lecture vi - physicsphysics.wm.edu/~ran/pages/fc/electronics lecture 6.pdf · 2020. 2. 28. ·...
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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
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
Midterm Exam
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• 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
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
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Coupl i ng Capac i tor
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• 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
A s imp le base -b ia sed amp l i f i e r
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• 𝑉𝑖𝑛• 𝑉𝐵𝐵• 𝑉𝐶• 𝑉𝑜𝑢𝑡• 𝑉𝐸
Waveform
VDB ampl i f ie r
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Calculate the dc voltages and currents
• Take a look at the simulation in Multisim
VDB waveform
Smal l S igna l Operat ion
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• 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
Smal l S igna l Operat ion
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• 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?
Analyz ing an ampl i f ie r
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• 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
Analyz ing an ampl i f ie r
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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
Analyz ing an ampl i f ie r
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• 𝐴𝑣 =𝑟𝑐
𝑟𝑒′
Define 𝑟𝑐 = 𝑅𝐶 ∥ 𝑅𝐿
Back to our first amplifier
The l oad i ng e f fec t o f i npu t impedance
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• 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
The l oad i ng e f fec t o f i npu t impedance
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• 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
What e l se do we need do i n t he lab?
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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
Di s c re te v s I n t egra ted C i r c u i t s
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• 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.
What have we learned today
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Coupling capacitor
Bypass capacitor
VDB Common Emitter amplifier
Loading effect
What to study for next time?Chapter 16
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Operational Amplifiers
741
Inverting op-amps
Noninverting op-amps
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