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Oct 7, 2013 (3 years and 8 months ago)

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EEEB273/EEEB 314 Electronics II


Operational Amplifier Circuits (Part 1)

Dr. Ungku Anisa, UNITEN, 2007

1

Operational Amplifier Circuits (Part 1)


A
Bipolar Op
-
Amp Circuit


The
741 op
-
amp:



Description of general circuit configurations



perform
detailed dc analysis



perform
detailed small
-
signal analysis


Circuit Description
































































Figure
1
: Equivalent circuit, 741 op
-
amp.


The 741 consists of input differential amplifier stage, gain stage, output
stage and separate bias circuit.

EEEB273/EEEB 314 Electronics II


Operational Amplifier Circuits (Part 1)

Dr. Ungku Anisa, UNITEN, 2007

2

Input Diff
-
Amp


Input stage (Q1 to Q7)



Input tran
sistors Q1 & Q2
: emitter followers


high
R
id



Q3 & Q4: common
-
base amplifier


large voltage gain



Output currents from Q1 & Q2 are input currents to Q3 & Q4.



Q5, Q6 & Q7 with R1, R2 & R3: active load for diff
-
amp



Output (single
-
sided) taken at collector of

Q4 & Q6



Dc output voltage at collector Q6 is at lower potential than inputs at
bases Q1 & Q2.



Dc voltage level shifts several times as signal passes through the
opamp


zero dc output voltage for zero differential input
.



Two null terminals: for appropria
te adjustments to achieve zero dc
voltage design goal.


Biasing (Q8 to Q12)



Q12, Q11 & R5: dc current biasing


provides
I
REF



Q11, Q10 & R4:
Widlar current source

for biasing of common
-
base
transistors (Q3 & Q4) and current mirror formed by Q8 & Q9.

To gain
stage

EEEB273/EEEB 314 Electronics II


Operational Amplifier Circuits (Part 1)

Dr. Ungku Anisa, UNITEN, 2007

3

Voltag
e breakdown protection



Q3 & Q4:

lateral pnp devices



larger breakdown voltage

(smaller current gain

)



Figure
2
: (a) Basic common
-
emitter differential pair with a large differential voltage
and (b) 741 input stage, with a large

differential voltage.





From Figure 2,
V
1

= 15 V,
V
2

= 0 V.



Fig 2(a): B
-
E of Q2 reverse biased by approx. 14.3V



npn have breakdown voltage of 3
-

6 V so Q2 suffers permanent
damage.



Fig 2(b): B
-
E of Q1 & Q3 are forward biased



series combination of B
-
E junction of Q2 & Q4 reverse biased by
13.6 V.



Breakdown voltage of lateral pnp is about 50 V



B
-
E of Q4 provides breakdown protection
.











EEEB273/EEEB 314 Electronics II


Operational Amplifier Circuits (Part 1)

Dr. Ungku Anisa, UNITEN, 2007

4

Gain Stage



Q16 & Q17: second gain stage



Q16: emitter follower


large
R
in




Q13: two transistors in paral
lel



Q13A: ¼ area of Q12




Q13B: ¾ area of Q12



Q13B: provides bias current for
Q17, and also acts as an active load
to produce high voltage gain




Q17: common
-
emitter



output voltage at collector of

Q17 is input to the output stage


signal undergo
es
another dc

level shift




Capacitor C1: internal feedback
compensation
(Miller
compensation)
for stability



connected between output and

input terminals of the gain stage


Output Stage



Q14 & Q20: class
-
AB circuit of
complementary emitter
-
follower



t
o provide low
R
out


and current gain (for driving large

load currents)



Output of gain stage connected to
base of Q22


emitter follower,
high
R
in
.



Q13A: bias currents for Q22, Q18
& Q19



Q18 & Q19


provide quiescent
bias current in output transistors
Q14
& Q20.



Q15 & Q21: short
-
circuit
protection



normally off.

To output
stage

From
input stage

From gain
stage

EEEB273/EEEB 314 Electronics II


Operational Amplifier Circuits (Part 1)

Dr. Ungku Anisa, UNITEN, 2007

5

Q15 & Q21 conduct only when output is inadvertently connected to
ground

and resulting in a very large output current.


An abbreviated data sheet for the 741 is shown in Table 13.1 (page 949,
Neam
en 3
rd

ed. textbook). A more complete data sheet for the 741 op
-
amp is
available in Appendix C (Neamen textbook).



