Feedback
ROCHESTER INSTITUTE OF TECHNOLOGY
MICROELECTRONIC ENGINEERING
Feedback Laboratory
Dr. Lynn Fuller
Webpage: http://people.rit.edu/lffeee/
Microelectronic Engineering
Rochester Institute of Technology
82 Lomb Memorial Drive
Rochester, NY 14623-5604
Tel (585) 475-2035
Email: Lynn.Fuller@rit.edu
MicroE webpage: http://www.microe.rit.edu
Rochester Institute of Technology
Microelectronic Engineering
© March 10, 2015 Dr. Lynn Fuller, Professor
3-10-16 Feedback_Lab.ppt
Page 1
Feedback
INTRODUCTION
In the two prior labs we investigated current sources and the
differential amplifier (Lab 1) and DC coupled amplifier and output
stage (Lab 2). The result was a high gain (Av=-800), low output
resistance (Ro ~ 100 ohms) DC amplifier. This lab will add global
feedback as shown in the schematic on the following page (a single
resistor from output back to the input). This configuration is called
Voltage Sampling – Parallel Mixing or Voltage-Shunt feedback. The
feedback will result in reduced but less sensitive voltage gain and will
modify the input and output resistance. We will predict the results
using hand calculations, and SPICE simulations. Then we will
construct the circuit and make measurements for the gain with
feedback and output resistance.
Rochester Institute of Technology
Microelectronic Engineering
© March 10, 2015 Dr. Lynn Fuller, Professor
Page 2
Feedback
MULTISTAGE DIRECT COUPLED AMPLIFIER
VCC = +12 V
10 F
RC1
RC2
RE1
Q3
Q2N3906
10 k 10 F
Q1
100 
vid
Q2
ii
Vo/Vin = -800
RE2
100 
~
Q4
Q2N3904
Q2N3904
Q6
100 
Rout
vo
6 mA
Q2N3906
Q5
Rpi = 0.026/IB
= 650 ohms
100 
Q7
Q2N3906
RC3
–VEE = –12 V
Rochester Institute of Technology
Microelectronic Engineering
© March 10, 2015 Dr. Lynn Fuller, Professor
Page 3
RL
Feedback
WITH FEEDBACK
Feedback
Rf
Source
+V
10k
Vsig +
-
100
ii
-
Vo
+
Amplifier
Rload = 1000
-V
Av= Vo/Vin = -800
Rin = Rpi + Rpi + 100 = 1400
Rout = 100 + few ohms (use 100)
Rochester Institute of Technology
Microelectronic Engineering
© March 10, 2015 Dr. Lynn Fuller, Professor
Page 4
Feedback
SMALL SIGNAL AC - VOLTAGE-SHUNT FEEDBACK
Select y-two port equivalent circuits and Norton equivalent circuits for voltageshunt feedback:
IS
Or
VS/RS
+
Ve
-
RS
y11A y12A VL y21A Ve
+
VL
-
RL
y22A
+
VL
-
+
Ve
y11B y12B VL y21B Ve
y22B
Rochester Institute of Technology
Microelectronic Engineering
© March 10, 2015 Dr. Lynn Fuller, Professor
Page 5
Feedback
SIMPLIFIED VOLTAGE-SHUNT FEEDBACK
From the previous page we combine current sources in parallel and
conductances in parallel. (the equivalent circuit is simplified)
+
VL
-
+
Ve
-
IS
Or
VS/RS
y11
y11 = y11A
+ y11B + 1/Rs
y12 = y12A +y12B
y12 VL
y21 Ve
y22
y22 = y22A + y22B + 1/RL
y21 = y21A +y21B
Rochester Institute of Technology
Microelectronic Engineering
© March 10, 2015 Dr. Lynn Fuller, Professor
Page 6
Feedback
VOLTAGE-SHUNT FEEDBACK SUMMARY
Voltage-Shunt
Input Admittance
YIf = y11 1- T
YOf = y22 1- T
Output Admittance
Loop Gain
y12y21
T= y y
11 22
1
ARf ~
= y12
~ Gain
I1
y-parameter
+
V2
y12 V2
Rochester Institute of Technology
Microelectronic Engineering
y21 V1
AVf = ARf (1/RS)
AIf =ARf (1/RL)
Current Gain
I2
+
V1
y11
Voltage Gain
Exact Gain
-y21
y11y22
ARf =
-y y
1 + y12y 21
11 22
Transresistance
I1 = y11 V1 + y12 V2
I2 = y21 V1 + y22 V2
y22
Reference: See Dr. Fuller’s Webpage
© March 10, 2015 Dr. Lynn Fuller, Professor
Page 7
Feedback
THE SOURCE
The source is represented by its Norton equivalent circuit.
10k
Vsig +
-
IS
Or
VS/RS
100
RS
Norton
Is = Vsig/10K
Rs = 10K // 100 = 99
Rochester Institute of Technology
Microelectronic Engineering
© March 10, 2015 Dr. Lynn Fuller, Professor
Page 8
Feedback
VOLTAGE-SHUNT FEEDBACK Y PARAMETERS
Find two port parameters for A block and B block.
