7. Fundamental Transistor Amplifier Configurationsaries.ucsd.edu/NAJMABADI/CLASS/ECE65/13-W/Slides/ECE65_W13-7...7. Fundamental Transistor Amplifier Configurations ... oFocus on fundamental

7. Fundamental Transistor Amplifier Configurations

Lecture notes: Sec. 5

Sedra & Smith (6th Ed): Sec. 5.4, 5.6 & 6.3-6.4 Sedra & Smith (5th Ed): Sec. 4.4, 4.6 & 5.3-5.4

ECE 65, Winter2013, F. Najmabadi

Issues in developing a transistor amplifier:

F. Najmabadi, ECE65, Winter 2013, Fundamental Amp Configuration (2/26)

1. Find the iv characteristics of the elements for the signal (which can be different than their characteristics equation for bias). o This will lead to different circuit configurations for bias versus signal

2. Compute circuit response to the signal o Focus on fundamental transistor amplifier configurations

3. How to establish a Bias point (bias is the state of the system when there is no signal). o Stable and robust bias point should be resilient to variations in µnCox (W/L),Vt (or β for BJT) due to temperature and/or manufacturing variability.

o Bias point details impact small signal response (e.g., gain of the amplifier).

What are amplifier parameters?


:Circuit theofGain Voltage sig

o

vvA =

∞→

=LRi

ovo v

vA :Gain loop-Open

:ResistanceInput i

ii i

vR =

0

: Amplifier of ResistanceOutput →

−=sigvo

oo i

vR

Output resistance is the Thevenin resistance between the output terminals!

:Amplifier theofGain Voltage i

ov v

vA =

In general Ri depends on RL and Ro depends on Rsig

Observations on the amplifier parameters


Avo is the maximum possible gain of the amplifier.

Value of Ro is important. o For Ro << RL , Av ≈ Avo

o For Ro = RL , Av = 0.5 Avo

o For Ro >> RL , Av ≈ 0 Prefer “small” Ro

vooL

L

i

ov A

RRR

vvA

+==

Value of Ri is important. o For Ri >> Ri , vi ≈ vsig

o For Ri = Rsig , vi = 0.5 vsig

o For Ri << Rsig , vi ≈ 0 Prefer “large” Ri

sigi

i

sig

i

RRR

vv

+=

vsigi

i

i

o

sig

i

sig

o ARR

Rvv

vv

vvA

+=×==

:Gain Overall

Some observation on single-transistor amplifiers


1. As we will discuss, there are many ways to bias a transistor. Thus, there are many practical single-transistor amplifier circuits.

o Fortunately, signal circuits always reduce to one of four fundamental configuration .

2. We compute the voltage gain and input resistance of these four fundamental configurations in the presence of an arbitrary load RL. Then:

3. Ro is calculated in a real circuit (with Rsig & vsig) once load is clearly identified.

vsigi

i

i

o

sig

i

sig

o ARR

Rvv

vv

vvA

+=×==

:Gain Overall

∞→= | :Gain loop-Open

LRvvo AA


Fundamental Transistor Amplifier Configurations

We are considering only signal circuit here!

Possible BJT amplifier configurations


Same as Common Base (vi does not change)

Common-Base Common-Collector

Common-Emitter with RE

Not Useful

Common-Emitter

PNP configurations are the same as those of NPN (because of similar small-signal model)


Common-Base Common-Collector Common-Emitter

Common-Emitter Common-Base Common-Collector

MOS fundamental configurations are analogous to BJTs


Common-Base Common-Collector Common-Emitter

Common-Source Common-Gate Common-Drain

Common Emitter Configuration


Signal Circuit:

Signal Circuit with BJT SSM: o ro and R’L are in parallel o vπ = vi

Common Emitter Configuration (Av & Ri)


ππ

rivR

rvi

i

ii

ii ==⇒=

)||(

)||(

Lomi

ov

Loimo

RrgvvA

Rrvgv

′−==

′−=

By KCL

Common Source Configuration


Signal Circuit:

Signal Circuit with MOS SSM: o ro and R’L are in parallel o vgs = vi

Common Source Configuration (Av & Ri)


0 ∞==⇒=i

iii i

vRi )||(

)||(

Lomi

ov

Loimo

RrgvvA

Rrvgv

′−==

′−=

By KCL

Common Source & Common Emitter Configurations are “similar”


Signal Circuit

Signal Circuit with transistor SSM

)||(

πrR

RrgvvA

i

Lomi

ov

=

′−==

)||(

∞=

′−==

i

Lomi

ov

R

RrgvvASimilar formula if

we let ∞→ πr

Note that Av & Ri are independent of vsig & Rsig

Common Emitter Configuration with RE


Signal Circuit:

Signal Circuit with BJT SSM:

Common Emitter Configuration with RE (Av & Ri)


)/1)(/(1

)/1)(/(1

π

ππ

π rRrRRrgrR

rRrRRgRg

vvA

EoL

Emi

EoLEm

Lm

i

ov +′+

+≈+′++

′−≈=

0)(

0)(

=−+−

+′

=−−−

+−

+

−=

eimo

eo

L

o

eimo

oeie

E

e

ei

vvgr

vvRv

vvgr

vvr

vvRv

vvv

π

π

Node voltage method:

Node ve

Node vo

1. Add the two node equations to get ve in terms of vo and vi

2. Substitute for ve in Node vo equation to find vo and gain

3. Compute ii in terms of node voltages. Then Ri = vi/ii

4. Lengthy calculations (See Notes).

Common Source Configuration with RS (Av & Ri)


