1 conventional amplifier collector emitter base rb1 rb2 rc rece rl vcc vin vout av = vout/vin = -...
TRANSCRIPT
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Conventional amplifier
Collector
Emitter
Base
Rb1
Rb2
Rc
Re Ce
RL
Vcc
Vin
Vout
Av = Vout/Vin = - (Rc//RL) / re
re = ac resistance of the emitter
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High-frequency transformer-coupled amplifier
Collector
Emitter
Base
Rb1
Rb2
RL
ReCe
Vcc
C1
Vin
Vout
f = 1 / (2 pi sqrt(L C1)
Q = f / B
L
Example 2.1
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Practical common emitter amplifier with better impedance matching
Collector
Emitter
Base
Rb1
Rb2
RL
ReCe
Vcc
C1
Vin
VoutL
Rd
Cd
Better impedance matching
Higher Q
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Common base RF amplifier
RL
ReCb
Vin
VoutL L
Vcc
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Wideband amplifier- Class A
Collector
Emitter
Base
Rb1
Rb2
RL
ReCe
Vcc
Vin
Vout
L
Linear amplifier
Generally used as single-ended audio amplifiers
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Wideband amplifier- Class B
Rb1
RL
Vcc
Vin
Vout
Compared to Class A:
Greater efficiency
Larger distortion
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Amplifier- Class C
Collector
Emitter
Base
RL
Vcc
Vin
Vout
L
High efficiency
Larger distortion
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Operating condition
Class C amplifiers would improve the efficiency by operating in nonlinear regime, however the input has to be a sinusoidal wave
Some means are needed to remove the distortion and restore the signal to its original sine shape
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Operation principle
The active device conducts for less than 180 degrees of the input cycle
The output resembles a series of pulses more than it does the original signal
The pulses can be converted back to sine waves by an output tuned circuit
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Circuit configuration
Collector
Emitter
Base
RL
Vcc
Vin
Vout
L
Input
Output
Nonlinear amplifier
Sine input -> nonlinear current output -> sine output
Fig. 2.12
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Pros and Cons of the Class C amplifiers
Pros:
• High efficiency, no current in absence of signal
Cons:
• The output tuned circuit must be adjusted fairly close to the operating frequency
• The amplification is nonlinear
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Comparison of three amplifiers
Class A B C
Conduction angle
360 180 < 180
Maximum efficiency
50% 78.5% 100%
Likely practical efficiency
25% 60% 75%
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Neutralization
Collector
Emitter
Base
Rb1
Rb2
RL
ReCe
Vcc
Vin
VoutL
Rd
Cd
Cn Neutralization capacitor
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Oscillator
A
B
Barkhausen criteria:
• A x B = 1
• Phase shift must total 0 or integer multiple of 360 degrees
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Using non-inverting amplifier
Hartley oscillator
B = N1 / (N1 + N2)
f = 1/2pi sqrt(LC)
N2
N1
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Using inverting amplifier (Hartley oscillator)
B = -N1 / N2 B = (N1 + N2) / N1
Example 2.2
N2
N1
N2
N1
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Colpitts oscillator (non-inverting amplifier)
B = Xc1 / Xct = C2 / (C1 + C2)
C2
C1
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Colpitts oscillator (inverting amplifier)
Example 2.3
B = -Xc1/Xc2 = - C2/C1
C2
C1
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Clapp oscillator
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Varactor tuned oscillator
Example 2.5
C=C0/sqrt(1+2V)
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Oscillation frequency of LC circuit
See MIT open course ware
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Another application of high Q filter
Before After
Clock recovery by strong filtering effect
PTL Oct
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Crystal
Crystal oscillators achieve greater stability by using a small slab of quartz as a mechanical resonator, in place of an LC tuned circuit
Cs Cp
Two resonance frequency related to Cs and Cp, respectively
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fT = f0 + k f0(T-T0)
Example 2.6
A portable radio transmitter has to operate at temperatures from –5 to 35 degrees. If the frequency is derived from a crystal oscillator with a temperature coefficient of +1ppm/degree C and it transmits at exactly 146 MHz at 20 degree, find the transmitting frequencies at the two extremes of the operating range
Temperature dependence
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Mixers
A mixer is a nonlinear circuit that combines two signals in such a way as to produce the sum and difference of the two input frequencies at the output
Any nonlinear device can operate as a mixer
Vout = A Vi + B Vi2 + C Vi3 + …
f1 f2f1+f2f1- f2
Second order effects
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Square law mixers
Vout = A Vi + B Vi2
If inputs are two frequencies,
the outputs will be:
Original frequencies, double frequencies, sum frequencies, and differential frequencies