DC Analysis



To determine dc bias currents.



Assumptions:

o

non
-
inverting and inverting input terminals at ground potential.

o

dc supply voltage
s are +15 V and
-
15 V.

o

V
BE

for npn =
V
EB

for pnp = 0.6 V.

o

In
most cases

dc base currents are neglected.


Bias Circuit and Input Stage


Figure
3
: Bias circuit and input stage portion of 741 op
-
amp circuit.


EEEB273/EEEB 314 Electronics II


Operational Amplifier Circuits (Part 1)

Dr. Ungku Anisa, UNITEN, 2007

6



The reference current
is established by Q12, Q11 & R5:


(1.1)





Current
I
C10

is obtained from the Widlar current source (Q11, Q10 &
R4):


(1.2)



where Q10 & Q11 are assumed matched.



Neglecting base currents,
I
C8

=
I
C9

=
I
C10
. Hence, the quiescent
collector currents in Q1 t
hrough Q4 are:


(1.3)




Assuming
dc currents

in the input stage are exactly
balanced
, dc
voltage at collector of Q6 = dc voltage at collector Q5, i.e.:


(1.4)




The dc level shifts through the opamp.



See Example 13.1.


Base Current Effects





in
Q3, Q4, Q8 & Q9 (lateral pnp’s)

may be small, hence their
base
currents

may
not

be
negligible
. But
base currents in npn’s are negligible
.




I
C10

establishes base currents in Q3 &
Q4

establish emitter currents
I
. At
collector of Q8:


(1.5)




since
I
C
8

=
I
C9
.

EEEB273/EEEB 314 Electronics II


Operational Amplifier Circuits (Part 1)

Dr. Ungku Anisa, UNITEN, 2007

7




Therefore,



(1.6)






Even if pnp transistor base currents are not negligible, bias currents in
Q1 and Q2 are, from eq. (1.6), very nearly:


(1.7)



Hence, previous assumption (eq. (1.3)) is correct.



Gain Stage






Q12 & Q13 form a current
mirror.



Q13B is scaled to 0.75 of Q12.
Hence, neglecting base currents:


(1.8)




The collector current in Q16:



(1.9)





See Example 13.2, Exercise 13.2.




Figure
4
: Reference circuit and gain stage, 741
op
-
amp.





1

EEEB273/EEEB 314 Electronics II


Operational Amplifier Circuits (Part 1)

Dr. Ungku Anisa, UNITEN, 2007

8

Output Stage
(Class AB configuration)



I
Bias

is supplied by Q13A



Input signal applied to base of
Q22 (emitter follower).



Q18 & Q19 establishes
2V
BE

drops

between base terminals of
Q14 & Q20



V
BB

produces quiescent

collector currents in Q14 and

Q20.




Output trans
istors
Q14 & Q20


biased
slightly in conducting

state
, i.e.‘on’ with no signal

present


removes cross
-
over

distortion (more in Ch 8!!).

Figure
5
: Basic output stage, 741 op
-
amp.




Q13A is scaled to 0.25 of Q12. Neglecting base c
urrents,


(1.10)


where
I
REF

is given by equation (1.1).




Neglecting base currents,
I
C22

=
I
Bias
.



The collector current in Q18 is:


(1.11)




Therefore,


(1.12)




Since
V
BB

remains almost constant:

o

as
v
1

increases, base voltage of Q14 increases and
v
O

increases

o

as
v
1

decreases, base voltage of Q20 decreases and
v
O

decreases



Hence,
small
-
signal gain of output stage is unity
.




See Example 13.3, Exercise 13.3.

EEEB273/EEEB 314 Electronics II


Operational Amplifier Circuits (Part 1)

Dr. Ungku Anisa, UNITEN, 2007

9

Short
-
Circuit Protection Circuitry




To protect Q14 from burnout due
to large current induced in
transistors if output terminal is
shorted to ground during a
positive signal.



R6 & Q15 limits current in Q14 in
event of short circuit.

o

If current in Q14 reaches
20mA,
V
R6

is 540 mV.

o

Hence, Q15 turns on, and
conducts excess current from
base of Q14 into i
ts collector.

o

Thus, current in Q14 is limited
to a maximum value.


Figure
6
: Output stage, 741 op
-
amp with
short
-
circuit protection devices.




