Transcript Differential Amplifier

Operational
Amplifier
Introduction
• Operation amplifier (op-amp) have high gain
amplifier and able to amplify signal with
frequency ranging from 0 to 1MHz.
• An op-amp is named so because it was
originally designed to perform mathematical
operations like summation, subtraction,
multiplication, differential and integration etc in
analogue computer.
• It has two input terminals, the inverting input
("-"), the non-inverting input ("+") and one output
terminal.
• A complete amplifier electronic circuit
may contains transistor, diode, resistor,
capacitor and others components and
constructed on a single silicon chip.
• The area is 5mm2 and thickness is less
than 0.5mm, it is protected by lace
plastic.
Symbol and IC configuration of op-amp
BASIC PRINCIPLES OF OP-AMP
Inverting input
Signal applied at
negative input
terminal will
appear
amplified but
phase
inverted at the
output terminal.
BASIC PRINCIPLES OF OP-AMP
Non inverting input
Signal applied at
positive input terminal
will appear amplified
and in phase at the
output terminal.
Open loop voltage
gain,
Where,
Vo = Output voltage
Vid = voltage different
on inverting input and
non inverting input.
Input Signal Modes
• In single ended input mode one input is grounded
and signal voltage is applied only to other input.
• If the input is applied to the non-inverting terminal,
the output signal will be in the same phase with the
input signal.
• In the single input mode connection, if the input is
given to the non-inverting terminal, then the
output will have a 1800 phase shift.
In the double ended
differential input mode,
the difference input is
amplified and in phase
with the input signal.
BLOCK DIAGRAM OF OP AMP
An op-amp is a high quality amplifier. It contains three stages,
which are connected in cascaded manner. Though designs
vary between products and manufacturers, all op-amps have
basically the same internal structure, which consists of three
stages:
• The first stage of an op-amp is a Differential amplifier
(double ended) provides low noise amplification, high
input impedance, usually a differential output.
• The second stage is an intermediate gain stage.
Voltage amplifier – provides high voltage gain, usually
single-ended output.
• The third stage is an Output amplifier stage – provides
low output impedance, current limiting and short circuit
protection circuitry.
Block diagram of operational amplifier
Configuration of
Differential Amplifier
Introduction
There are four configurations of differential
amplifier in the op-amp circuit.
1. Double ended input, balance output.
2. Double ended input, unbalance output.
3. Single ended input, balance output.
4. Single ended input, unbalance output.
Each configuration is categories based on
several factors:
1.
Input signal quantity that use in circuit
connection;
– Input : 2 input signal is used, so it is
called 2 input
– Input: 1 input signal is used, so it is
called single input.
2. How output voltage being measured.
– If the voltage is measured between 2
collectors, the output will be balance.
– If the voltage is measured at a collector
and refer to ground, the output will not
balance.
Double ended input, balance output
Double ended input, unbalance output.
Single ended input, balance output
Single ended input, unbalance output.
DC analysis of differential amplifier
Differential Amplifier
Input bias current
• The average of the currents flowing into both
inputs.
• It can calculated as follows:
I BIAS 
I1  I 2
2
• The concept of input bias current is illustrated
below:
Input offset current
• The difference of the two input currents when the
output voltage is zero.
I OS  I 1  I 2
• The concept of input offset current is illustrated
below:
Input offset voltage
• A small dc voltage appears at the output when no
differential input voltage applied.
• Its primary cause is a slight mismatch of the
base-emitter voltages of the differential amplifier
input stage of an op-amp.
• Typical values of input offset voltage are in the
range of 2 mV or less. In the ideal case, it is 0V.
Common-Mode Gain (Acm)
• Same voltage source is applied
at both terminals
• Output voltage is ideally zero
due to differential voltage is
zero
• Practically, a small output
signal can still be measured
+
V
o

V
i
~
Common-mode rejection ratio (CMRR)
• The common mode rejection ratio (CMRR) is the measure
of the device's ability to reject common mode signals
• It is the ratio of open loop gain, Aol to common-mode gain,
Acm.
Aol
CMRR 
Acm
• The CMRR is often expressed in decibels (dB) as
 Aol 