Signal Circuit

Let

SE RRr

→∞→π

)/1)(/(1

)/1)(/(1

π

ππ

π

rRrRRrgrR

rRrRRgRg

vvA

EoL

Emi

EoLEm

Lm

i

ov

+′++≈

+′++′

−≈=

∞=

′++′

−≈=

i

oLSm

Lm

i

ov

RrRRg

RgvvA

)/(1

Common Base Configuration (Gain)


Signal Circuit:


)||(

)||(1

Lomv

Loo

om

i

ov

RrgA

Rrr

rgvvA

′≈

′+==

io

om

Lo

o

imo

io

L

o

i

vr

rgRr

v

vgr

vvRv

vv

+=

′

=−+−

+′

−=

1||

0)(

π


Node vo

Common Base Configuration (Ri)


1

|||| om

Loxi rg

RrrRrR+

′+== ππ

om

o

x

ix

Loxomi

i

Lxomxi

rgRr

ivR

Rrirgvvv

Rirvgiv

+′+

==

′+=+−=

′++=

1

)()1(

)(

π

πKVL:

xi

ii

x

i

x

iix

ii

x

ix

RrivR

Rrv

Rv

rvi

rvi

ivR

||

||

π

πππ

==

=+=+=

=Define

KCL:

By KCL

Common Gate Configuration (Av & Ri)


Signal Circuit

1

||

)||(

om

Loi

Lomi

ov

rgRrrR

RrgvvA

+′+

=

′≈=

π

Let ∞→ πr

1

)||(

om

Loi

Lomi

ov

rgRrR

RrgvvA

+′+

=

′≈=

Common Collector Configuration (Emitter Follower)


Signal Circuit:


)||(1

)||( Lom

Lom

i

ov Rrg

RrgvvA

′+′

==

imim

momLo

o

oimo

oio

L

o

oi

vgvrg

gvrgRr

v

vvgrv

rvv

Rv

vvv

≈

+=

++

′

=−−+−

+′

−=

ππ

π

π

1111||

0)(


Node vo

1>>= βπrgm

vi

ii

vioi

i

Ar

ivR

Arv

rvvi

−==

−×=−

=

1

)1(

π

ππ

)||()||( LoLomi RrrRrrgrR ′+=′+= βπππ

Common Drain Configuration (Source Follower)


Signal Circuit

)||()||(

)||(1

)||(

LoLomi

Lom

Lom

i

ov

RrRrrgRRrg

RrgvvA

′=′=

′+′

==

βπ

Let ∞→ πr∞=

′+′

==

i

Lom

Lom

i

ov

RRrg

RrgvvA

)||(1

)||(

BJT Basic Amplifier Configurations (PNP circuits are identical)


Common Emitter ),||( πrRRrgA iLomv =′−=

Common Base

om

LoiLomv rg

RrrRRrgA+

′+=′=

1|| ),||( π

Common Collector/ Emitter Follower

)||(

)||(1

)||(

Loi

Lom

Lomv

RrrRRrg

RrgA

′+=

′+′

=

βπ)/1)(/(1

)/1)(/(1

π

ππ

π

rRrRRrgrR

rRrRRgRgA

EoL

Emi

EoLEm

Lmv

+′++≈

+′++′

−≈

Common Emitter with RE

MOS Basic Amplifier Configurations (PMOS circuits are identical)


Common Source with RS ,

/1∞=

′++′

−= ioLSm

Lmv R

rRRgRgA

Common Drain/Source Follower

,)||(1

)||(∞=

′+′

= iLom

Lomv R

RrgRrgA

),||( ∞=′−= iLomv RRrgA

Common Source

Common Gate

om

LoiLomv rg

RrRRrgA+

′+=′=

1 ),||(

Observations of Transistor Amplifiers (1)


Common-Emitter has a high gain of and a “medium” (several k).

o Minus sign in the gain reflects a 180o phase shift in the output.

Common-Base also has a high gain of but a “low” Ri (several hundred Ω) which significantly affects the overall circuit gain.

)||( Lomv RrgA ′−= πrRi =

)||( Lomv RrgA ′≈

Common-Source has a high gain of (but lower than the BJT analog, CE amplifier). It has an infinite Ri.

Common-Gate also has a high gain of but a “low” Ri (several hundred Ω).

)||( Lomv RrgA ′−=

)||( Lomv RrgA ′≈

CE and CS configurations are the main gain cells in ICs. CB and CG configurations have superior high-frequency response (discussed in ECE102).

Observations of Transistor Amplifiers (2)


Common-Emitter with RE has a much lower gain compared to a CE amplifier (i.e., no RE ) but has a much larger Ri . o Amplifier gain is also much less sensitive to BJT parameters

(i.e., β).

o It is used primarily in discrete circuits because it does not need a by-pass capacitor (will be discussed later).

Common-Source with RS has a much lower gain compared to a CS amplifier (i.e., no RS ). It has an infinite Ri .

Common-Collector (emitter follower) and Common-Drain (source follower) configurations have a gain ≤ 1 . They have a large Ri (infinite for CD) and a low Ro (as we will see later). They are usually configured to get a gain close to 1 and used either as a “buffer” or as a “current amplifier” to drive a load.

*Buffers are discussed later in the context of multi-stage amplifiers

7. Fundamental Transistor Amplifier Configurationsaries.ucsd.edu/NAJMABADI/CLASS/ECE65/13-W/Slides/ECE65_W13-7...7. Fundamental Transistor Amplifier Configurations ... oFocus on fundamental

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