The with anop-amp: a Consider theinvertingamplifier— is alsoreferred tobyengineers the /321 Coe n pnLo anlcue 1/5 ClosedandOpen LoopGainlecture 2/13/2011 Jim Stiles The Univ. of Kansas Dept. of EECS of Dept. ofKansas Univ. The Jim Stiles feedback open
- circuit amplifierconstructed voltagegainof thisamplifier: Closed-Loop and
Open-Loop Gain
v
in
A
closed loop gain R
vo
1
=
−
R R 1 2
v v
-
+
of the
- +
idea
R
l
2
feedback amplifier
v
out
oc .
back In this case,the the /321 Coe n pnLo anlcue 2/5 ClosedandOpen LoopGain lecture A: Q: 2/13/2011 Jim Stiles The Univ. of Kansas Dept. of EECS of Dept. ofKansas Univ. The Jim Stiles
Theterm“closedloop”refers toloopformedbythe
feed-back Closed loop
pathisformed bythefeedback
v
in
(i.e.,feedback) path oftheamplifier. ? What doesthat mean?
feed-forward
R
1
A closedloop
pathisformedbythe
v v
-
+
- + idea resistor
R
l
2
R 2
.
Fee Path
op-amp d
- feed-forward
forward forward
v
out
oc Close
Fee Path
, whilethe
d d
-
-Loo
back
p
pathand
feed-
/321 Coe n pnLo anlcue 3/5 ClosedandOpen LoopGain lecture If theloopis 2/13/2011 Jim Stiles The Univ. of Kansas Dept. of EECS of Dept. ofKansas Univ. The Jim Stiles open loopcaseisreferredtoas the
v
in
broken
R i
1
1 , thenwesaytheloopis“open”. Thegain(
An openloop
v v
-
+
open-loop gain - +
idea
R i
2
l
2
.
v
Ope out
oc
v
n
o
-Loo
v i
) forthe
p
which of courseis the /321 Coe n pnLo anlcue 4/5 ClosedandOpen LoopGain lecture For example,inthecircuitweknow that: 2/13/2011 Jim Stiles The Univ. of Kansas Dept. of EECS of Dept. ofKansas Univ. The Jim Stiles Once we“close” theloop, we haveanamplifier witha Combining, wefindthe
v v v ii
12
u op out
− + oc ==
−= =− = viR =−
0
in vv Av 0 () 11 +− Open andclosedloopgains 0
open-circuit voltage gain
open-loop gain
v
in
A
AA
closed pnop open = =
ofthisamplifier to be:
v
v v
vR
out
oc
in out
oc R in
i
1 1
=−
=− R 2 1 ofthis inverting amplifier.
closed-loop
v v
-
+
- +
idea
R
l i
2
2
gain:
v
out
oc
5/5 ClosedandOpen LoopGain lecture Note that the 2/13/2011 Jim Stiles The Univ. of Kansas Dept. of EECS of Dept. ofKansas Univ. The Jim Stiles Note that is toohigh!)into a gain
A * large that theamplifier iseasily * often gain. Moreover,asinthiscase, the * open-loop gain typically areselectedtoprovide
op . Conversely, the As aresult,the The closed-loopgainisdetermined bytwo closing only Feedback isawonderfulthing closed involvestheop-ampgain! thefeedbackloop turnsagenerally very . -loop gain( usefulone(the gainisjustright)! open op-amp gain -loop gain(
− RR 21 saturated significant isoften alternativelyreferredtoas the ) does
open-loop gainofafeedbackamplifier -A op ) obviously not . gain( explicitlyinvolvethe op-amp resistor
useless
A vo does > 1 values,which ), albeit notso involve theop-amp amplifier (thegain