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Electrical > Op Amps > Flashcards

Op Amps Flashcards

1
Q

What is an amplifier?

A

An amplifier is a device used to increase the amplitude or power level of a small signal but without distorting it

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2
Q

Passive components ____ or ____ signal power

Active components ____ external power and ____

A

Passive components absorb or transfer signal power

Active components accept external power and amplify

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3
Q

Passive components __ ___ need power for them to work

Active components __ need power for them to work

A

Passive components do not need power for them to work

Active components do need power for them to work

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4
Q

For a voltage amplifier the Gain = ?

A

Gain = Vout / Vin

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5
Q

Gain is a ____ so has no ____

A

Gain is a ratio so has no units

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6
Q

To calculate the voltage delivered to the load we need to know the amplifiers parameters…(3 things)

A
  1. Input resistance of amplifier rin
  2. Output resistance of amplifier rout
  3. Open circuit voltage gain Av
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7
Q

If we know the internal resistance of the source Rs , then the voltage across the input terminal of amplifier will be…(equation)

A
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8
Q

What is Rs?

A

The internal resistance of the source

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9
Q

Rs and rin form a…?

A

Voltage divider

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10
Q

What does a voltage divider ciruit look like?

A
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11
Q
  • The larger the value of rin, the closer the value of Vin is to…?
  • This is the reason that in practice ___ ____ ____ is always required
A
  • 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
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12
Q

What is the symbol for open circuit voltage gain and what is it larger than?

A

Av - always larger than 1, very, very large!

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13
Q

What is Open circuit voltage gain?

A

It is the voltage gain if the amplifier is not loaded i.e. if RL is replaced with an ∞ resistance

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14
Q

If a load is connected to the amplifier, the voltage across the load is…(equation)

A
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15
Q

A ____ output resistance is preferred in an amplifier

A

A small output resistance is preferred in an amplifier

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16
Q

What is IC and give an example

A

Intergrated circuit and an Op amp

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17
Q

Define inverting input

A

the phase angle of the output will be 180° different from that of the input applied at this terminal.

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18
Q

The ____ ____ is often omitted from circuit diagrams for simplicity, but it must be understood to be present at all times

A

The ground line is often omitted from circuit diagrams for simplicity, but it must be understood to be present at all times

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19
Q

A perfect amplifier should have…(5 things)

A
  • 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
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20
Q

An ideal op-amp should have…(5 things)

A
  • 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
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21
Q

What is the current golden rule?

A

Because rin≈∞, virtually no current can flow into input

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22
Q

What is the voltage golden rule?

A

Because Vout ≈Av(Vp-Vn) and Av ≈∞, Vp-Vn ≈0, i.e. virtually no voltage difference between inputs

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23
Q

Comparison between ideal and real op-amps

A
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24
Q

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

A

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

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25
Q

V0 in a feedback amplifier = ?

A

Vo = Av x Vs

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26
Q

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 = ?

A
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27
Q

The new voltage gain Av ’ of the amplifier with feedback…(equation)

A
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28
Q

If ß Av>>1, then A’v = ?

A
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29
Q

What is ß?

A

The feedback fraction

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30
Q

Feedback voltage, Vn = ?

A
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31
Q

G = ? = ? = ?

A
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32
Q

ß = ?

A
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33
Q

G is called the ____ ____and also the ____-____ ____ ____. It is almost entirely determined by the ____ ____ and is hardly affected by small changes in ___

A

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

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34
Q

Op-amp input resistance = ?

A
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35
Q

In a feedback amplifier, the input resistance = ?

A
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36
Q

Rin = ? and = ?

A

Rin = Vin / iin and

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37
Q

In feedback amplifier, Vs = ?

A
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38
Q

β < 1 ?

A

Feedback Fraction​

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39
Q

The “closed-loop gain”, G, of this feedback system = ?

A

Feedback equation

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40
Q

Ohm’s law = ?

A

V =IR

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41
Q

KVL = ?

A

V4 - V1 - V2 - V3 = 0

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42
Q

KCL = ?

