BJT Configurations Voltage  Gain

Current  Gain

Power  Gain

X

X

X

X

X

Common Emitter Common Collector Common Base

X

Common emitter: hgh input impedance, for general amplification of voltage, current and power from low power, high impedance sources.

• LTspice • Bipolar Junction Transistor

Common collector: aka "emitter follower" for high input impedance and current gain without voltage gain, as in an amplifier output stage. Common base: low input impedance for low impedance sources, for high frequency response. Grounding the base short circuits the Miller capacitance from collector to base and makes possible much higher frequency response.

Acnowledgements: Neamen, Donald: Microelectronics Circuit Analysis and Design, 3rd Edition

6.101 Spring 2014

X

Lecture 3

1

6.101 Spring 2014

Lecture 3

General Configuration

2

Transistor Configurations TRANSISTOR AMPLIFIER CONFIGURATIONS

Common Emitter

+15V

RL

+15V

R2

RL

R2

+

R2

+15V

+ +

Vin

Common Base

Common Collector

6.101 Spring 2014

R1 RE

VOUT

-

[a] Common Emitter Amplifier

Lecture 3

3

6.101 Spring 2014

-

+ Vin

+

+

R1

RE

-

VOUT -

-

[b] Common Collector [Emitter Follower] Amplifier

Lecture 3

+

+

+ Vin

+

VOUT

+

+

+

R1

RE

-

[c] Common Base Amplifier

4

Load Line – Operating Point

Variation of Collector Current with β Two Resistor Biasing

+20 V

R2

910

ICQ

+

2N3904

vout R1

91

• • •

Find  Vout open circuit voltage: 20V Find ICQ max = 20/(910 +91) = ~20ma Draw load line.  

IC 

BFC -

IC 

• • • •

6.101 Spring 2014

 F VB  0.7V  RB   F RE

Variation of Collector Current with Beta 6

Two Resistor One Resistor

 F 10.4  0.7V  22k   F 2200

For RE = 0, just choose Q at ½ VCC for  maximum swing. For RE > 0, set Q at ½ [VCC – VRE].  For ICQ = 10 mA, VRL = 9.1V,  VRE = 0.91V, VCE = 10V.   For ICQ = 10.5mA, VRL = 9.6V,  VRE = 0.96V, VCE = 9.5V

Lecture 3

5

6.101 Spring 2014

ib

3.7 mA

2.9 mA

50

4.0 mA

4.0 mA

100

4.2 mA

5.0 mA

200

4.3 mA

5.4 mA

300

IC=0.6 mA

IC=2.5 mA

Lecture 3

6

IC

F

3.7 mA

50

4.0 mA

100

4.2 mA

200

4.3 mA

300

The base is connected to the emitter through with R3 and C2 . Since the AC gain between the base and the emitter is almost 1, the voltage at both ends of the resistor R3 is almost the same [the capacitor is a short circuit at signal frequencies]. In real life, the voltage gain is say 0.995 from base to emitter. The AC current through R3 is therefore (1−0.994) ÷ 4.7kΩ = 1.1 µA.

IC=0.6 mA

33K

IC

Bootstrapping*

Base Current – Resistor Divider

68K

F

IC

Result: stiff biasing with high input resistance at signal frequency.

Make ib small compared to the current through R2

*Horowitz and Hill Figure 2.65

See handout: Transistor bias stability 6.101 Spring 2014

Lecture 3

7

6.101 Spring 2014

Lecture 3

8

Common Emitter with Emitter Degeneration

Commom Emitter – Hybrid π TRANSISTOR AMPLIFIER CONFIGURATIONS WITH HYBRID- EQUIVALENT CIRCUITS COMMON EMITTER AMPLIFER

 0 g m r

+15V

RL

RB C

2N3904

+

gm 

IC

vout + vin _

VTH  26mv

R’s=RB // RS

+

IB

Rs

I CQ VTH

_

vout   o ib RL   o RL   ; vin ib R 's  r   o  1RE  R 's  r   o  1RE

v

Av 

g m v

if R 's  r   o  1RE ;

then Av  RL / RE

R’s=RB // RS b

c + r

Rs + vin

ib

RB

RL

vout   oib RL   o RL   vin ib R's  r  R's  r

if R'S r ; then Av 

e _

_

6.101 Spring 2014

vout

Av 

Lecture 3

  o RL

o

• • •

  g m RL

gm

9

6.101 Spring 2014

Input resistance (β+1)RE Voltage  gain reduced by (1+gm RE) Voltage gain less dependent on β (linearity)

Lecture 3

10

AC Coupled vs DC Coupled Amplifiers

Gain vs Frequency

• AC Coupling – Advantage: easy cascading with DC blocking  capacitor, bias stability and stage independent – Disadvantage: lot’s of R’s  and C’s, no DC gain,  need large C for low freqency

• DC coupling – Same gain at DC – Fewer R’s C’s

6.101 Spring 2014

Lecture 3

11

6.101 Spring 2014

Lecture 3

12

Expanded Hybrid π

Miller Effect* – Common Emitter

rx

CM  C [1  g m ( RC RL )] * Agarwal & Lang Foundations of Analog & Digital Electronics Circuits p 861 6.101 Spring 2014