CMRR  20log
 Acm 
and with common devices having ratings between 60 dB
to 120 dB.
• The higher the CMRR is, the better the devices.
Example
1.
A certain op-amp has an open-loop voltage
gain of 100,000 and a common-mode gain of
0.2. Determine the CMRR and express it in
decibels.
2.
Determine the CMRR and express it in dB for
an op-amp with an open-loop voltage gain of
85,000 and a common-mode gain of 0.25.
3.
An op-amp has a CMRR of 90dB. If its
differential voltage gain is 30000, calculate its
common-mode gain.
INTERNAL CIRCUITRY OF OP-AMP:
+Vcc
Q4
Input 1
Q1
Q7
Q5
Q2
output
Input 2
Q8
Q3
Push-pull amplifier
Q6
-Vcc
Differential amplifier
Input stage
Voltage amplifier
Intermediate/Gain stage
Push-pull amplifier
Output level
Internal circuitry of an Op-amp
Internal block diagram of an Op-amp
Input Stage: Differential Amplifier:
- an op-amp is usually matched transistors configured as a dual-input Differential amplifier(DA)
- output of this input stage taken from across the outputs (collector) of the paired transistors
- this balanced output is fed into another dual-input DA in the intermediate stage (gain stage)
Intermediate/Gain Stage: Voltage Amplifier
- the output of this intermediate/gain stage is taken from just one of the transistors (single-ended
output/unbalanced)
- the DC level at the output of this stage is high with respect to ground, so a level-shifting circuit
such as an emitter follower is used to shift it down closer to ground
Output Stage : Push-pull amplifier
- Act as buffer to connect 2nd level and output differential amplifier not affected by load
- Consists of a push-pull amplifier: which increases the swing of the output voltage and enhances
the load current capacity of the op-amp (prepare enough current to trigger load at the output terminal).
Push-pull Amplifier
Push-pull Amplifier
• When the input signal is positive, the npn transistor
has a positive voltage at the emitter compared with
the 0 V on the far side of the load. Conventional
current flows through the load to the 0 V line. The
current is “pushed” through the load.
• If the input is negative, the collector of the pnp
transistor is negative compared to the zero point,
and conventional current flows from the 0 V
through the load to the pnp transistor. The
current is "pulled" through the load.
IDEAL CHARACTERISTICS OF OP-AMP
a) Voltage gain =

-the gain of the op-amp without positive or negative feedback.
-ideal op-amp, Aol is taken to be infinite value.
-Typical values of Aol range from 20,000 to 200,000 in real devices.
b) Input impedance =

-Input impedance is the ratio of input voltage to input current
Avo = Vo
Vid
Vin
Z in 
I in
- When Zin is infinite, the input current Iin=0.
- High-grade op-amps can have input impedance in the Tera Ω range.
c) Output impedance = 0
- An ideal op-amp has infinite input impedance and zero output impedance.
- when Iin is zero if Rin is equal to infinity.
Model of an op-amp
Vo   AoVi 
RL
Ro  RL
If the output resistance Ro is very small, there is no drop in output voltage due
to the output resistance of an op-amp.
d) Input offset voltage = 0
-
input offset voltage of an op-amp is equal to the output for zero input voltage
divided by the open-loop voltage gain of the amplifier.
Vos = Vo
Aol
-
output voltage of an op-amp should be zero when the value of an applied voltage
at both the input terminals is zero.
in practical op-amp, found that the output voltage does exist for zero input
voltage.
it cause by small imbalances within the op-amp.
e) Offset current = 0
- caused by difference in bias currents are needed by both input transistors in the
Op-Amp.
- It happen due to unsuccessful matching between transistor β – input transistor
1   2
I OS  I B1  I B 2
f) Bandwidth =

- The bandwidth of an amplifier is the range of frequencies for which the amplifier
gives "satisfactory performance".
Comparison of Operational Amplifier
Characteristic
The Op-Amp
Configurations
Non-inverting amplifier
 Rf 
vin
vo  1 
R1 

Inverting amplifier
 Rf
vo  
 R1

vin

Differential amplifier
 Rf
vo  1 
R1

Rf 
 R3

v2 
v1 
R1 
 R2  R3
Summing Amplifier
 v1 v2 v3 
vo   R f  
 
 R1 R2 R3 
Integrator
Differentiator
Comparator
vo  Ao v1  v2 