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Analogue Electronics II EMT 212/4 Chapter 1 Operational Amplifier Semester 1 2008/09 By: Nordiana Mohamad Saaid 1 1.0 Operational Amplifier 1.1 Introduction 1.2 Ideal Op-Amp 1.3 Op-amp Input Modes 1.4 Op-amp Parameters 1.5 Operation Single-mode Differential-mode Common-mode operation 1.6 Op-Amps Basics 1.7 Datasheet 2 1.1 Introduction Typical IC packages IC packages placed on circuit board 3 1.1 Introduction Definition The operational amplifier or op-amp is a circuit of components integrated into one chip. A typical op-amp is powered by two dc voltages and has one inverting(-) input, one non-inverting input (+) and one output. Op-amps are used to model the basic mathematical operations ; addition, subtraction, integration, differentiation etc in electronic analog computers. Other operations include buffering and amplification of DC and AC signals. 4 1.1 Introduction Two Power Supply Op-amp schematic symbol +V : Positive PS -V : Negative PS One Output Terminal Two Input Terminals Inverting input Non-inverting input 5 1.1 Introduction Applications of Op-Amp To provide voltage amplitude changes (amplitude and polarity) Comparators Oscillators Filters Sensors Instrumentation amplifiers 6 1.1 Introduction Stages of an op-amp INPUT STAGE OUTPUT STAGE GAIN STAGE 7 1.1 Introduction Typical op-amp packages 8 1.1 Introduction The 741 op-amp Real o-amp : 741 Literally a Black Box 9 1.2 Ideal Op-Amp Practical Op-Amp Ideal Op-Amp 10 1.2 Ideal Op-Amp Properties Ideal Op-Amp Practical Op-Amp Infinite input impedance Input impedance 500k-2M Zero output impedance Output impedance 20-100 Infinite open-loop gain Open-loop gain (20k to 200k) Infinite bandwidth Bandwidth limited (a few kHz) Zero noise contribution Noise contribution Zero DC output offset Non-zero DC output offset Both differential inputs stick together 11 1.2 Ideal Op-Amp Infinite Input Impedance Input impedance is measured across the input terminals It is the Thevenin resistance of the internal connection between the two input terminals. Input impedance is the ratio of input voltage to input current. Vi Zi Ii When Zi is infinite, the input current is zero. The op amp will neither supply current to a circuit nor will it accept current from any external circuit. In real op-amp, the resistance is 500k to 2M 12 1.2 Ideal Op-Amp Zero Output Impedance Looking back into the output terminal, we see voltage source with an internal resistance. The internal resistance of the op-amp is the output impedance of op-amp This internal resistance is in series with the load, reducing the output voltage available to the load Real op-amps have output impedance in the range of 20100 . 13 1.2 Ideal Op-Amp Vout AvVin Infinite Open-Loop Gain Open-Loop Gain, A is the gain of the op-amp without feedback. In the ideal op-amp, A is infinite In real op-amp, A is 20k to 200k 14 1.2 Ideal Op-Amp Infinite Bandwidth The ideal op-amp will amplify all signals from DC to the highest AC frequencies In real op-amps, the bandwidth is rather limited This limitation is specified by the Gain-Bandwidth product, which is equal to the frequency where the amplifier gain becomes unity Some op-amps, such as 741 family, have very limited bandwidth, up to a few kHz only 15 1.2 Ideal Op-Amp Zero Noise Contribution in an ideal op amp, all noise voltages produced are external to the op amp. Thus any noise in the output signal must have been in the input signal as well. the ideal op amp contributes nothing extra to the output noise. In real op-amp, there is noise due to the internal circuitry of the op-amp that contributes to the output noise 16 1.2 Ideal Op-Amp Zero Output Offset The output offset voltage of any amplifier is the output voltage that exists when it should be zero. The voltage amplifier sees zero input voltage when both inputs are grounded. This connection should produce a zero output voltage. If the output is not zero then there is said to be an output voltage present. In the ideal op amp this offset voltage is zero volts, but in practical op amps the output offset voltage is nonzero (a few miliVolts). 17 1.2 Ideal Op-Amp Both Differential Inputs Stick Together this means that a voltage applied to one inverting inputs also appears at the other non-inverting inputs. If we apply a voltage to the inverting input and then connect a voltmeter between the non-inverting input and the power supply common, then the voltmeter will read the same potential on non-inverting as on the inverting input. 