G=Vout/Vin
What is bandwidth?
This is the range of frequencies over which the gain is greater than 70% of the maximum gain
What is gain-bandwidth product?
GBP=gain x bandwidth
GBP is a constant of the op amp. If the gain doubles, the bandwidth halves and vice versa
What is saturation?
If the output voltage is too large, the output of the op−amp saturates. The saturation voltage is limited by the power supply. Solutions are to have a larger power supply and reduce the gain of the amplifier or the input voltage.
What is slew rate?
This is the rate at which Vout changes when input changes.
What is the equation to calculate slew rate?
Slew rate=ΔVout/Δt
What is open-loop gain?
there is no feedback – no signal path between the output of the op-amp and its inputs.
What is the ideal open loop gain?
Infinite,implying that a significant
output voltage would result from an input voltage of virtually zero
What is virtual earth?
when one input is connected directly to 0 V, the other sits at 0 V provided that the output is not saturated.
What is input impedance?
‘Resistance’ reduces the flow of electricity in a circuit. ‘Impedance’ can be considered as the equivalent for AC circuits. It determines the current drawn from an input device
What is the ideal value for input impedance?
infinite
What is output impedance?
the amplifier provides an output to the remainder of the circuit. We do not want to
lose any of the signal in the amplifier itself
What is the ideal value for output impedance?
Zero
What is common-mode rejection ratio?
MEasures the ability to reject input signals common to both input leads
What is the ideal vale for CMRR?
infinite
What is feedback?
‘Feedback’ means adding a fraction of the output signal to the input signal.
What is positive feedback?
When the fraction of output signal is returned in phase with the input signal, it is called ‘positive feedback’.
What is negative feedback?
When it is out of phase with the input
signal, it is called negative feedback.
How is the feedback supplied in an amplifier circuit?
With a feedback resistor connected between the input and output.
Draw an inverting amplifier
-Non-inverting input grounded
-Rin on the inverting input
-Feedback resistor between inverting input and the output
How is gain calculated on an inverting op amp?
G=-Rf/Rin
Is the gain always positive or negative on an inverting op amp?
negative
For an inverting op amp, the gradient of the graph is equal to what?
The gain
On an inverting op amp, if the output reaches its maximum value, it is said to have reached what kind of saturation?
Positive
On an inverting op amp, if the output reaches its minimum value, it is said to have reached what kind of saturation?
Negative saturation
For an inverting op amp, if the input voltage is switched to AC, how does this change the way the graph looks
The output has the same frequency but inverted
How is gain calculated for a non-inverting amplifier?
G=1+Rf/Rin
Is gain always positive or negative for a non-inverting op amp?
Positive
Draw a non-inverting op amp circuit
-Vin through the non-inverting input.
-Feedback resistor connected between vout and the inverting input
-R1 grounded and connected to the inverting input
On a non-inverting op amp, when does positive saturation occur?
When the output reaches the maximum possible value
On a non-inverting op amp, when does negative saturation occur?
When the output reaches the minimum possible value
Name the two types of op amp distortion
Clipping and slew rate
What is clipping distortion?
This occurs when the peak Vout is larger than the saturation voltage
State 3 solutions for clipping?
Higher the power supply
Lower the gain
Lower the input voltage
What is slew rate distortion?
This is where the output cannot change fast enough to replicate input at high frequencies
How can slew rate be avoided?
Purchasing an op amp with a greater slew rate
The slew rate needs to be=2πfVp
What is the gain of a voltage follower?
- This means Vout=Vin
Draw a voltage follower circuit
-Vin through the non-inverting input
-Inverting input connected directly to the output providing feedback
No resistors
Draw a summing amplifier circuit
-3 different inputs, which all have their own resistor.
-The resistors are all connected to the inverting input
-Rf is also connected to the inverting input’
-non-inverting is grounded
How do you calculate gain of a summing amplifier?
Vout=-Rf(V1/R1 + V2/R2 +…
The voltage follower is a form of what op amp?
Non-inverting
What is the ideal feedback resistance of a voltage follower?
0Ω
What is a comparator?
A comparator is a sub-system that compares two voltages. Its output indicates which is larger.
Does a comparator have feedback?
No
How is the comparator powered?
By a single-rail power supply, with the negative power terminal
What is used to amplify the difference in input voltages?
Full open loop gain
True or false: The output of the of a comparator will always be saturated
True
Vout:G0-(V1-V2)
For a comparator.If the non-inverting input is greater than the inverting input, what saturation will the output be?
Positive saturation
For a comparator.If the inverting input is greater than the non-inverting input, what saturation will the output be?
Negative saturation
On a comparator which input is the reference voltage?
The inverting input
On a typical comparator circuit, what is usually connected via the non-inverting input?
A sensor circuit. An LDR or thermistor can be used to change the output voltage response
What is reference voltage in a comparator?
Reference voltage is a fixed voltage
