BJT Differential Pair - Weber State University€¦ · range of BJT diff pair is smaller than the MOS diff pair (2) It can be used for . fast switching (ECL logic) by current steering:

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ECE 3120 Microelectronics II Dr. Suketu Naik

Chapter 8Differential andMultistage Amplifiers

PART B: BJT Diff Pair with Passive Load

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Advantages:1)

2)

Disadvantages:1)

2)

3)

Differential Pair

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BJT Differential Pair

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8.3 BJT Differential PairBJT Differential Pair Two matched transistors (Q1

and Q2) joined and biased by a constant current source Iwhich is implemented by current mirror

Q1 and Q2 must stay in active region

How does the diff. pairwork?

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1) Input Common-mode Range

2) Large Signal Operation

3) Small Signal Operation

4) Common-mode Rejection Ratio (CMRR)

5) Input Offset Voltage

Important Parameters and Characteristics

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Input Common-Mode RangeDC Common-mode voltage (VCM)

What is the range of input voltage within which the transistors will stay in active region?

VCM_max= 0.4+VCC - 0.5αIRC

VCM_min= -VEE+VCS+VBE

VC

+VCS-

Min voltage need to keep the current mirror in active region

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Large Signal Operation(1) Note that the linear range of BJT diff pair is smaller than the MOS diff pair

(2) It can be used for fast switching (ECL logic) by current steering: e.g. current flows entirely in one branch then switches to the other branch; requires only 4VT

(3) The difference input signal, vid should be less than VT/2 to linear amplification

Current Steering

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Large Signal OperationHow to increase the linear range?

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Differential Gain=2 x single stage gain Ad

If the input differential voltage = vid /2 - (-vid/2)=vid

then Differential gain=Ad= |vod / vid |= gm(RC | |ro)

Small Signal Operation

e

idec

idmc

id

Tc

rvii

vgi

vVIi

2

2

22

αα

α

==

=

=

TT

Cm V

IVIg

2α

==

Bias voltage (DC)+ small signal (ac)

Virtual ground

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∆

−=

+∆

−=C

C

EE

C

eEE

Ccm R

RRR

rRRA

22α

( )

∆==

C

CEEm

cm RRRg

AA

CMRR d 2

CMRR is the ratio of differential gain over common-mode gain

Common-Mode Rejection Ratio (CMRR)CMRR: Figure-of-merit for noise rejection

𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪(𝒅𝒅𝒅𝒅) = 𝟐𝟐𝟐𝟐𝟐𝟐𝟐𝟐𝟐𝟐𝟏𝟏𝟐𝟐𝑨𝑨𝒅𝒅𝑨𝑨𝒄𝒄𝒄𝒄

𝑨𝑨𝒅𝒅 = −𝟐𝟐𝒄𝒄𝑪𝑪𝒄𝒄

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Input Offset Voltage

Apply a small voltage of opposite polarity to cancel the offset

Device mismatches cause a finite dc voltage at the output

VOS is smaller than MOS diff pair

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List of Problems

BJT Diff Pair

p8.34: input common mode range of BJT differential amplifier

ex8.13: BJT diff pair: differential gain, CMRR

p8.49 (simulate): design of BJT differential amplifier

p8.62 (simulation only): NPN differential amplifier

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BJT Differential Pair - Weber State University€¦ · range of BJT diff pair is smaller than the MOS diff pair (2) It can be used for . fast switching (ECL logic) by current steering: