9. Discrete Transistor Amplifiers Lecture notes: Sec. 6 Sedra & - - PowerPoint PPT Presentation

▶
9 discrete transistor amplifiers
SMART_READER_LITE
LIVE PREVIEW

9. Discrete Transistor Amplifiers Lecture notes: Sec. 6 Sedra & - - PowerPoint PPT Presentation

Apr 08, 2024 283 likes •582 views

9. Discrete Transistor Amplifiers Lecture notes: Sec. 6 Sedra & Smith (6 th Ed): Sec. 5.6, 5.8, 6.6 & 6.8 Sedra & Smith (5 th Ed): Sec. 4.6, 4.8, 5.6 & 5.8 ECE 65, Winter2013, F. Najmabadi How to add signal to the bias Bias &

slide-1
SLIDE 1
  • 9. Discrete Transistor Amplifiers

Lecture notes: Sec. 6 Sedra & Smith (6th Ed): Sec. 5.6, 5.8, 6.6 & 6.8 Sedra & Smith (5th Ed): Sec. 4.6, 4.8, 5.6 & 5.8

ECE 65, Winter2013, F. Najmabadi

slide-2
SLIDE 2

How to add signal to the bias

  • F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (2/42)

Bias & Signal vGS = VGS + vgs Bias & Signal vDS = VDS + vds

  • 1. Direct Coupling
  • Use bias with 2 voltage supplies
  • For the first stage, bias such that

VGS = 0

  • For follow-up stages, match bias

voltages between stages

  • Difficult biasing problem
  • Used in ICs
  • Amplifies “DC” signals!
  • 2. Capacitive Coupling
  • Use a capacitor to separate bias

voltage from the signal.

  • Simplified biasing problem.
  • Used in discrete circuits
  • Only amplifies “AC” signals
slide-3
SLIDE 3

Capacitive coupling is based on the fact that capacitors appear as open circuit in bias (DC)

  • F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (3/42)
  • At a high enough frequency, Zc = 1/ (ωC), becomes small (effectively, capacitors

become short circuit).

  • Mid-band parameters of an Amplifier.*
  • At low frequencies, Zc cannot be ignored. As Zc depends on frequency, amplifier is

NOT linear (for an arbitrary signal) for these low frequencies. (We do NOT want to

  • perate the amplifier in these frequencies!)
  • Capacitors introduce a lower cut-off frequency for an amplifier (i.e., amplifier

should be operated above this frequency).

In ECE102, you will see that transistor amplifiers also have an “upper” cut-off frequency

Real Circuit Bias Circuit Signal Circuit

slide-4
SLIDE 4

How to Solve Amplifier Circuits

  • F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (4/42)

1. Find Bias and Signal Circuits. 2. Bias circuit (signal = 0):

  • Capacitors are open circuit.
  • Use transistor large-signal model to find the bias point.
  • Use bias parameters to find small-signal parameters (rπ , gm , ro ).

3. Signal Circuit (IVS becomes short, ICS becomes open circuit):

  • Assume capacitors are short to find mid-band amplifier parameters.
  • Replace diodes and/or transistors with their small-signal model.
  • Solve for mid-band amplifier parameters (Av , Ri , Ro ).
  • For almost all circuits, we can use fundamental amplifier configurations,

instead of solving signal circuits.

  • Include impedance of capacitors to find the lower cut-off frequency
  • f the amplifier.
slide-5
SLIDE 5

Emitter-degeneration bias circuits have similar signal circuits

  • F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (5/42)

Bias with one power supply (voltage divider) Bias with two power supplies The same circuit for

2 1 || B B B

R R R =

Signal Circuits

We will solve this configuration

slide-6
SLIDE 6

By-pass capacitors

  • F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (6/42)

Basic CE Configuration

  • There is no RE in the basic Common-Emitter

configuration.

  • However, RE is necessary for bias in discrete circuits.
  • Use a by-pass capacitor

Real Circuit Bias Circuit: Cap is open, RE stabilizes bias Signal Circuit: Capacitor shorts RE

slide-7
SLIDE 7

Discrete Common-Emitter Amplifier

  • F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (7/42)

Standard Bias Circuit:* Caps are open circuit Real Circuit CE amplifier: Input at the base Output at the collector

* Bias calculations are NOT done here as we have done them before.

slide-8
SLIDE 8

Signal circuit of the discrete CE Amplifier

  • F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (8/42)

Real Circuit Short caps Zero bias supplies Rearrange

slide-9
SLIDE 9

Discrete CE Amplifier (Gain)