I2
I1
y-parameter
I1
+
V1
-
-800V1
Rin
I1 = y11 V1 + y12 V2
I2 = y21 V1 + y22 V2
+
V2
-
+
V1
-
I2
+ Rout +
V2
-
y11B y12B V2 y21B V1
y11A = 1/1400 = 7.14E-4
y12A = 0 (almost always)
y21A = 800/100 = 8
y22A = 1/100 = 1E-2
y22B
y11B = 1/10K = 1E-4
y12B = -1/10K = -1E-4
y21B = -1/10K = -1E-4
y22B = 1/10K = 1E-4
Rochester Institute of Technology
Microelectronic Engineering
© March 10, 2015 Dr. Lynn Fuller, Professor
Page 9
Feedback
VOLTAGE-SHUNT FEEDBACK
Find the combined parameters
y11 = y11A + y11B + 1/RS
y12 = y12A + y12B
y21 = y21A + y21B
y22 = y22A + y22B + 1/RL
= 7.14e-4 + 1e-4 + 1/99
= 0 + -1e-4
= 8 -1e-4
=1e-2 + 1e-4 + 1e-3
= 1.09e-2
= -1e-4
=8
= 1.11e-2
Compute quantities of interest
Gain with feedback (transresistance)
-8
-y21
(1.09e-2)(1.11e-2)
y11y22
ARf =
-y12y21
1+
y11y22
=
1+
= -8680 ohms
-8(-1e-4)
(1.09E-2)(1.11e-2)
Voltage gain with feedback
Rochester Institute of Technology
Microelectronic Engineering
AVf = ARf (1/RS) = -8680 (1/99) = -87.7
© March 10, 2015 Dr. Lynn Fuller, Professor
Page 10
Feedback
VOLTAGE-SHUNT FEEDBACK
Current Gain with Feedback
AIf = ARf (1/RL) = -8680 (1/1000) = -8.68
Approximate Gain
ARf ~ = 1/y12 = -1/1e-4 = -10000 ohms
AVf ~ = -10000 (1/RS) = -101
AIf ~ = -10000 (1/RL) = -10
Loop gain = T = -
y12y21
y11y22
= (-1e-4)(-8)/(1.09E-2)(1.11E-2) = -6.61
Rochester Institute of Technology
Microelectronic Engineering
© March 10, 2015 Dr. Lynn Fuller, Professor
Page 11
Feedback
VOLTAGE-SHUNT FEEDBACK
y12y21
)
y11y22
Input admittance YIf = y11 (1- T) = y11 ( 1 -
YIf = (1.09E-2)(1- -6.61) = 82.9 mS
Input impedance ZIf = 1/YIf = 12 ohms
Note: this ZIf = 12 ohms is equal to the 99 ohms RS in parallel with Zin’
the amplifier input impedance.
So 99//Zin’ = 12 therefore we can find Zin’ = 13.7 ohms
IS
Or
VS/RS
13.7 ohms
RS 99
Zin’
Rochester Institute of Technology
Microelectronic Engineering
© March 10, 2015 Dr. Lynn Fuller, Professor
Page 12
Feedback
VOLTAGE-SHUNT FEEDBACK
Output Impedance Zof = 1/Yof
Yof = y22 (1-T) = y22 (1- -6.61) = 84.5 mS
Zof = 1 / Yof = 11.8 ohms
Note: this includes the 1000 ohm RL
Yof = 1/Rout with feedback +1/RL
Rout with feedback = 1/83.5 mS or 12.0ohms
Rochester Institute of Technology
Microelectronic Engineering
© March 10, 2015 Dr. Lynn Fuller, Professor
Page 13
Feedback
SUMMARY
Amplifier Transresistance with no Feedback
Vo/Iin = Av x Rin = 800 x 1400 = 1.12Meg
Amplifier Voltage Gain with no Feedback
Input Resistance with no Feedback
Output Resistance with no Feedback
1.12Meg ohms
Amplifier Transresistance with Feedback
Amplifier Voltage Gain with Feedback
Input Resistance with Feedback
Output Resistance With Feedback
-8680 ohms
-87.7 V/V
13.7 ohms
12.1 ohms
Approximate Transresistance with Feedback
Approximate Voltage Gain with Feedback
-10,000 ohms
-101 V/V
-800 V/V
1400 ohms
100 ohms
Rochester Institute of Technology
Microelectronic Engineering
© March 10, 2015 Dr. Lynn Fuller, Professor
Page 14
Feedback
REFERENCES
1. Sedra and Smith, 5.1-5.4
2. Device Electronics for Integrated Circuits, 2nd Edition, Kamins
and Muller, John Wiley and Sons, 1986.
3. The Bipolar Junction Transistor, 2nd Edition, Gerald Neudeck,
Addison-Wesley, 1989.
Rochester Institute of Technology
Microelectronic Engineering
© March 10, 2015 Dr. Lynn Fuller, Professor
Page 15