lecture 2&3: Amplifiers
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Transcript of lecture 2&3: Amplifiers
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lecture 2&3:Amplifiers
The heart of EE
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Lecture outline
Ideal Op-Amp
Negative Feedback
Application to Filters
Linear systems
Bode plots
Op-amp nonidealities
Positive feedback
Reading:1)Hambley Ch 1,2, section 8.1
2) Lab 2 handout
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Amplifiers
They are everywhere: Music, Computers, Sensors
Used to turn a small voltage into a bigger voltage that can be manipulated easily.
Some filtering is used as well.
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amplifiers give gain
Simple amp-1 input and 1 output
Gain, A=Vout/Vin
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Example
If the amplifier above gives an output voltage of 1000V with an input voltage of 50V, what is the gain?
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ideal operational-amplifier(op-amp)
Inputs draw no current-infinite input impedace
Vout=A(Vplus-Vminus) A-open loop gain.
A is ideally infinity-How is this useful?
Output can provide as much voltage/current as needed-zero output impedance
http://www.youtube.com/watch?v=TQB1VlLBgJE
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negative feedback
Negative feedback (NF) tries to reduce the difference
with NF, Vplus=Vminus ALWAYS
summing point constraints
virtual ground.
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Inverting amplifier
Input goes into Vminus input-INVERTING input
Gain, Ainv=-R2/R1, gain is negative because inverting
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inverting amplifier
Vplus=Vminus
Inputs draw no current
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Non-inverting amplifier
Input goes into Vplus input-NON-INVERTING input
Gain, Ainv=1+R2/R1, gain is positive
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unity gain buffer
Gain is 1 i.e. Vin=Vout
Used to isolate one side from the other
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Real op-amps
Output voltage determined by rails (power supply)
Open loop gain not infinity
Inputs draw small amount of current-nA’s or less
Quad LM324Single LM741
http://www.national.com/mpf/LM/LM324.html#Overview
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bandpass Filter amplifier
f1=0.3Hz, f2=10Hz
High freq., cap is short, low freq., cap is open
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frequency<f1
all caps are open.
What is the gain?
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F1<Frequency<F2
C1 is short. C2 is open.
What is the gain?-midband gain.
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frequency>f2
All caps are shorts
What is the gain?
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linear systems
T(s) has zeros when T(s)=0
T(s) has poles when T(s)=infinity
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linear systemsAny voltage signal can be represented by a sum of sinusoidal voltage signals-Fourier/Laplace theorems
If s=jω, the input voltage is represented by:
V0exp(jωt)= V0exp(st)
Allows us to use algebra instead of differential eqns.
RLC circuit, for example.
t
ttt
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filter op-amp
What is T(s)?
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filter op-amp
zero at s=0
poles at 1/R1C1 and 1/R2C2
What happens at the zero? At the poles?
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bode plots
Frequency response on log/log axes
x-axis frequency on log scale
y-axis gain in decibels (dB)
gain in dB=20log|Vo/Vi|
20dB/decade will appear often.
Can sketch quickly without doing too much math
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bode plot rules
Zeros of transfer function
+20dB/decade increase in gain starting at zero
phase contribution at zero frequency is +45°
Total phase change is 90° attained at 10 times zero freq.
Poles of transfer function
-20dB/decade decrease in gain starting at zero
phase contribution at pole frequency -45°
Total phase change is -90° attained at 10 times pole freq.
See Hambley section 8.1
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bode plot errors
Bode magnitude plot only has straight lines
not true near break frequencies ~3dB error
Bode phase plot only has straight lines
not true for phase near break frequencies ~5° error
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Bode plot of filter
Zero at s=0, poles at s=1/R1C1 and 1/R2C2
Gain at midband =R2/R1or 20log(R2/R1) dB
midband, inverted output so phase=180°
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bode plot of filter
180
135
90
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op-amp non-idealities
Slew rate -maximum rate of change of voltage at output-arises from compensating capacitor
i.e. dv/dt|max
If input voltage changes too quickly, op-amp can’t keep up-leads to distortion of output.
Gain-bandwith product-high frequency, lower gain.
AOLfOL=ACLfCL HINT for prelab! Eqn 2.39
Unity gain frequency
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Positive feedback
Amplifies differences, so output is always +/-Vcc
http://en.wikipedia.org/wiki/Schmitt_trigger
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Schmitt trigger
Switches between 2 output rails +/-M.
Switch when inputs equal each other
Use resistors to control this
Inverting and non-inverting Schmitt triggers
Inverting SchmittStandard Schmitt
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inverting schmitt trigger
Vtrigger=+/-VR2/(R1+R2)
Input into inverting input
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Schmitt application
To minimize switching too often in noisy environment
e.g. Thermostat-switch only when T=+/-1degree
Does this represent inverting or non-inverting Schmitt?
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EE SHORT: Tubes vs. Op-Amps
Old amplifiers used tubes-BIG!
New amplifiers use op-amps. Compact, high performance
http://www.youtube.com/watch?v=PbJ1GZMi_ho
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more transfer functions & Timer circuitsfrequency response cont.