lecture vi - physicsphysics.wm.edu/~ran/pages/fc/electronics lecture 6.pdf · 2020. 2. 28. ·...
TRANSCRIPT
![Page 1: Lecture vi - Physicsphysics.wm.edu/~ran/Pages/fc/Electronics lecture 6.pdf · 2020. 2. 28. · Lecture vi. What are we doing every week? Design •Pre-lab Homework •Lecture Simulation](https://reader035.vdocuments.us/reader035/viewer/2022063020/5fe161e7c7b3ee0a37436004/html5/thumbnails/1.jpg)
E lectron ics
S p r i n g 2 0 2 0 R a n Ya n g
http://physics.wm.edu/~ran/
L e c t u r e v i
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What are we doing
every week?
Design
• Pre-lab Homework
• Lecture
Simulation
• Pre-lab Homework
• Lecture
Build in real world
• Lab
Trouble Shooting
• Lab
• Simulation
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Midterm Exam
Ran Yang || [email protected]
• We do have class on 3/2 Monday
• We do have lab 6 on 3/3, 3/4 and 3/5
• The exam will happen in the first 50 minutes of the lab 6
• Talk to me if you have accommodation pleaseTime
• Lab 1 to Lab 5 homework/lab , Textbook Chapter 1 to 7, and Chapter 14
• As long as you understand your design homework and your lab procedure you are good to go
• Circuit design, circuit analysis & troubleshooting and physics/mathContents
• Closed book & closed notes
• A pencil, an eraser and a ruler
• A scientific calculator with its battery full
• If you use a scientific calculator on your phone, the phone must be set to airplane mode
• I do not provide a formula sheet
Rules
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B ipo la r Junc t i on Tran s i s to r Amp l i f i e r sR e a d C h a p t e r 7 & 8
Ran Yang || [email protected]
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Coupl i ng Capac i tor
Ran Yang || [email protected]
• The capacitor effectively blocks dc voltage and transmits ac voltage.
• frequency is high , the capacitive reactance is much smaller than the resistance.
• almost all the ac source voltage appears across the resistor.
• It couples or transmits the ac signal to the resistor.
How it works
• They allow us to couple an ac signal into an amplifier without disturbing its Q point (operation).
• Its reactance must be much smaller than the resistance at the lowest frequency of the ac source.
Coupling capacitors are important
• 𝑋𝐶 < 0.1𝑅
Good coupling
𝑍 = 𝑅2 + 𝑋𝐶2
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A s imp le base -b ia sed amp l i f i e r
Ran Yang || [email protected]
• 𝑉𝑖𝑛• 𝑉𝐵𝐵• 𝑉𝐶• 𝑉𝑜𝑢𝑡• 𝑉𝐸
Waveform
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VDB ampl i f ie r
Ran Yang || [email protected]
Calculate the dc voltages and currents
• Take a look at the simulation in Multisim
VDB waveform
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Smal l S igna l Operat ion
Ran Yang || [email protected]
• The ac voltage on the base produces the ac emitter current
• This ac emitter current has the same frequency as the ac base voltage.
• The ac emitter current is not a perfect replica of the ac base voltage because of the curvature of the graph.
• Since the graph is curved upward, the positive half-cycle of the ac emitter current is elongated (stretched) and the negative half-cycle is compressed.
• This stretching and compressing of alternate half-cycles is called distortion.
• It is undesirable in high-fidelity amplifiers because it changes the sound of voice and music.
Distortion
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Smal l S igna l Operat ion
Ran Yang || [email protected]
• by keeping the ac base voltage small.
• reduce the peak value of the base voltage, the movement of the instantaneous operating point reduces.
• The smaller this swing or variation, the less the curvature in the graph.
• If the signal is small enough, the graph appears to be linear
Reduce Distortion
• negligible distortion for a small signal.
• When the signal is small, the changes in ac emitter current are almost directly proportional to the changes in ac base voltage because the graph is almost linear.
