6.012 - Electronic Devices and Circuits

Lecture 23 - The Cascode and the µA741 - Outline • Announcements

Handouts - Lecture Outline and Summary; CascodeDesign Problem - Due tomorrow

• Review - Bounding mid-bandOCTC/SCTC methods: ti = RiCi; estimate tHI as S ti (Largest t dominates)

wi = 1/RiCi; estimate wLO as Swj (Largest w dominates)

- The Miller effect: why C m and Cgd are so important The concept: a capacitor shunting a gain stage looks larger by (1 + |Av|) Examples: the Miller effect magnifies Cm and Cgd in CE/S stages

no significant Miller effect impact on CB/G or E/SF stages the Miller effect is used to stabilize the mA 741

• The Marvelous cascodeConcept and wHI: getting larger bandwidth from CE + CBOutput resistanceThe costs

• The µA741A nice context in which to look at: 1) applying the Miller effect; 2) a

bipolar chip; 3) dissecting a real multistage amplifier; 4) seeing some novel circuits in use (cascode, Darlington, push-pull, current mirror load, Widlar current source)

Clif Fonstad, 11/03 Lecture 23 - Slide 1

Q16

Q17

- 1.5V

+

-VBIASx + vins5

vOUT+

-+

-VBIASy + vins5

+ 1.5VFall 2003 Design Problem Analysis The fourth stage voltage gain: This follower stage sees a large load and has avoltage gain that is essentially 1: ª 1Av 4

The fifth stage voltage gain: Because this follower sees a load of only100 Ohms (RL) its gain may be less than RL

one. To analyze it we begin by recognizingthat Q16 and Q17 both see the same inputsignal, albeit riding on different DC biases:

The L.E.C. is: And the gain is:

(g+ g )v

m16

m17

+ + g 16 v go16 [gp16 + gp17 + gm16 + gm17 ] RL+ g 17 ª+ go17

Av 5-v inS5 1 + [gp16 + gp17 + gm16 + gm17 ] RL+ RL vout [gm16 + gm17 ] RL RLª =

- - 1 + [gm16 + gm17 ] RL RL + kT 2qIC16

We find that go16 and go17 Retaining g 16 and g 17

can be neglected. will give a slightly Lecture 23 - Slide 2 Clif Fonstad, 11/03 higher value for Av5.

Review of OCTC and SCTC methods:

l w

l |A |

wb wcwdwa

w w *

w4 w5 w2 w1 w3

wHI * wHI

Mi

og

og vd

LO LO

d-band Range

• OCTC: an estimate for wHI

1. wHI* is a weighted sum of w's associated with device capacitances: (add RC's and invert)

2. Smallest w (largest RC) dominates wHI* 3. Provides a lower bound on wHI

• SCTC: an estimate for wLO 1. wLO* is a weighted sum of w's associated with bias capacitors:

(add w's directly) 2. Largest w (smallest RC) dominates wLO* 3. Provides a upper bound on wLO

Clif Fonstad, 11/03 Lecture 23 - Slide 3

The Miller effect (general) Cm

Consider an amplifier shunted by a capacitor, and consider how

Avthe capacitor looks at the input vOUT = A vvINand output terminals:

vIN

(1 - A v)v IN -+iIN Cm i N = Cm d{[1-Av)v N}/dt I I+ + = [1-Av]Cm dv N/dt Iv IN vOUT = A vvIN - - Recall: Av is < 0

At the input: At the output: + +(1 - A v)Cm [(1 - A v)/Av]Cm

vOUT v IN Cm --

Cin looks much bigger than Cm Cout looks like Cm

Clif Fonstad, 11/03 Lecture 23 - Slide 4

Stages with little or no Miller effect V+ Common-base/ go

-gate stages:

CO

+ vout

(gm + gmb)vsg

+

-

v in = v

+

-

vv t +

-

rt

C Csg out

gs gd glCI -

+ vINIBIAS - No capacitor the Miller position

V-

Emitter-/Source-follower stages: V+

+ COvin- +

voutIBIAS -

V

C

Cµ

gp +

-

vp gmvp go

gl

+

-

v in +

-

v

v t +

-

rt

out

- Voltage gain of 1 means Clif Fonstad, 11/03 no Miller multiplier Lecture 23 - Slide 5

