Discussion Notes – Week 5
Recall some key equations
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IC = IS
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IC = β IB IC = α IE … α =
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IB =
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IE = IC + IB … I E =
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gm =
(1+
=> VBE = VT ln
… IS =
IS
rπ =
IS
ro=
Bipolar Amplifiers
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Amplifier is a device that produces an output(voltage or current) that is a magnified
version of the input(voltage or current)
Input and output impedances are particularly important
o For an ideal voltage amplifier, input must act as a voltmeter – able to sense a
voltage without any disturbance. Therefore, ideal input impedance in infinite. At
the output, it must behave as a voltage source – delivering constant voltage to any
load impedance. Therefore, ideal output impedance is zero.
Finding the Input/Output Impedance
o 1) Set all of the independent sources to their equivalent zero => replace voltage
source with short circuit and current source with open circuit
o 2) Apply a test voltage(vx, small signal) and test current(ix, small signal) and
measure
Examples – HW4, Prob. 1(b), 2(c)
Biasing
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Proper biasing is required to ensure operation in the forward active region => small
signal parameters depend on the bias conditions
o 1) Perform DC analysis to find the operating point(VEB and VCE => under what
condition is it forward active region?) and small signal parameters
o 2) Set all sources to their zero equivalent and perform small signal analysis
Note: When iteration is required, it is relatively safe to assume VBE=0.7V to begin with
Resistive Divider Biasing
o Example 5.9 Razavi (use of Thevenin)
R
VCC => with assumption that the base current is negligible(relatively
o Vx =
R R
accurate since input impedance is typically extremely high)
o IC = IS
= IS
=> even though resistance does not vary much,
exponential dependence on resistance still makes it undesirable
o Multiple iteration often required
Biasing with Emitter Degeneration
o Example 5.10 Razavi (just the set-up)
o Presence of RE helps to absorb the error in VBE(variation in VBE results in a linear
potential drop across RE)
Self-biasing
o Collector voltage provides the necessary Vx and IB.
o Higher potential VC guarantees forward active region
Bipolar Amplifier Topologies – Common Emitter
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Voltage gain – Av = -gmRC – is negative
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The voltage gain of CE stage is limited by the supply voltage VCC
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Input/output impedance is found using identical method as above
Common Emitter Degeneration
o Resistor connected to the emitter of CE stage
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o Overall gain is reduced, but improvement in linearity and input impedance(Rin=rπ
=> Rin= rπ + (β+1)RE
Multiple Stage CE
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o We can reduce Q2 to its equivalent impedance using the method described above
o Example – HW4, Prob. 4 (a), (b)