Op Amps Flashcards
What is an amplifier?
An amplifier is a device used to increase the amplitude or power level of a small signal but without distorting it
Passive components ____ or ____ signal power
Active components ____ external power and ____
Passive components absorb or transfer signal power
Active components accept external power and amplify
Passive components __ ___ need power for them to work
Active components __ need power for them to work
Passive components do not need power for them to work
Active components do need power for them to work
For a voltage amplifier the Gain = ?
Gain = Vout / Vin
Gain is a ____ so has no ____
Gain is a ratio so has no units
To calculate the voltage delivered to the load we need to know the amplifiers parameters…(3 things)
- Input resistance of amplifier rin
- Output resistance of amplifier rout
- Open circuit voltage gain Av
If we know the internal resistance of the source Rs , then the voltage across the input terminal of amplifier will be…(equation)
What is Rs?
The internal resistance of the source
Rs and rin form a…?
Voltage divider
What does a voltage divider ciruit look like?
- The larger the value of rin, the closer the value of Vin is to…?
- This is the reason that in practice ___ ____ ____ is always required
- The larger the value of rin, the closer the value of Vin is to Vs
- This is the reason that in practice high input impedance is always required
What is the symbol for open circuit voltage gain and what is it larger than?
Av - always larger than 1, very, very large!
What is Open circuit voltage gain?
It is the voltage gain if the amplifier is not loaded i.e. if RL is replaced with an ∞ resistance
If a load is connected to the amplifier, the voltage across the load is…(equation)
A ____ output resistance is preferred in an amplifier
A small output resistance is preferred in an amplifier
What is IC and give an example
Intergrated circuit and an Op amp
Define inverting input
the phase angle of the output will be 180° different from that of the input applied at this terminal.
The ____ ____ is often omitted from circuit diagrams for simplicity, but it must be understood to be present at all times
The ground line is often omitted from circuit diagrams for simplicity, but it must be understood to be present at all times
A perfect amplifier should have…(5 things)
- High gain
- High input impedance so that any signal could be supplied to it without a loading problem
- Low output impedance so that the power supplied by the amplifier is not limited
- Zero gain for common-mode input signal
- Infinite bandwidth
An ideal op-amp should have…(5 things)
- Infinite voltage gain, Av ≈∞
- Infinite input impedance, rin ≈∞
- Zero output impedance, rout ≈0
- Zero input offset voltage (i.e. exactly zero out if zero in)
- Infinite bandwidth
What is the current golden rule?
Because rin≈∞, virtually no current can flow into input
What is the voltage golden rule?
Because Vout ≈Av(Vp-Vn) and Av ≈∞, Vp-Vn ≈0, i.e. virtually no voltage difference between inputs
Comparison between ideal and real op-amps
In a system, the ____ is compared with the desired ____
If there is a difference between the two, ____ is used to drive the output close to its intended value
In a system, the output is compared with the desired output
If there is a difference between the two, feedback is used to drive the output close to its intended value
V0 in a feedback amplifier = ?
Vo = Av x Vs
If the feedback amplifier circuit is modified such that a fraction β of the output is fed back into the input, then the new output becomes;
Vs = ?
V’o = ?
The new voltage gain Av ’ of the amplifier with feedback…(equation)
If ß Av>>1, then A’v = ?
What is ß?
The feedback fraction
Feedback voltage, Vn = ?
G = ? = ? = ?
ß = ?
G is called the ____ ____and also the ____-____ ____ ____. It is almost entirely determined by the ____ ____ and is hardly affected by small changes in ___
G is called the feedback fraction and also the closed-loop voltage gain. It is almost entirely determined by the feedback circuit and is hardly affected by small changes in Av
Op-amp input resistance = ?
In a feedback amplifier, the input resistance = ?
Rin = ? and = ?
Rin = Vin / iin and
In feedback amplifier, Vs = ?
β < 1 ?
Feedback Fraction
The “closed-loop gain”, G, of this feedback system = ?
Feedback equation
Ohm’s law = ?
V =IR
KVL = ?