Example 2.7
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Diode mixers
The V-I curve for a typical silicon signal diode is nonlinear
Diode mixers can operate between reverse and forward biased states
Or they can operate with a small forward bias
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Transistor mixers
Collector
Emitter
Base
Rb1
Rb2
RL
Re
Vcc
f1
Vout
L
f2
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Balanced mixers
A multiplier circuit, where the output amplitude is proportional to the product of two input signals, can be used as a balanced mixer
V1 = sinω1t
V2 = sinω2t
Vo = V1 x V2 = 0.5 x [cos(ω1t - ω2t) – cos(ω1t + ω2t)]
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Applications of balanced mixers
AM Modulation
Data (…01101001…)
Carrier
Output Signal
AM de-modulation
Signal input
Local oscillator
Output Signal
Filter
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Detection schemes
Signal input
Output Signal
Filter
Self-mixing homodyne detection
Homodyne and heterodyne detection
One example of heterodyne detection
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Phase detector using mixer
Signal input
The DC output depends on the phase of the two paths
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Phase locked loop
Phase detector
LPF Amp VCO
OutputInput
Capture range
Lock range
Example 2.8
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Simple frequency synthesizer
Phase detector
LPF Amp VCO
OutputInput
/ N divider
FM and AM channel spacing
Example 2.9
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A practical example – 29M to 10G synchronization circuit
200
10
MAV11100
Clk ResetD1D0
74F163Counter
29MHzpulsein
5V
MAV11
D Q_
Clk Q74F74
D Flip-Flop
4.84 MTTL
Output
7K
4.7u150 150
4.7u
15V
MRV901
4K
1K
29MHz / 6 circuit
29MHz amplification, digitization and frequency division circuit (All capacitors are 0.1uF).
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2K
10K
10K 100K
+6V 5
8
UPG506B 14GHz divide by 8 Prescalar
2.2V Zener
1000UF
10GHVCO
Splitter
+15V To 10G laser
1.5K
1.5K
+17V
10 dBm 2-10 dBm
1
UPB1502 1.25GHz divide by 128 Prescalar
2
3
4
8
7
6
5
-15~0 dBm
74F86 XOR gate
4.84 MHz TTL Input
74F74 f/2
5M to 10G synchronization circuit
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Spectrum of 4.827MHz square signal wave. Span: 500Hz, RB: 30Hz.
Spectrum of 4.827MHz square signal wave. Span: 500Hz, RB: 30Hz.
Experimental results
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Pre-scaling
Phase detector
LPF Amp VCO
OutputInput
Fixed /M
Programmable /N
Fixed /Q
Example 2.10
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Frequency translation
The movement of a block of frequencies is called a frequency translation
Two configurations:
Synthesizer with frequency shifting
Synthesizer with mixer in the loop
Example 2.11
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Transmission lines
Coaxial cables (solid dielectric, air dielectric)
Parallel line cables (television twin-lead, open-wire line, shielded twin-lead)
Twisted pair cable
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Two models of short transmission line section
Balanced line
Unbalanced line
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Step and pulse response of lines
Characteristic impedance: the ratio of voltage to current through the transmission line with a step signal
Concept of matched line
Characteristic impedance Z0 = sqrt[(R + jwL) / (G + jwC)]
Many lines approach Z0 = sqrt(L/C)
Example 14.1, 14.2
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Reflection (step input)
Open end scenario
Short end scenario
Pulse input…
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Some definitions
Γ = Vr/Vi: reflection efficient
Γ = (ZL – Z0) / (ZL + Z0)
Meaning of the above equation:
1. To have zero reflection, ZL has to be equal to Z0
2. By measuring Γ, ZL can be derived to probe the internal characteristic of the load
Example 14.13
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An example to know the internal parameters of a tunable laser
Lp
Cp
Rp
Cs
Rsub
D
Parasitics PN junction
Rs
SourceTransmission
line
S11
S11 = (ZL – Z0) / (ZL + Z0)
Parameters Reflector biased at 10 mA
Is (A) 1.79E10-5
q 4.47
Rp (ohm) 0.1
Rs (ohm) 0.1
Rsub (ohm) 1.0
Cp (pF) 4.58
Cs (pF) 355
Lp (nH) 21.4
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Voltage driver is better than current driver
Current response Optical response
Y. Su et al, IEEE PTL Sept. 2004
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Wave propagation
In a matched line, a sine wave moves down the line and disappear into the load. Such a signal is called a traveling wave
Example 14.5
RF Phase shifter
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Standing waves
The interaction between the incident and reflected waves causes what appears to be a stationary pattern of waves on the line, which are called standing waves
SWR = Vmax/Vmin
For a matched line, the SWR = 1
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Relation between Γ and SWR
SWR = (1+ |Γ|) / ( 1 - |Γ|)
If ZL >Z0,
SWR = Z0 / ZL
Example 14.6
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