Maximum current in Q20 limited by R7, Q21 & Q24:

o

Large output current results in
V
R7
, whi
ch turns on Q21.

o

Excess current in Q20 will be shunted by Q21 & Q24.


















EEEB273/EEEB 314 Electronics II


Operational Amplifier Circuits (Part 1)

Dr. Ungku Anisa, UNITEN, 2007

10

Small
-
Signal Analysis


Input Stage



Effective impedance at base of Q3
& Q4 is ideally infinite, i.e. open
circuit, due to constant
-
current
biasing at base of Q3 & Q4.



R
a
ct
1

effective resistance of active
load.



R
i2

input resistance of gain stage.



The small
-
signal differential
voltage gain is:




(1.13)




where I
CQ

= quiescent collector

current in each of the transistors Q1


to Q4.


Figure
7
: Simplified ac equivalent circuit of 741 op
-
amp input stage.




Effective resistance of active load (Widlar current source):


(1.14)




Input resistance of gain stage:


(1.15)

where


(1.16)




See Example 13.4, Exercise 13.4.




EEEB273/EEEB 314 Electronics II


Operational Amplifier Circuits (Part 1)

Dr. Ungku Anisa, UNITEN, 2007

11

Gain Stage

Figure
8
: The ac equivalent circuit, gain stage
of 741 op
-
amp.




R
act2

effective resistance of active
load



R
i3

input resistance of output stage.



From Figure 8, the small
-
signal
voltage gain can be developed
directly.



Input base current to Q
16:


(1.17)



where
R
i2

= input resistance to the gain stage.




The base current into Q17:


(1.18)





The output voltage:


(1.19)


where
R
o17

= output impedance looking into collector of Q17.



Hence, combining (1.17), (1.18) and (1.19), the small
-
signal

voltage
gain:


(1.20)





Effective resistance of active load is the resistance looking into
collector of Q13B:


(1.21)


EEEB273/EEEB 314 Electronics II


Operational Amplifier Circuits (Part 1)

Dr. Ungku Anisa, UNITEN, 2007

12



The input resistance of the output stage
R
i3

can be determined from

Figure 9.



**Important!

Assume only
either

Q20

OR
Q14 is conduc
ting.



Assumption:

pnp output transistor
Q20 is active

and npn output
transistor
Q14 is cut
-
off
.



Q22 is an emitter follower.



A load resistance
R
L

is included.



Hence, input resistance of output
stage:


(1.22)




Resistance
R
19

is given by:



R
19

= R
e19
+ R
e18
+ R
13A


However, (
R
e19
+ R
e18
)

< R
13A
,


thus:


(1.23)





The output transistor Q20 is also an emitter follower, therefore:


(1.24)


where
R
L

>> R
7
.




See Example 13.5, Exercise 13.5 (in these examples R
8

is neglected in
calculation of R
O1
7
).








Figure
9
: The ac equivalent circuit, 741
op
-
am
p output stage, for calculating
input resistance.

EEEB273/EEEB 314 Electronics II


Operational Amplifier Circuits (Part 1)

Dr. Ungku Anisa, UNITEN, 2007

13

Overall Gain



The
product of individual gain factors

(since loading effects were
considered in voltage gain of each stage).


(1.25)




It is assumed that
A
V3

≈ 1
. Typical voltage gain values of the 741 op
-
amp is in the range of 200,000.


Output
Resistance



Ac equivalent circuit of figure 10 is referred, assuming
Q20 is
conducting

and
Q14 is cutoff
.


Figure
10
: The ac equivalent circuit, 741 op
-
amp output stage, for calculating output
resistance.




The output resistance:


(1.26)



where:


(1.27)





As before,
R
e19

+
R
e18

small
compared to
R
c13A
, so
R
c19


R
c13A
.



Also:


(1.28)


where:

(1.29)



(1.30)




See Example 13.6, Exercise 13.6.



EEEB273/EEEB 314 Electronics II


Operational Amplifier Circuits (Part 1)

Dr. Ungku Anisa, UNITEN, 2007

14

Problem
-
Solving Technique: Op
-
Amp Circuits


1.

DC Analysis:



Identify the bias portion of op
-
amp circuit



Determine the reference current.



Determine the bias currents
in the individual building blocks of the
overall circuit.


2.

AC Analysis:



Analyse the small
-
signal properties of the building blocks
individually.



Loading effects of follow
-
on stages must be taken into account in the
analysis of each building block.