--

Multi-stage amplifier analysis/design - special pair stages

gl (= gLOAD + gnext )

: ii

"l i illi l

in i i lg i in i ill b.

li l l iin i l

V+

VBIAS

vout +

-

CO

+

-

vin + Q2

Q1

g 2

+

-

v 2 g v 2

g

+

-

v in

+ vg 1

+

vg v

g

The Cascode A common-em tter/source stage w th a common-base/gate stage as the

oad" to avo d the M er effect. The common-em tter stage vo tage

ga s -1 because t sees a oad of m2, but ts current ga s st

The common-base stage has vo tage ga n of gm2/g , a though ts current ga s on y 1.

m2o2 out eb1

m1 eb1

o1

The combination has the same voltage gain and current gain of a common-emitter stage, with no Miller effect.

-IBIAS

V-

CE

Clif Fonstad, 11/03 Lecture 23 - Slide 6

- - - - - - - - - - - -

Multi-stage amplifier analysis/design - special pair stages

The Cascode, cont.: A very large output resistance is another i

g 2

+

-

v 2 = 0

g v 2 = 0 g

+

-

v in = 0

+

-

v tg 1

+

-v

g v

git

mportant feature of the cascode

m2o2 eb1

m1 eb1

o1

Two node equations give us: it = go1[vt - veb1]- gm1veb1 = [go2 + gp1]veb1

Eliminating veb1 gives us our result:

rout ≡ vtest [go1 + go2 + gp1 + gm1] [gp1 + gm1] =

[b +1] i

= ª = [b +1] r01 test go1[go2 + gp1] go1 gp1 go1

For a MOSFET cascode we find: v K2 VA 2 K2rout ≡ test [go1 + go2 + gm1] ª

gm1= = =r01 Vv,oc 2 r01K1itest go1 go2 go1 go2 K1 [VGS 2 -VT 2 ] Clif Fonstad, 11/03 Lecture 23 - Slide 7

lti lifi l i i

i it: ll ic

Mu -stage amp er ana ys s/des gn - The µA741

Clif Fonstad, 11/03 Lecture 23 - Slide 8

The c rcuA fu schemat

Multi-stage amplifier analysis/design - The µA741

ip: i l IC

i

i

i

i

Image courtesy of Fairchild Semiconductor. Used with permission.

The chA b po ar

Trans stors

Capac tor

Res stors

Bond ng pads

Clif Fonstad, 11/03 Lecture 23 - Slide 9

Multi-stage amplifier analysis/design - The µA741 Emitter-follower/

Figuring the circuit out: common-base cascode

ll ic i lifi ic

li

di ial i

ll

The fu schemat A s mp ed schemat

Dar

fferent ga n stage

Push-pungton common-Current mirror load emitter gain stage output

Clif Fonstad, 11/03 Lecture 23 - Slide 10

Multi-stage amplifier analysis/design - The µA741

The capacitor: What is it there for?

To stabilize the amplifier.

How it works:A dominant pole iscreated that insuresthat the gain will fallbelow one before thephase shift reaches360˚, because if thegain is greater than oneand the phase shift if360˚ there will bepositive feedback andthe amplifier willoscillate.

Clif Fonstad, 11/03 Lecture 23 - Slide 11 Figure 14.4 in the course text.

6.012 - Electronic Devices and Circuits

Lecture 23 - The Cascode and the µA741 - Summary

• The Marvelous cascode Concept and wHI: current gain from a CE stage and voltage gain from

a CB to circumvent the Miller effect Output resistance: significantly larger than CE alone. This is valuable

in circuits using modern MOSFET's with small VA's The costs: the added device increases the voltage distance away from

the rails and limits voltage swings

• The µA741The Miller effect can be used to stabilize a high gain circuit On an IC, resistors and capacitors are expensive, transistors are cheap A real multistage amplifier uses the same building blocks we do Specialty circuits are widely used: cascode

Darlington push-pull current mirror load Widlar current source)

Clif Fonstad, 11/03 Lecture 23 - Slide 12