  • F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (9/42)

) || || (

L C

  • m

i

  • R

R r g v v − =

Fundamental CE configuration  Signal input at the base  Signal output at the collector  No RE ) || (

L

  • m

i

  • R

r g v v ′ − =

L C L

R R R || = ′

slide-10
SLIDE 10

Discrete CE Amplifier (Ri)

  • F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (10/42)

||

π

r R R

B i =

|

π

r R R

CE i

= = Fundamental CE configuration

π

r R =

slide-11
SLIDE 11

Discrete CE Amplifier (Ro)

  • F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (11/42)

1) Set vsig = 0

||

  • C
  • r

R R =

=

π

v

Controlled current source becomes open circuit because gm vπ = 0

2) Replace transistor with its SSM

slide-12
SLIDE 12

Discrete CE and CS Amplifiers

  • F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (12/42)

|| ) || || (

  • D
  • G

i L D

  • m

i

  • r

R R R R R R r g v v = = − =

i

  • sig

i i sig

  • v

v R R R v v × + = || || ) || || (

  • C
  • B

i L C

  • m

i

  • r

R R r R R R R r g v v = = − =

π

∞ → π r

slide-13
SLIDE 13

Discrete CS Amplifier with RS

  • F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (13/42)

Real Circuit

Signal Circuit

Short caps Zero bias supplies CS amplifier with RS Input at the gate Output at the drain

Bias Circuit

Caps open

slide-14
SLIDE 14

Discrete CS Amplifier with RS (Gain)

  • F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (14/42)

/ ) || ( 1 ) || (

  • L

D S m L D m i

  • r

R R R g R R g v v + + − = Fundamental CS configuration with RS

L D L

R R R || = ′  Signal input at the gate  Signal output at the drain  RS ! / 1

  • L

S m L m i

  • r

R R g R g v v ′ + + ′ − =

slide-15
SLIDE 15

Discrete CS Amplifier with RS (Ri)

  • F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (15/42)

G i

R R =

|

/

∞ = =

RS CS i

R R ∞ = R Fundamental CS configuration with RS

slide-16
SLIDE 16

Discrete CS Amplifier with RS (Ro)

  • F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (16/42)

1) Set vsig = 0  Since ig = 0, vg = 0 and gate is grounded 2) Replace transistor with its SSM

slide-17
SLIDE 17

Discrete CS Amplifier with RS (Ro)

  • F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (17/42)

[ ]

) 1 ( ||

S m

  • D
  • R

g r R R + =

 Attach vx , compute ix (Ro = vx /ix )

S

  • m
  • x

y S

  • m
  • y

x S y S y

  • m
  • y

x S y

  • gs

m y x S y gs

R r g r v i R r g r i v R i R i r g r i v R i r v g i v R i v ) 1 ( ] ) 1 ( [ ) ( ) ( + + = + + = + − − = + − = − = KVL: )] 1 ( [ || ] [ || ] ) 1 ( [ || ) 1 (

S m

  • D

x S

  • m
  • D

x x S

  • m
  • D

x x S

  • m
  • x

D x y D x x

R g r R v R r g r R v i R r g r R v i R r g r v R v i R v i + = + ≈ + + = + + + = + = KCL: By KCL

slide-18
SLIDE 18

Discrete CE and CS Amplifiers with RE / RS

  • F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (18/42)

[ ]

) 1 ( || / ) || ( 1 ) || (

S m

  • D
  • G

i

  • L

D S m L D m i

  • R

g r R R R R r R R R g R R g v v + = = + + − =

i

  • sig

i i sig

  • v

v R R R v v × + =                 + + + =       + + + = + + + − =

sig B E E

  • C
  • L

C E E B i E

  • L

C E m L C m i

  • R

R R r R r R R r R R R R r R R r R r R R R g R R g v v || 1 || ] / ) || [( 1 || ) / 1 ]( / ) || [( 1 ) || (

π π π

β β

slide-19
SLIDE 19

Discrete CB Amplifier

  • F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (19/42)

Real Circuit

Signal Circuit

Short caps Zero bias supplies CB amplifier Input at the gate Output at the drain

Bias Circuit

Caps open Capacitor CB is necessary. Otherwise, Amp gain drops substantially.

slide-20
SLIDE 20

Discrete CB Amplifier (Gain)

  • F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (20/42)