V4 - V1 - V2 - V3 = 0
KCL = ?
i1 +i3 - i1 - i4 = 0
Current Golden rule = ?
ip = in = 0
Voltge Golden rule = ?
vp = vn
What is the virtual ground/earth?
When the voltage at a point is almost zero, but it is not connected to ground
A real earth ____ voltage whereas a virtual earth can’t
A real earth absorbs voltage whereas a virtual earth can’t
A minus sign implies that the polarity of incoming voltage is ____, so it’s an ____ ____
A minus sign implies that the polarity of incoming voltage is reversed, so it’s an inverting amplifier
If Rs =Rf , then the amplifier has a ____ ____ (=1) and the incoming signal is ____
If Rs =Rf , then the amplifier has a unity gain (=1) and the incoming signal is inverted
In an open circuit, the input resistance of the op-amp is…
…infinite
What is this equation called and what is the 1 +Aß called?
Feedback equation and the feedback factor
For op-amps with high open voltage gain, G = ? = ? = ?
If A is sufficiently high then Aβ…?
If A is sufficiently high then Aβ >> 1
In this circuit, R2 and R1 form a ____ ____; they define VN as a fraction of ____
In this circuit, R2 and R1 form a potential divider**; they define VN as a fraction of **Vout
Comparing a general feedback amplifier circuit with a non-inverting feedback amplifier
In an inverting feedback amplifier, the open loop voltage gain, A, is ____ (as opposed to ____ as postulated for an ideal op-amp)
And the voltage golden rule is…
In an inverting feedback amplifier, the open loop voltage gain, A, is finite (as opposed to infinite as postulated for an ideal op-amp)
…not fullfilled, i.e. Vp does not = Vn
For an ideal Op-amp
CGR = ?
VGR = ?
In an op-amp circuit without feedback, A is given by…(explanation and equation)
…the ratio of the output voltage to the difference between the two input voltages
G = Vout / Vin = - ??? (big equation)
What happens in this equation if A is large?
The denominator reduces to unity as A gets large
The open loop gain of most commercial op-amps is usually higher than ___ , therefore the assumption of ideal behaviour of op-amp is ____
The open loop gain of most commercial op-amps is usually higher than 104 , therefore the assumption of ideal behaviour of op-amp is acceptable
From ohms law for a real Op-amp??
G is the closed-loop voltage gain of the circuit
If A>>G
Rin ≈ R1
The input resistance in an ____ feedback circuit will become ____ if the real op-amp characteristics are taken into account
The input resistance in an inverting feedback circuit will become smaller if the real op-amp characteristics are taken into account
In the example of a non-inverting feedback amplifier (lecture 2) we found that ____ negative feedback tends to ____ the input impedance
In the example of a non-inverting feedback amplifier (lecture 2) we found that series negative feedback tends to increase the input impedance
A voltage-derived feedback gives rise to a circuit with ____ output resistance
A voltage-derived feedback gives rise to a circuit with low output resistance
When both inputs are connected to the ground like in this circuit, in an ideal op-amps we expect Vout = 0, however Vout ≠ 0. What is this called?
This dc output voltage, when both input terminals are grounded, is called output offset voltage
Does an ideal op-amp exist?
What would a typical op-amp display?
The input circuit of an op-amp is a ____ _____ of bipolar transistors (eg. LM741)
The input circuit of an op-amp is a symmetrical arrangement of bipolar transistors (eg. LM741)
Transistors need a small… for correct operation
Transistors need a small dc input current, IB, for correct operation
What is IB ?
IB is called bias current
In an op-amp, it is difficult to make two input transistors identical, IB into one input is always ____ from the IB into another
In an op-amp, it is difficult to make two input transistors identical, IB into one input is always different from the IB into another
IB = ?
The difference in these two currents is defined as ____ ____ (Equation as well)
Its value quantifies the ____ between the input transistors
The difference in these two currents is defined as offset current
Its value quantifies the mismatch between the input transistors.
If an adjustable voltage source, Vos is connected across the input terminals of a real op-amp:
This external voltage source is adjusted until the output voltage is brought to zero, Vout=0
What is the input offset voltage?
The input offset voltage must exist across the two inputs in order to bring op-amp’s output voltage to ZERO.