Lecture 3

13

6.101 Spring 2014

log scale

 q  g m    IC  kT  0  h fe (datasheet)

AV (dB)

R C

V2

0 -3dB

slope = -6 dB / octave slope = -20 dB / decade

C  Cob (datasheet) gm  fT (transit frequency datasheet) 2 (C  C )

log f

1 1  j XC V j C   Av  2  j RC  1 V1 R   j X C R  1 j C Av 

1 sRC  1

High frequency cutoff f hi 

fHI or f-3dB Degrees

0o

1 2RC

gm  C 2 fT r rx (low frequency) : datasheet or estimate 50 100

C 

PHASE LAG

-45o -90o

(high frequency) : estimate  25 fHI or f-3dB

6.101 Spring 2014

Lecture 2

14

Hybrid‐π Parameters

Low Pass Filter LPF

V1

Lecture 3

log f

15

6.101 Spring 2014

Lecture 3

16

2N3904 CE configuration, VCC +15v

β

Use max for worst case cu

6.101 Spring 2014

Lecture 3

17

6.101 Spring 2014

hfe and High Frequency Limits

Lecture 3

18

Common Base Configuration

Small signal current gain versus frequency, hfe, of a BJT biased in a common emitter configuration: ib 

vbe  vbe jC r

h fe 

g m vbe g m r    ib 1  jr C 1  jr C

For hfe =1 = fT, (transit frequency )

hT 

gm where C  (c  c ) 2C

For 2N3904*, IC =1ma, VCE=10V , cπ=25pF, cμ=2pF

fT 

0.04mho  240 MHz 2 27 pF

for a gain of g m RL  100 f h 

1 1   320kHz 2 r g m RL c 2 2.5 K(100)2 pF

Miller effect reduces high frequency limit! *Lundberg, Kent: Become One with the Transistor p29 6.101 Spring 2014

Lecture 3

19

6.101 Spring 2014

Lecture 3

20

Common Collector – Emitter Follower Biasing

Common Collector (Emitter Follower)  0 g m r gm 

I CQ VTH

+15V

7.5 mA

VTH  26mv

R2 2N3904

1.0 k

7.5 mA

B

R’s=RB // RS Av 

v g m v

 o  1 RE  o  1ib RE vout   ; vin ib R 's  r   o  1RE  R's  r   o  1RE

if R's  r   o  1RE ;

• •

6.101 Spring 2014

21

Common Collector – Emitter Follower Biasing

7.5 mA R2

IDivider A

8.1 V

2N3904

R1

1.0 k B

7.5 mA 2N3904

Buffer with unity gain High input resistance driving low  output resistance (current gain).

Lecture 3

+15V

+15V

then Av  1

•

With R1 = 24kΩ,  R2 = 16 kΩ, the current  through the voltage divider is 15 ÷ [40  kΩ] = 375 µA.

•

The 75 µA base current is 20% of 375 µA.

•

With R1 = 2 kΩ, will need a divider  current that is ~ 4.1 mA. (75 µA is only  ~2% of 4.1 mA, which is negligible)

•

The voltage drop across R2 will be [15 V – 8.1 V] = 6.9 V;  R2 = 1.7 kΩ

•

But input impedance will be low = ~890Ω

•

Use bootstrapping configuration

IB RB 7.5 V

VB

R1    R1  R2 

RB = R1||R2,   VB = 15

VB = IBRB + 0.6V + 7.5V VB = [75 µA x 10k] + 0.6V + 7.5V VB = 750 mV + 0.6V + 7.5V VB = 8.9V [15 R1] ÷ [R1 + R2] = 8.9V 15 R1 = 8.9 x [R1 + R2] [15−8.9] R1 = 8.9 R2 R1 = 1.44 R2 [R1 x R2] ÷ [R1 + R2] = 10 kΩ [1.44R2 x R2] ÷ [1.44 R2 + R2] = 10kΩ R2 = 16.9 kΩ (use 16 kΩ) R1 = 1.44 R2 = 24.4 kΩ  (use 24 kΩ)

6.101 Spring 2014

Lecture 3

22

Low Frequency Hybrid‐ Equation Chart

7.5 mA

High gain applications Moderate input resistance High output resistance

= 24.4 kΩ  (use 24 kΩ)

6.101 Spring 2014

Β = 100, iB = 7.5ma/100 =‐ 75µa Using Thevenin equivalent,  

A

R1

• •

Lecture 3

23

6.101 Spring 2014

Lecture 3

Unity gain, low output resistance High input resist.

High gain, better high frequency response Low input resistance

24

Lab 2 •

Hands on introduction to  – diodes, zener diodes – bjt – operational amplifiers (op‐amps) – power supplies

•

Lab instruments – current tracer – HP 428B Current Probe/Conduction Angle

•

Size resistor wattage accordingly!

•

Be careful with electrolytic capacitors!

6.101 Spring 2014

Lecture 3

25