Fundamental CB Configuration

L C L

R R R || = ′  Signal input at the emitter  Signal output at the collector

) || || (

L C

  • m

i

  • R

R r g v v + =

) || (

L

  • m

i

  • R

r g v v ′ + =

slide-21
SLIDE 21

Discrete CB Amplifier (Ri)

  • F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (21/42)

1 ) || ( || ||       + + =

  • m

L C

  • E

i

r g R R r r R R

π

1 || |

  • m

L

  • CB

i

r g R r r R R + ′ + = =

π

Fundamental CB configuration

1 ||

  • m

L

  • r

g R r r R + ′ + =

π

slide-22
SLIDE 22

Discrete CB Amplifier (Ro)

  • F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (22/42)

1) Set vsig = 0 2) Replace transistor with its SSM

slide-23
SLIDE 23

Discrete CB Amplifier (Ro)

  • F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (23/42)

{ }

)] || || ( 1 [ ||

sig E m

  • C
  • R

R r g r R R

π

+ =

 Attach vx , compute ix (Ro = vx /ix )

1 1 1 1 1 1 1

) 1 ( ] ) 1 ( [ ) ( ) ( || || R r g r v i R r g r i v R i R i r g r i v R i r v g i v R i v r R R R

  • m
  • x

y

  • m
  • y

x y y

  • m
  • y

x y

  • m

y x y sig E

+ + = + + = + − − = + − = − = =

π π π

KVL: )] 1 ( [ || ] [ || ] ) 1 ( [ || ) 1 (

1 1 1 1

R g r R v R r g r R v i R r g r R v i R r g r v R v i R v i

m

  • C

x

  • m
  • C

x x

  • m
  • C

x x

  • m
  • x

C x y C x x

+ = + ≈ + + = + + + = + = KCL: By KCL

slide-24
SLIDE 24

Discrete CB and CG Amplifiers

  • F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (24/42)

i

  • sig

i i sig

  • v

v R R R v v × + =

{ }

)] || || ( 1 [ || 1 ) || ( || || ) || || (

sig E m

  • C
  • m

L C

  • E

i L C

  • m

i

  • R

R r g r R R r g R R r r R R R R r g v v

π π

+ =       + + = + =

{ }

)] || ( 1 [ || 1 ) || ( || ) || || (

sig S m

  • D
  • m

L D

  • S

i L D

  • m

i

  • R

R g r R R r g R R r R R R R r g v v + =       + + = + = ∞ → π r

slide-25
SLIDE 25

Discrete CD Amplifier (Source Follower)

  • F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (25/42)

Real Circuit

Signal Circuit

Short caps Zero bias supplies CD amplifier Input at the gate Output at the source

Bias Circuit

Caps open

slide-26
SLIDE 26

Discrete CD Amplifier (Gain)

  • F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (26/42)

Fundamental CD Configuration  Signal input at the gate  Signal output at the source

) || || ( 1 ) || || (

L S

  • m

L S

  • m

i

  • R

R r g R R r g v v + =

L S L

R R R || = ′ ) || ( 1 ) || (

L

  • m

L

  • m

i

  • R

r g R r g v v ′ + ′ =

slide-27
SLIDE 27

Discrete CD Amplifier (Ri)

  • F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (27/42)

G i

R R =

| ∞ = =

CD i

R R ∞ = R Fundamental CD Configuration

slide-28
SLIDE 28

Discrete CD Amplifier (Ro)

  • F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (28/42)

1) Set vsig = 0 2) Replace transistor with its SSM 3) Attach vx , compute ix (Ro = vx /ix )

  • m

S

  • m

S x x

  • x

m x S x x

  • x

gs m S x x x gs

r g R R r g R v i r v g v R v i r v v g R v i v v || ) / 1 ( || || ) / 1 ( || / 1 = = + + = + − = − = KCL:

* Because 1/gm << ro

m S

  • g

R R 1 || ≈

 Since ig = 0, vg = 0 and gate is grounded

slide-29
SLIDE 29

Discrete CC and CD Amplifiers

  • F. Najmabadi, ECE65, Winter 2013, Discrete Amplifiers (29/42)

|| 1 || ) || || ( 1 ) || || (

  • m

S

  • G

i L S

  • m

L S

  • m

i

  • r

g R R R R R R r g R R r g v v = = + =

i

  • sig

i i sig

  • v

v R R R v v × + =

[ ]

  • sig

B E

  • L

E

  • B

i L E

  • m

L E

  • m

i

  • r

R R r R R R R r r R R R R r g R R r g v v || || || ) || || ( || ) || || ( 1 ) || || ( β β

π π

+ = + = + =