• if the ac base voltage is a small enough sine wave, the ac emitter current will also be a small sine wave with no noticeable stretching or compression of half-cycles.
Why is this important?
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Analyz ing an ampl i f ie r
Ran Yang || [email protected]
• The simplest way to analyze an amplifier is to split the analysis into two parts: a dc analysis and an ac analysis.
• we mentally open all capacitors. The circuit that remains is the dc-equivalent circuit.
• The most important current in the dc analysis is the dc emitter current.
The DC-Equivalent Circuit
• Reducing the dc voltage source to zero is equivalent to shorting it.
• Therefore, to calculate the effect of the ac source, we can short the dc voltage source.
• From now on, we will short all dc voltage sources when analyzing the ac operation of an amplifier.
• this means that each dc voltage supply point acts like an ac ground.
AC Effect of a DC Voltage Source
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Analyz ing an ampl i f ie r
Ran Yang || [email protected]
The circuit will get complicated quickly, another separated class
• 𝐴𝑣 =𝑣𝑜𝑢𝑡
𝑣𝑖𝑛=
𝑖𝑜𝑢𝑡𝑟𝑜𝑢𝑡
𝑖𝑖𝑛𝑟𝑖𝑛
• 𝑣𝑖𝑛 = 𝑖𝑏𝛽𝑟𝑒′
• 𝑣𝑜𝑢𝑡 = 𝑖𝑐 𝑅𝐶 ∥ 𝑅𝐿 = 𝑖𝑏𝛽 𝑅𝐶 ∥ 𝑅𝐿
• 𝐴𝑣 =𝑅𝐶∥𝑅𝐿
𝑟𝑒′
AC Voltage gain (Skipping Equation deriving)
• 𝐴𝑣 =𝑟𝑐
𝑟𝑒′
Define 𝑟𝑐 = 𝑅𝐶 ∥ 𝑅𝐿
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Analyz ing an ampl i f ie r
Ran Yang || [email protected]
• 𝐴𝑣 =𝑟𝑐
𝑟𝑒′
Define 𝑟𝑐 = 𝑅𝐶 ∥ 𝑅𝐿
Back to our first amplifier
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The l oad i ng e f fec t o f i npu t impedance
Ran Yang || [email protected]
• an ac voltage source vg has an internal resistance of RG.
• When the ac generator is not stiff, some of the ac source voltage is dropped across its internal resistance. As a result, the ac voltage between the base and ground is less than ideal.
• The ac generator has to drive the input impedance of the stage zin(stage).
• This input impedance includes the effects of the biasing resistors R1 and R2, in parallel with the input impedance of the base zin(base).
Input Impedance
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The l oad i ng e f fec t o f i npu t impedance
Ran Yang || [email protected]
• When the generator is not stiff, the ac input voltage vin is less than vg.
• With the voltage-divider theorem, we can write:
Input voltage
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What e l se do we need do i n t he lab?
Ran Yang || [email protected]
Our function generator doesn’t provide very small signal
We can use the built-in -20dB attenuator to make the signal 10 times smaller
The lowest I can get out of our function generator is 200mV, not small enough
We need to build an attenuator (resistor network) to make the signal another 10 to 20 times smaller
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Di s c re te v s I n t egra ted C i r c u i t s
Ran Yang || [email protected]
• Discrete means that all components, such as resistors, capacitors, and transistors, are separately inserted and connected to get the final circuit.
• A discrete circuit differs from an integrated circuit (IC), in which all the components are simultaneously created and connected on a chip, a piece of semiconductor material.
• Later chapters will discuss the op amp, an IC amplifier that produces voltage gains of more than 100,000
The standard way to build a discrete transistor amplifier.
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What have we learned today
Ran Yang || [email protected]
Coupling capacitor
Bypass capacitor
VDB Common Emitter amplifier
Loading effect
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What to study for next time?Chapter 16
Ran Yang || [email protected]
Operational Amplifiers
741
Inverting op-amps
Noninverting op-amps
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