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12-1

EE105 – Fall 2015 Microelectronic Devices and Circuits

Prof. Ming C. Wu

wu@eecs.berkeley.edu

511 Sutardja Dai Hall (SDH)

12-2

Basic Single-Transistor Amplifier Configurations

MOSFET

BJT

2

12-3

Two-Port Model of Amplifiers

Gv =Rin

Rsig + RinAv0

RLRo + RL

In addition to gain, it's important to have properinput and output resistances.

Procedure to find Ro :Ground input, remove Rsig

Apply a test current sourceat output (conceptually, not experimentally),find voltage at output terminal.

R0 =vxix

12-4

Common-Source (CS) Amplifier

Using hybrid-pi model,it is almost trivial to solve:Av = −gmRD

With load resistance, RL :Since RL is in paralle with RDGv = −gm RD || RL( )Rin =∞R0 = RD

Previously, using MOSFET equation(on p. 11-4), we derived analytically :AV = −knVOVRD(The process is combersome, tedious,and will go quickly out of hand with more complex circuits)

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12-5

Common-Emitter (CE) Amplifier Again, using hybrid-pi model,it can be easily solved:Av = −gmRCWith load resistance, RL :Since RL is in paralle with RD

Gv = −gm RC || RL( ) rπRsig + rπ

Rin = rπRo = RC

12-6

T-Equivalent Circuit Model for MOSFET (without ro)

No change in circuit since the additional dependent current source has the same current

No change in circuit since no current will flow through the new connected path

Same current with the dependent current source replaced by equivalent resistance

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12-7

CS with Source Resistance (Source Degeneration)

You can analyze the circuit usinghybrid-pi model. (Try it !)However, whenever there is a resistor connected to source, it is much easier to use the "T-model"

v0 = −iRD

vi = i1gm

+ RS"

#$

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&'

Av = −RD

1gm

+ RS= −

gmRD1+ gmRS

Grain reduced by 1+ gmRS( ) compared with standard CS amplifierRin =∞Ro = RD

12-8

CS with Source Resistance (Source Degeneration)

Note :

Av = −RD

1gm

+ RS= −

Total resistance in DrainTotal resistance in Source

RS provides negative feedback, which(1) stablize drain current(2) increse linearity by keeping vgs small

vgs = vi

1gm

1gm

+ RS= vi

11+ gmRS

(3) Increase usable bandwidth

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12-9

T-Model for BJT Hybrid-pi Model

T-Model without r0

T-Model with r0

12-10

CE with Emitter Resistance (Emitter Degeneration)

v0 = −icRCvi = ie 1/ gm + Re( )

Av = −RC

1/ gm + Reicie≈ −α

gmRC1+ gmRe

where α = icie=

ββ +1

(usually α ≈1)

Note: Rin is boosted signficantly:

Rin =ie 1/ gm + Re( )

ib= β +1( ) 1

gm+ Re

#

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&

'(

Rin ≈ rπ + β +1( )ReRo = RC

Gv = −αgmRC

1+ gmReRin

Rsig + Rin

#

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&

'((

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12-11

CE with Emitter Resistance (Emitter Degeneration)

Effect of Re (called "Emitter Degeneration"):(1) increase input resistance by (1+ gmRe )(2) reduce voltage gain by (1+ gmRe )(3) reduce vbe by (1+ gmRe ) → lower nonlinear distortion(4) voltage gain less dependent on β(5) improved high frequency response

12-12

Common-Gate (CG) Amplifier CG Amplifier CG Amplifier with T-Model

Rsig connected to source→Use the "T-model" v0 = −iRDvi = −i(1 / gm )

Av =RD

1/ gm= gmRD

Rin =1/ gm

vi = vsigRin

Rsig + Rin= vsig

1/ gmRsig +1/ gm

Gv = gmRD1/ gm

Rsig +1/ gm=

RDRsig +1/ gm

Ro = RD

Low input resistance

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12-13

Common-Base (CB) Amplifier

Rsig connected to Emitter→Use the "T-model" v0 = −iRCvi = −i(1 / gm )

Av =RC

1/ gm= gmRC

Rin =1/ gm

vi = vsigRin

Rsig + Rin= vsig

1/ gmRsig +1/ gm

Gv = gmRC1/ gm

Rsig +1/ gm=

RCRsig +1/ gm

Ro = RC

Low input resistance

12-14

Unity-Gain Buffer Voltage Amplifier Driving low impedance load directly:

Driving low impedance load with unity-gain buffer:

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12-15

Common-Drain (CD) Amplifier (Source Follower)

Source Follower Source Follower with T-Model

RL connected to source→Use the "T-model"

v0 = v0RL

RL +1/ gm

Av =RL

RL +1/ gm→1 when RL >>1/ gm

Rin =∞

Gv =RL

RL +1/ gmRo =1/ gm Low output resistance

12-16

Common-Collector (CC) Amplifier (Emitter Follower)

Emitter Follower Emitter Follower with T-Model

RL connected to Emitter→Use the "T-model"

v0 = v0RL

RL +1/ gm

Av =RL

RL +1/ gm→1 when RL >>1/ gm

Rin = (β +1)(RL +1/ gm )

Gv =RL

RL +1/ gm(β +1)(RL +1/ gm )

Rsig + (β +1)(RL +1/ gm )Ro =1/ gm Low output resistance

High input resistance

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12-17

Input and Output Resistance of Emitter Follower

Intuition:

•  Resistance on emitter side is multiplied by (1+β) when “seen” from base (input side)

–  High input resistance

•  Resistance on base side is divided by (1+β) when “seen” from emitter side

–  Low output resistance

12-18

Comparison of Different Amplifier Configurations

•  MOS has higher input (∞) impedance

•  BJT has higher gain (gm)

•  IC uses MOS, discrete circuits favors BJT

•  CS / CE provides the bulk of the gain

•  CD / CC used as voltage buffer in output stage

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12-19

Summary of MOSFET Amplifiers

12-20

Summary of BJT Amplifiers