Amplification
and Feedback
Amplifiers
• Make a signal (voltage or current) bigger
• Voltage amplifier:
Voltage out = Voltage in x G (gain)
• Small signals: Sensor outputs, radio
reception, microelectronics
• Big signals: Provide mechanical energy
(motors, speakers), radio transmission,
heating
The ideal amplifier
Vin
G
Vout = G x Vin
Amplification devices
• Mechanism in which a
small signal controls a
large signal
• Water analogy: turning
the faucet (small signal)
controls large flow of
water (big signal)
Vacuum tubes
• Electrons “boil” off of heated cathode
• Voltage on grid(s) control current reaching
anode
Anode
electrons
Heater
Cathode
Transistor
• Invented 1947
• Made of semiconductors – silicon,
germanium, gallium arsenide
• Layered structure – creates junctions
The first transistor –
Bell Labs, 1947
Transistors
• Voltage between base
and emitter controls
current between
collector and emitter
• Types: bipolar (NPN,
PNP), JFET, MOSFET
~ 1 µm
Approximate transistor behavior –
the Ebers-Moll equations
• Explicit temperature dependence
• Parameters (αF, αR, IES, ICS) dependent upon
manufacturing details, temperature, etc.
Simple device – complicated
behavior
• The physical laws which describe
electronic behavior are not simple
• Want to make electronic components “act”
simple to make them easier to apply
• Need to build a complicated device to get
simple behavior!
How to make electronics “act”
simple?
• Emphasize use of
components which
have close to ideal
behavior
Vin
R1
– Resistors, capacitors
• Use circuits which are
inherently selfcompensating for
nonideal behavior
– Ex: voltage divider
Vout
R2
Modern operational amplifiers
~ 1 mm
LT1006
LMH6642
What’s a good “building block” for
amplification?
• One option: Fixed gains (10, 100, etc)
– Instrumentation amplifier
Vin
G
Vout = G x Vin
• Another option: Make G…really big, then
“throw away” gain you don’t need
Operational amplifiers
V+
+
Vout=G x (V+-V-)
V-
-
• G very large (AD820: G ~= 1 million (DC, low load))
• Inputs draw little current (AD820: ~10 pA)
Feedback amplifier
• Feedback: output “feeds back” to input
Vin
+
-
Noninverting
Feedback
Amplifier
Vout
G
R1
R2
• Op amps are always used with feedback
• How does this work?
Rationale for use of feedback
amplifiers
• Amplifier performance is dictated largely
by the behavior of simple passive
components (e.g. resistors, capacitors)
• Pioneers: H. S. Black, H. Nyquist, H. W.
Bode, Ball Laboratories, 1920’s
“Black’s patent application was delayed for more than nine years in
part because the concept was so contrary to established beliefs
that the Patent Office initially did not believe it would work. They
treated the application in the same manner as one for a perpetual
motion machine.” -- W. M. Siebert, Signals and Systems
Basic op amp configurations
• Noninverting
Vin
+
Vout = Vin (R1+R2)/R2
R1
R2
• Inverting
R2
Vin
R1
Vout = -(R2/R1) Vin
+
Quick analysis of op amp circuits
• Because G is so large, the difference between V+ and Vmust be very small
• To a good approximation V+=VVin
+
Vout
R1
R2
• Ex: V- is the output of a voltage divider: V- = Vout *
R2/(R1+R2). V+ = V-, so Vout = V+ (R1+R2)/R2.
Other op amp configurations
• Endless variety of
circuits for performing
different functions:
filtering (high pass,
low pass, band pass,
band reject),
integration,
differentiation,
calculating logarithms,
square roots, etc.
• Use different feedback
components and
interconnections
Ex: Sallen-key lowpass filter
Buffer amplifier
+5V
Vin
3
+
7
6
2
-
Vout
4
• Gain of 1 – why use it at all?
Output impedance
+
• Ideal voltage source:
Voltage independent of
load
• Real voltage source:
Acts like an ideal voltage
source in series with a
resistor
Ideal
Vout
+
R
RL
• Small resistance = “low
output impedance”
Real
Ex: Voltage divider
Vin
Vin
R1
+5V
R1
Vout
3
+
7
6
R2
RL
No buffer amplifier: Vout
depends on RL
R2
2
-
Vout
4
With buffer amplifier: Vout
Independent of RL
RL
Buffer amplifier – gain of 1
+5V
Vin
3
+
7
6
2
-
Vout
4
• Buffer amplifier decreases the output impedance
of a signal
• Makes it easier to transmit signals between parts
of a circuit – enables modular construction
Op amp packaging
• Generally 1, 2, or 4 to a package
• Standard pinouts
Single
Dual
Quad
Other op amp parameters
• Speed (slew rate, bandwidth) – what type of
signal can be amplified
– DC, audio, video
• Precision (input offset voltage) – how closely
does the op amp equalize its inputs
• Power (supply current, output current) – how
much power does the amplifier require, and how
much power can it deliver to a load
Input and output range
• Op amp can only amplify signals within a range
set by its power supply – can limit applications
Dual supply
5V
5V
-5V
Power
Supply
Single supply
Input
range
Output
range
0V
Power Input
Supply range
Output
range
• “Single supply” op amps have input and output
voltage range extending to the negative supply