Download Differential Amplifier Model: Basic

VENUS INTERNATIONL COLLAGE OF
TECHNOLOGY GANDHINAGAR
Prepared by
Satapara Nirav
130810109083
Differential
Amplifier
Model:
Basic
Represented by:
A
= open-circuit voltage gain
vid = (v+-v-) = differential input signal voltage
Rid = amplifier input resistance
Ro = amplifier output resistance
The
signal developed at the amplifier output is
in phase with the voltage applied at the + input
(non-inverting) terminal and 180° out of phase
with that applied at the - input (inverting)
terminal.
The Inverting Amplifier
Configuration
• The positive input is grounded.
• A “feedback network” composed of resistors R1 and R2 is connected
between the inverting input, signal source and amplifier output node,
respectively.
Voltage Gain
The negative voltage gain
implies that there is a 1800
phase shift between both dc
and sinusoidal input and
output signals.
 The gain magnitude can be
greater than 1 if R2 > R1
 The gain magnitude can be
less than 1 if R1 > R2
 The inverting input of the
op amp is at ground
potential (although it is not
connected directly to
ground) and is said to be at
virtual ground.

vs  isR  i R  vo  0
1 2 2
vs  isR  i R  vo  0
1 2 2
R
vo
vs
Av 
 2
is 
vs
R
R
1
1
Input and Output Resistances
Rout is found by applying a test current
(or voltage) source to the amplifier
output and determining the voltage (or
current) after turning off all
independent sources. Hence, vs = 0
vx  i R  i R
2 2 11
But i1=i2
vx  i ( R  R )
1 2 1
vs
R   R since v  0
in i
1
s

Since v- = 0, i1=0. Therefore vx = 0
irrespective of the value of ix .
Rout  0
Inverting Amplifier
Input and Output Resistances
Rout is found by applying a test current
(or voltage) source to the amplifier
output and determining the voltage (or
current) after turning off all
independent sources. Hence, vs = 0
vx  i R  i R
2 2 11
But i1=i2
vx  i ( R  R )
1 2 1
vs
R   R since v  0
in i
1
s

Since v- = 0, i1=0. Therefore vx = 0
irrespective of the value of ix .
Rout  0
Inverting Amplifier
Input and Output Resistances
Rout is found by applying a test current
(or voltage) source to the amplifier
output and determining the voltage (or
current) after turning off all
independent sources. Hence, vs = 0
vx  i R  i R
2 2 11
But i1=i2
vx  i ( R  R )
1 2 1
vs
R   R since v  0
in i
1
s

Since v- = 0, i1=0. Therefore vx = 0
irrespective of the value of ix .
Rout  0
The Unity-gain Amplifier or “Buffer”




This is a special case of the non-inverting amplifier, which is also called
a voltage follower, with infinite R1 and zero R2. Hence Av = 1.
It provides an excellent impedance-level transformation while
maintaining the signal voltage level.
The “ideal” buffer does not require any input current and can drive
any desired load resistance without loss of signal voltage.
Such a buffer is used in many sensor and data acquisition system
applications.
The Summing Amplifier

Since the negative amplifier
input is at virtual ground,
v
v
1
i 
i  2 i   vo
1 R
2 R
R
1
2 3
3
Since i-=0, i3= i1 + i2,
R
R
vo  3 v  3 v
R 1 R 2
1
2
 Scale factors for the 2 inputs
can be independently adjusted
by the proper choice of R2
and R1.
 Any number of inputs can be
connected to a summing
junction through extra
resistors.
 This circuit can be used as a
simple digital-to-analog
converter. This will be
illustrated in more detail,
later.
Instrumentation Amplifier
NOTE
Combines 2 non-inverting amplifiers
with the difference amplifier to
provide higher gain and higher input
resistance.
R
vo   4 (va  v )
b
R
3
va  iR  i(2R )  iR  v
2
1
2 b
v v
i 1 2
2R
1
R  R 
vo   4 1 2 (v  v )
R  R  1 2
3
1
Ideal input resistance is infinite
because input current to both op
amps is zero. The CMRR is
determined only by Op Amp 3.