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
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
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
4
Q
For a voltage amplifier the Gain = ?
A
Gain = Vout / Vin
5
Q
Gain is a ____ so has no ____
A
Gain is a ratio so has no units
6
Q
To calculate the voltage delivered to the load we need to know the amplifiers parameters…(3 things)
A
- Input resistance of amplifier rin
- Output resistance of amplifier rout
- Open circuit voltage gain Av
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
8
Q
What is Rs?
A
The internal resistance of the source
9
Q
Rs and rin form a…?
A
Voltage divider
10
Q
What does a voltage divider ciruit look like?
A
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
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!
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
14
Q
If a load is connected to the amplifier, the voltage across the load is…(equation)
A
15
Q
A ____ output resistance is preferred in an amplifier
A
A small output resistance is preferred in an amplifier
16
Q
What is IC and give an example
A
Intergrated circuit and an Op amp
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.
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
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
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
21
Q
What is the current golden rule?
A
Because rin≈∞, virtually no current can flow into input
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
23
Q
Comparison between ideal and real op-amps
A
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
25
V0 in a feedback amplifier = ?
Vo = Av x Vs
26
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 = ?
27
The new voltage gain Av ’ of the amplifier with feedback...(equation)
28
If ß Av\>\>1, then A'v = ?
29
What is ß?
The feedback fraction
30
Feedback voltage, Vn = ?
31
G = ? = ? = ?
32
ß = ?
33
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_**
34
Op-amp input resistance = ?
35
In a feedback amplifier, the input resistance = ?
36
Rin = ? and = ?
Rin = Vin / iin and
37
In feedback amplifier, Vs = ?
38
β \< 1 ?
Feedback Fraction
39
The “closed-loop gain”, G, of this feedback system = ?
Feedback equation
40
Ohm's law = ?
V =IR
41
KVL = ?
V4 - V1 - V2 - V3 = 0
42
KCL = ?
i1 +i3 - i1 - i4 = 0
43
Current Golden rule = ?
ip = in = 0
44
Voltge Golden rule = ?
vp = vn
45
What is the virtual ground/earth?
When the voltage at a point is almost zero, but it is not connected to ground
46
A real earth ____ voltage whereas a virtual earth can't
A real earth **_absorbs_** voltage whereas a virtual earth can't
47
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_**
48
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_**
49
In an open circuit, the input resistance of the op-amp is...
...infinite
50
What is this equation called and what is the 1 +Aß called?
**Feedback equation** and the **feedback factor**
51
For op-amps with high open voltage gain, G = ? = ? = ?
52
If A is sufficiently high then Aβ...?
If A is sufficiently high then Aβ \>\> 1
53
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_**
54
Comparing a general feedback amplifier circuit with a non-inverting feedback amplifier
55
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
56
For an **ideal Op-amp**
CGR = ?
VGR = ?
57
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**
58
G = Vout / Vin = - ??? (big equation)
59
What happens in this equation if A is large?
The denominator reduces to unity as A gets large
60
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_**
61
From ohms law for a real Op-amp??
62
G is the **closed**-**loop** **voltage** **gain** of the circuit
If **A\>\>G**
**Rin ≈ R1**
63
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
64
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
65
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
66
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**
67
Does an ideal op-amp exist?
What would a typical op-amp display?
68
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)
69
Transistors need a small... for correct operation
Transistors need a small **_dc input current, IB_**, for correct operation
70
What is IB ?
IB is called **bias current**
71
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
72
IB = ?
73
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.
74
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
75
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.
76
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
77
What is the **Input offset current (Ios)**
**Input Offset Current (Ios):** the difference of the two input bias currents
78
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
79
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_**”
80
Define **current limit**
**Current limit** indicates the maximum current that an op-amp can deliver to the load
81
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**
82
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**
83
SR = ?
SR = dv/dt
84
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;
## Footnote
**SR \> ωVp**
85
The **open-loop gain** of an **op-amp** appears in the calculations for
* The voltage gain
* The input impedance
* The output impedance
86
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
87
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.
88
What does a **bode diagram** or **bode plot** represent?
It represents the **response** of a **given component** (or **circuit**) over a **frequency range**
89
What can be used to **express** the **voltage gain**?
**Decibel (dB)** can be used to express the voltage gain
90
The gain of most op-amps ____ \_\_ ('\_\_\_\_ \_\_") at ____ frequencies
The gain of most op-amps **_falls of (“rolls off”)_** at **_high_** frequencies
91
Define **bandwidth**
**Bandwidth:** freq at which the gain is reduced by a factor of sqrt(2)
(with respect to its value at DC)
92
What is **"cut-off" frequency** also known as?
**3dB point** or **bandwidth**
93
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)_**
94
What has a higher bandwidth than an open-loop amplifier?
A closed-loop amlifier
95
Give some limitations of a real op-amp
* Limitated gain bandwidth
* Slew rate
96
Op-amps perform simple operations such as...
...addition, subtraction, multiplication, differentiation and integration
97
What does a **differentiator in electronics** do?
The “**differentiator**” in electronics denotes a circuit that performs a mathematical operation (differentiation) on the input voltage
98
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
99
Ic = ?
100
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”
101
What would Vout look like?
102
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
103
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
104
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”
105
Equation for feedback voltage?
Vf = *B*Vout
106
Equation for input voltage to the op-amp?
Vs = Vin - *B*Vout