What is the Input Bias Current (IB):
Input Bias Current (IB): the average of the current flowing into both inputs. Typical range: from nA down to pA
What is the Input offset current (Ios)
Input Offset Current (Ios): the difference of the two input bias currents
What is the Input Offset Voltage (Vos)
Input Offset Voltage (Vos): the voltage that must be applied between two input terminals to set the output voltage to zero. For an ideal op-amp, input offset voltage is zero
When a circuit drives the op-amp output beyond its swing voltage, we say that the op-amp is ____ or “__ ____”
When a circuit drives the op-amp output beyond its swing voltage, we say that the op-amp is saturated or “in saturation”
Define current limit
Current limit indicates the maximum current that an op-amp can deliver to the load
What’s the worst situation in a circuit to do with currents? And how can it be resolved?
The worst situation: when the output is short circuited
Therefore the short-circuit current is used to define the current limit
Define the speed limit, - Slew Rate (SR)
Term used to describe how fast the output of an op-amp can respond to changes in input voltage
SR = ?
SR = dv/dt
So for an op-amp to reproduce a sine wave without distortion, its SR needs to be ____ than sine wave’s ____ ____ ____, that is;
So for an op-amp to reproduce a sine wave without distortion, its SR needs to be bigger than sine wave’s maximum change rate, that is;
SR > ωVp
The open-loop gain of an op-amp appears in the calculations for
- The voltage gain
- The input impedance
- The output impedance
For most purposes, we assume the ____-____ ____ of an op-amp is very large, this is true at ____ ____ but not so true at ____ ____
For most purposes, we assume the open-loop gain of an op-amp is very large, this is true at low frequencies but not so true at high frequencies
How does the op-amp gain depend on the input frequency?
Real Op amps have a frequency dependent open loop gain. Their differential voltage gain tends to decrease as the input frequency increases.
What does a bode diagram or bode plot represent?
It represents the response of a given component (or circuit) over a frequency range
What can be used to express the voltage gain?
Decibel (dB) can be used to express the voltage gain
The gain of most op-amps ____ __ (‘____ __”) at ____ frequencies
The gain of most op-amps falls of (“rolls off”) at high frequencies
Define bandwidth
Bandwidth: freq at which the gain is reduced by a factor of sqrt(2)
(with respect to its value at DC)
What is “cut-off” frequency also known as?
3dB point or bandwidth
Cut-off frequency multiplied by gain is ____ the same value for a specific op-amp
This constant value is called the ____ ____ ____ (___)
Cut-off frequency multiplied by gain is always the same value for a specific op-amp
This constant value is called the Gain BandWidth Product (GBW)
What has a higher bandwidth than an open-loop amplifier?
A closed-loop amlifier
Give some limitations of a real op-amp
- Limitated gain bandwidth
- Slew rate
Op-amps perform simple operations such as…
…addition, subtraction, multiplication, differentiation and integration
What does a differentiator in electronics do?
The “differentiator” in electronics denotes a circuit that performs a mathematical operation (differentiation) on the input voltage
The current flowing through a capacitor, Ic , is proportional to the…
The current flowing through a capacitor, Ic , is proportional to the rate of voltage change dV/dt and the capacitance C
Ic = ?
Vout for an ideal op-amp = ?
- The output voltage is the derivative of the input
- Negative sign indicates that the circuit is still an inverter
- RC is known as the “time constant” or, in this circuit, as the “gain factor”
What would Vout look like?
Vout across the capacitor = ?
- Output voltage is proportional to the integral of the input voltage
- Circuit is an inverter
- Example: Used to integrate the signal from an accelerometer
What can you say about this equation? (3 things)
- Output voltage is proportional to the integral of the input voltage
- Circuit is an inverter
- Example: Used to integrate the signal from an accelerometer
What can you say about this equation? (3 things)
- The output voltage is the derivative of the input
- Negative sign indicates that the circuit is still an inverter
- RC is known as the “time constant” or, in this circuit, as the “gain factor”
Equation for feedback voltage?
Vf = BVout
Equation for input voltage to the op-amp?
Vs = Vin - BVout
