CHAPTER 3

OPERATIONAL AMP

OPERATIONAL AMPLIFIERLearning Objectives: To understand the general Op-amp circuit

design To understand the differential amplifier To understand more stages in gain amplifier To understand the push pull amplifier To understand the ideal op-amp To understand the op-amp

configurations

INTRODUCTION• Op-amp has high gain amplifier and able to

amplify signal with frequency ranging from 0 to 1MHz.

• designed to perform mathematical operations like summation, subtraction, multiplication, differential and integration etc in analogue computer

• has 2 input terminal :

- Inverting input terminal

- Non inverting input terminal

• only has 1 output terminal

• a complete amplifier electronic circuit may contains transistor, diode, resistor, capacitor and others components and constructed on a single silicon ship.

• the area is 5mm2 and thickness is less than 0.5mm, it is protected by lace plastic.

SYMBOL-

Construction:

- 2 input terminal

- 1 output terminal

CONFIGURATION

TYPE & EXAMPLES

• Common usage– Have range from 0 to 1 MHz– Examples: µA709 , 101, 741, 301

• For DC usage and low achievement– Have high input impedance– Low offset input voltage– Examples : h0052, 108

• For AC usage and high achievement– Have wide range– Example: H0063

• For high power and voltage– Use single line power supply and can trigger

load directly– Examples ; H0004, H0021, M124

• Programmable– Special op-amp that can be programmable– Example: 4250

• As mathematic operation in analogue/calculator

- Summation, multiplication, division, integration and differentiation.

• Waveform generator– Uses in Wein Bridge Oscillator for generate

sinus waveform.– Use differential and integrator circuit to

produce square wave and triangular waveform.

OP-AMP APPLICATIONS

• RC active filter– To filter signal which allow signal pass at

certain frequency only.

• Signal gain – To amplify audio/radio frequency @ signal– To amplify digital signal that send across long

distance

• Op-amp IC 741 is considered as industry standard op-amplifier.

DIFFERENTIAL AMP

• Differential Amplifier - BJT Type - Re function as high

impedance to input signal at input terminal. (Re = ideal)

- If there are same input ,Vin1

=Vin2 , amplifier produce small gain, Av=0

- If the input different, Vin1 ≠

Vin2 , amplifier have high gain, Av = high.

- Output id depended on differentiate for 2 input.

• Differential Amplifier – FET Type

Differences Between FET & BJT

FET BJT

Similarity

Both as amplifier

Connection : Common BaseGate / Base – as input

Drain / Collector – as output

Differences

FET with common base connection has high input resistance almost 100MΩ

FET - low internal noise rate compare to common transistor. It always use in hi-fi amplifier and receiver

in FM radio

Differences

Control voltage controlled current controlled

Current Carrier

only 1 majority current carrier

both current carrier

CONFIGURATION IN OP-AMP CIRCUIT

• Double ended input, balance output

• Double ended input, unbalance output

• Single ended input, balance output

• Single ended input, unbalance output

DEFFERENTIATE THE CONFIGURATION

• Each configuration is categories based on several factors:

i. 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

DEFFERENTIATE THE CONFIGURATION

ii. 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.

OP AMP INTERNAL BLOCK DIAGRAM

• Consist of 3 level :– 1st level – input level– 2nd level – Centre level– 3rd level – output level

• It is designed in 3 levels to avoid problem when the signal is weak and avoid from noise such as static interference, signal interference from power supply.

OP AMP INTERNAL BLOCK DIAGRAM

• Op-amp internal block diagram.

High Input Impedance Differential Amplifier

• 2 input, balance output. • High impedance to each input signal• Low gain to Common Mode Rejection Ratio (CMRR)

such as humming and noise.• High gain to useful signal• Reduce offset input voltage• Reduce ‘drift’ in circuit by using direct coupling.• ‘Drift’ is undesired signal that amplified together with

useful signal• This level most important because the impedance for

input is set and CMRR and offset voltage is reduced.

High Gain Voltage Amplifier (More stage of gain)

• 2 input, balance output.

• High voltage gain

• High drive current gain to trigger output and not burden to input.

Low Output impedance Differential Amplifier

• Known as class B push pull amplifier @ push pull emitter follower.

• Consist of single input-balance output• Act as buffer to connect 2nd level and

output amplifier differential amplifier not affected by load.

• Prepare enough current to trigger load at output terminal

• Prepare low impedance output.

DIFFERENTIAL AMPLIFIER INTERGRATED CIRCUIT

• Consist of 4 cascade amplifier.

• Differential amplifier with ‘Double ended output’– Av1 formed by Q1 and Q2 while Q3 is used

as ‘constant-current’ terminal for supply ‘high common mode rejection’

– Q3 is stable from temperature effect by 2 resistors and 2 diodes.

• Differential amplifier with single ended output– Output from differential amplifier Q1 and Q2

will trigger both Q4 and Q5– Q4 and Q5 have emitter source.

• Single ended output from second pair (from Q5) will trigger output stage through Q6.

• Feedback resistance for stable the output voltage refers to ground.

• Q7 and Q8 operation as phase splitter for give maximum voltage oscillation ( almost same as supply voltage for minimize output impedance)

• Transistor Q8 will amplify the signal.

TERM DEFINITION• Open Loop Gain

– Amplifier gain without feedback.– Ideal Op-amp in open loop gain = ∞ , Vo for

op-amp is too large compare to Vid

– Vid normally too small and practically can assume no different between inverting and non- inverting input.

Common Mode Gain– Gain – when both input terminal have same signal– Should be when Vid =0, Vo= 0– Practically, when Vo have value Acm will have small value.– Knows as CMRR, Common Mode Rejection Ratio– CMRR = Closed loop gain, Av

• Common mode gain, Acm– Normally Acm << 1– CMRR larger is better– Unit : dB– Av CMRR (dB) = 20 lg Acm

• = 20 lg CMRR

– For op-amp 741, CMRR = 90dB

• Offset Voltage

– undesired small signal– Generate by amplifier and voltage is produces

between input terminals even though the input is connected to 0 volt.

– It happen due to unsuccessful matching between basic transistor in op amp circuit.

• Vos – a few milivolt.

• Offset Current

– Different between current is needed by two input transistor in op amp

– It happen due to unsuccessful matching between transistor β – input transistor,

• β1≠ β2

• Ios = Iβ1 - Iβ2

• Virtual Ground

– Both op amp input terminal always refer as adder point or virtual ground

– It happen/occur due to :• Current always low• Voltage as input terminal too low compared to voltage at

others when op amp used without feedback. This low voltage is predicted as input terminal that connected to ground.

– Virtual ground is a point where the current flow and voltage at that point is zero.

• Maximum rate for Op-Amp

• Op amp will damage if the maximum rate is exceeded :– Power supply ± 18V– Operating temperature range, 0-700C– Input voltage, maximum different between +ve and –ve

terminal : 30V– Common mode input voltage, 12V, at power supply

15V– Internal power dissipation, 310mW– Saving temperature range, 55-1250C

Ideal and Actual Characteristic of Op Amp

OP-AMP CONFIGURATIONS

• Inverting amplifier (Group I)

• Non-inverting amplifier (Group H)

• Summing amplifier (Group G)

• Differential amplifier and subtractor amplifier (Group F)

• Integrator (Group E)

• Differentiator (Group C & Group D)

• Comparator (Group A & Group B)