A

i1 +i3 - i1 - i4 = 0

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43
Q

Current Golden rule = ?

A

ip = in = 0

44
Q

Voltge Golden rule = ?

A

vp = vn

45
Q

What is the virtual ground/earth?

A

When the voltage at a point is almost zero, but it is not connected to ground

46
Q

A real earth ____ voltage whereas a virtual earth can’t

A

A real earth absorbs voltage whereas a virtual earth can’t

47
Q

A minus sign implies that the polarity of incoming voltage is ____, so it’s an ____ ____

A

A minus sign implies that the polarity of incoming voltage is reversed, so it’s an inverting amplifier

48
Q

If Rs =Rf , then the amplifier has a ____ ____ (=1) and the incoming signal is ____

A

If Rs =Rf , then the amplifier has a unity gain (=1) and the incoming signal is inverted

49
Q

In an open circuit, the input resistance of the op-amp is…

A

…infinite

50
Q

What is this equation called and what is the 1 +Aß called?

A

Feedback equation and the feedback factor

51
Q

For op-amps with high open voltage gain, G = ? = ? = ?

A
52
Q

If A is sufficiently high then Aβ…?

A

If A is sufficiently high then Aβ >> 1

53
Q

In this circuit, R2 and R1 form a ____ ____; they define VN as a fraction of ____

A

In this circuit, R2 and R1 form a potential divider**; they define VN as a fraction of **Vout

54
Q

Comparing a general feedback amplifier circuit with a non-inverting feedback amplifier

A
55
Q

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…

A

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

56
Q

For an ideal Op-amp

CGR = ?

VGR = ?

A
57
Q

In an op-amp circuit without feedback, A is given by…(explanation and equation)

A

…the ratio of the output voltage to the difference between the two input voltages

58
Q

G = Vout / Vin = - ??? (big equation)

A
59
Q

What happens in this equation if A is large?

A

The denominator reduces to unity as A gets large

60
Q

The open loop gain of most commercial op-amps is usually higher than ___ , therefore the assumption of ideal behaviour of op-amp is ____

A

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

61
Q

From ohms law for a real Op-amp??

A
62
Q

G is the closed-loop voltage gain of the circuit

If A>>G

A

Rin ≈ R1

63
Q

The input resistance in an ____ feedback circuit will become ____ if the real op-amp characteristics are taken into account

A

The input resistance in an inverting feedback circuit will become smaller if the real op-amp characteristics are taken into account

64
Q

In the example of a non-inverting feedback amplifier (lecture 2) we found that ____ negative feedback tends to ____ the input impedance

A

In the example of a non-inverting feedback amplifier (lecture 2) we found that series negative feedback tends to increase the input impedance

65
Q

A voltage-derived feedback gives rise to a circuit with ____ output resistance

A

A voltage-derived feedback gives rise to a circuit with low output resistance

66
Q

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?

A

This dc output voltage, when both input terminals are grounded, is called output offset voltage

67
Q

Does an ideal op-amp exist?

What would a typical op-amp display?

A
68
Q

The input circuit of an op-amp is a ____ _____ of bipolar transistors (eg. LM741)

A

The input circuit of an op-amp is a symmetrical arrangement of bipolar transistors (eg. LM741)

69
Q

Transistors need a small… for correct operation

A

Transistors need a small dc input current, IB, for correct operation

70
Q

What is IB ?

A

IB is called bias current

71
Q

In an op-amp, it is difficult to make two input transistors identical, IB into one input is always ____ from the IB into another

A

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

72
Q

IB = ?

A
73
Q

The difference in these two currents is defined as ____ ____ (Equation as well)

Its value quantifies the ____ between the input transistors

A

The difference in these two currents is defined as offset current

Its value quantifies the mismatch between the input transistors.

74
Q

If an adjustable voltage source, Vos is connected across the input terminals of a real op-amp:

A

This external voltage source is adjusted until the output voltage is brought to zero, Vout=0

75
Q

What is the input offset voltage?

