Lecture 2) | Electronics II

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Electronics II

(ELE 343 – Lecture 2)

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Amplifier Example I

➢ The keys in solving this problem are recognizing the AC

ground between R1 and R2, and Thevenin transformation of

the input network.

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Current Sources

▪ Ideal Current Sources

II V

V

I

[I-V curve]

(Definition)

A current source is an electronic circuit that delivers or absorbs an electric current

which is independent of the voltage across it.

What is the output impedance for the current source?

➔ Ideally, it’s infinite.

What happens if the output impedance is not infinite?

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Current Sources

▪Why do we need a Current Source?

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Current Sources

▪Current source by using BJTs

➢ Ideally, IC does not depend on the collector to emitter voltage.

➢ This property allows the transistor to behave as a constant current source when its base-emitter voltage is fixed.

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➢ The claim that collector current does not depend on VCE isnot accurate.

➢ As VCE increases, the depletion region between base andcollector increases. Therefore, the effective base widthdecreases, which leads to an increase in the collectorcurrent.

➢ With Early effect, collector current becomes larger thanusual and a function of VCE.

Early Effect (Channel-length Modulation)

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Early Effect (Channel-length Modulation)

Early effect

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Early Effect (Channel-length Modulation)

▪Small Signal Model

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➢ With Early effect, collector current becomes larger than

usual and a function of VCE.

Early Effect (Channel-length Modulation)

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Output Impedance of Degenerated Stage

➢ Emitter degeneration boosts the output impedance by afactor of 1+gm(RE||r).

➢ This improves the gain of the amplifier and makes thecircuit a better current source.

➢ Rout?

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Boosted Output Impedances

➢ Pros & Cons of RE / RS

Pros

Large output impedance

➔ Being close to ideal CS

Cons

Narrow operating range

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Bipolar Cascode Stage

➢ Rout?

Degeneration Tr

Cascode Tr

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Example 9.1

➢ If Q1 and Q2 are biased at a collector current of 1 mA, determine the

output resistance.

Assume β = 100 and VA = 5 V for both transistors.

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Max. Bipolar Cascode Output Impedance

➢ The maximum output impedance of a bipolar cascode is

bounded by the ever-present r between emitter and ground

of Q1.

➢ Rout,max?

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Example: Output Impedance

➢ Typically, r is smaller than rO, so in general it is impossible

to double the output impedance by degenerating Q2 with a

resistor.

➢ We wish to increase the output resistance of the bipolar cascode

of Fig. 9.2(a) by a factor of two through the use of resistive

degeneration in the emitter of Q2. Determine the required value of

the degeneration resistor if Q1 and Q2 are identical.

➢ Rout and RoutA?

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Example: Mixed BJT/MOS Cascode

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1

➢ RoutA? ➢ RoutB?

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PNP Cascode Stage

Degeneration Tr

Cascode Tr

➢ Rout?

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Another Interpretation of Bipolar Cascode

➢ Instead of treating cascode as Q2 degenerating Q1, we can

also think of it as Q1 stacking on top of Q2 (current source)

to boost Q2’s output impedance.

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False Cascodes

➢ When the emitter of Q1 is connected to the emitter of Q2, it’s

no longer a cascode since Q2 becomes a diode-connected

device instead of a current source.

➢ Rout?

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MOS Cascode Stage

➢ Rout?

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Example: MOS Cascades

➢ Design an NMOS cascode for an output impedance of 500

kΩ and a current of 0.5 mA. For simplicity, assume M1 and

M2 are identical (they need not be).

Assume μnCox = 100 μA/V2 and λ = 0.1V−1.

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Another Interpretation of MOS Cascode

➢ Similar to its bipolar counterpart, MOS cascode can be thought ofas stacking a transistor on top of a current source.

➢ Unlike bipolar cascode, the output impedance is not limited by .

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Example: Parasitic Resistance

➢ RP will lower the output impedance, since its parallel

combination with rO1 will always be lower than rO1.

➢ During manufacturing, a large parasitic resistor RP has

appeared in a cascode as shown in the figure.

Determine the output resistance.

➢ Rout?