A

The input offset voltage must exist across the two inputs in order to bring op-amp’s output voltage to ZERO.

76
Q

What is the ​Input Bias Current (IB):

A

Input Bias Current (IB): the average of the current flowing into both inputs. Typical range: from nA down to pA

77
Q

What is the Input offset current (Ios)

A

Input Offset Current (Ios): the difference of the two input bias currents

78
Q

What is the Input Offset Voltage (Vos)

A

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

79
Q

When a circuit drives the op-amp output beyond its swing voltage, we say that the op-amp is ____ or “__ ____”

A

When a circuit drives the op-amp output beyond its swing voltage, we say that the op-amp is saturated or “in saturation”

80
Q

Define current limit

A

Current limit indicates the maximum current that an op-amp can deliver to the load

81
Q

What’s the worst situation in a circuit to do with currents? And how can it be resolved?

A

The worst situation: when the output is short circuited

Therefore the short-circuit current is used to define the current limit

82
Q

Define the speed limit, - Slew Rate (SR)

A

Term used to describe how fast the output of an op-amp can respond to changes in input voltage

83
Q

SR = ?

A

SR = dv/dt

84
Q

So for an op-amp to reproduce a sine wave without distortion, its SR needs to be ____ than sine wave’s ____ ____ ____, that is;

A

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

85
Q

The open-loop gain of an op-amp appears in the calculations for

A
  • The voltage gain
  • The input impedance
  • The output impedance
86
Q

For most purposes, we assume the ____-____ ____ of an op-amp is very large, this is true at ____ ____ but not so true at ____ ____

A

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

87
Q

How does the op-amp gain depend on the input frequency?

A

Real Op amps have a frequency dependent open loop gain. Their differential voltage gain tends to decrease as the input frequency increases.

88
Q

What does a bode diagram or bode plot represent?

A

It represents the response of a given component (or circuit) over a frequency range

89
Q

What can be used to express the voltage gain?

A

Decibel (dB) can be used to express the voltage gain

90
Q

The gain of most op-amps ____ __ (‘____ __”) at ____ frequencies

A

The gain of most op-amps falls of (“rolls off”) at high frequencies

91
Q

Define bandwidth

A

Bandwidth: freq at which the gain is reduced by a factor of sqrt(2)

(with respect to its value at DC)

92
Q

What is “cut-off” frequency also known as?

A

3dB point or bandwidth

93
Q

Cut-off frequency multiplied by gain is ____ the same value for a specific op-amp

This constant value is called the ____ ____ ____ (___)

A

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)

94
Q

What has a higher bandwidth than an open-loop amplifier?

A

A closed-loop amlifier

95
Q

Give some limitations of a real op-amp

A
  • Limitated gain bandwidth
  • Slew rate
96
Q

Op-amps perform simple operations such as…

A

…addition, subtraction, multiplication, differentiation and integration

97
Q

What does a differentiator in electronics do?

A

The “differentiator” in electronics denotes a circuit that performs a mathematical operation (differentiation) on the input voltage

98
Q

The current flowing through a capacitor, Ic , is proportional to the…

A

The current flowing through a capacitor, Ic , is proportional to the rate of voltage change dV/dt and the capacitance C

99
Q

Ic = ?

A
100
Q

Vout for an ideal op-amp = ?

A
  • 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”
101
Q

What would Vout look like?

A
102
Q

Vout across the capacitor = ?

A
  • Output voltage is proportional to the integral of the input voltage
  • Circuit is an inverter
  • Example: Used to integrate the signal from an accelerometer
103
Q

What can you say about this equation? (3 things)

A
  • Output voltage is proportional to the integral of the input voltage
  • Circuit is an inverter
  • Example: Used to integrate the signal from an accelerometer
104
Q

What can you say about this equation? (3 things)

A
  • 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”
105
Q

Equation for feedback voltage?

A

Vf = BVout

106
Q

Equation for input voltage to the op-amp?

A

Vs = Vin - BVout

Electrical (5 decks)

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