18 1.3 Op-Amp Input Modes Single-Ended Input Mode Input signal is connected to ONE input and the other input is grounded. Non- Inverting Mode Inverting Mode input signal at –ve terminal input signal at +ve terminal output same polarity as the applied input signal output opposite in phase to the applied input signal 19 1.3 Op-Amp Input Modes Differential Input Mode TWO out-of-phase signals are applied with the difference of the two amplified is produced at the output. Vout Ad Vd Vd Vin1 Vin2 20 1.3 Op-Amp Input Modes Common Mode Input Two signals of same phase, frequency, and amplitude are applied to the inputs which results in no output (signals cancel). But, in practical, a small output signal will result. This is called common-mode rejection. This type of mode is used for removal of unwanted noise signals. 21 1.4 Op-Amp Parameters COMMON-MODE REJECTION (CMRR) COMMON-MODE INPUT VOLTAGE INPUT OFFSET VOLTAGE INPUT BIAS CURRENT INPUT IMPEDANCE INPUT OFFSET CURRENT OUTPUT IMPEDANCE SLEW RATE 22 1.4 Op-Amp Parameters Common-Mode Rejection Ratio (CMRR) The ability of amplifier to reject the common-mode signals (unwanted signals) while amplifying the differential signal (desired signal) Ratio of open-loop gain, Aol to common-mode gain, Acm The open-loop gain is a datasheet value CMRR Aol Acm A CMRR 20 log ol Acm The higher the CMRR, the better, in which the open-loop gain is high and common-mode gain is low. CMRR is usually expressed in dB & decreases with frequency 23 1.4 Op-Amp Parameters Common-Mode Input Voltage The range of input voltages which, when applied to both inputs, will not cause clipping or other output distortion. Input Offset Voltage Ideally, output of an op-amp is 0 Volt if the input is 0 Volt. Realistically, a small dc voltage will appear at the output when no input voltage is applied. Thus, differential dc voltage is required between the inputs to force the output to zero volts. This is called the Input Offset Voltage, Vos. Range between 2 mV or less. 24 1.4 Op-Amp Parameters Input Bias Current Ideally should be zero The dc current required by the inputs of the amplifier to properly operate the first stage. Is the average of both input currents 25 1.4 Op-Amp Parameters Input Impedance Is the total resistance between the inverting and noninverting inputs. Differential input impedance : total resistance between the inverting and non-inverting inputs Common-mode input impedance: total resistance between each input and ground 26 1.4 Op-Amp Parameters Input Offset Current Is the difference of input bias currents Input offset current Offset voltage I os I1 I 2 Vos I1Rin I 2 Rin I1 I 2 Rin Thus, error Vos I os Rin Vout( error) Av I os Rin 27 1.4 Op-Amp Parameters Output Impedance Ideally should be zero Is the resistance viewed from the output terminal of the opamp. In reality, it is non-zero. 28 1.4 Op-Amp Parameters Slew Rate Is the maximum rate of change of the output voltage in response to a step input voltage. Vout SlewRate t where Vout Vmax (Vmax ) 29 1.4 Op-Amp Parameters Slew Rate It’s a measure of how fast the output can “follow” the input signal. 30 1.4 Op-Amp Parameters Example Determine the slew rate: SlewRate Vout t SlewRate 9V (9V ) 18V / s 1s 31 1.5 Operation Differential Amplifier Circuit Types of Op-amp Operation If an input signal is applied to either input with the other input is connected to ground, the operation is referred to as ‘singleended.’ If two opposite-polarity input signals are applied, the operation is referred to as ‘double-ended.’ If the same input is applied to both inputs, the operation is called ‘common-mode.’ 32 1.5 Operation Differential Amplifier Circuit Basic amplifier circuit 33 1.5 Operation Differential Amplifier Circuit DC bias of differential amplifier circuit DC ANALYSIS VBE I E RE (VEE ) 0 IE VEE VBE RE I C1 I C 2 IE 2 since VB 0 VC1 VC 2 VCC I C RC VCC IE RC 2 34 1.5 Operation Differential Amplifier Circuit Example • Calculate the dc voltages and currents 35 1.5 Operation Differential Amplifier Circuit Example Solution IE VEE VBE RE IE 9V 0.7V 2.5mA 3.3k I C1 I C 2 I E 2.5m 1.25mA 2 2 VC VCC I C RC VC 9V (1.25m)(3.9k ) 4.1V 36 1.5 Operation Differential Amplifier Circuit AC ANALYSIS Single-Ended Connection to calculate : Av1 = Vo1 / Vi1 37 1.5 Operation Differential Amplifier Circuit AC ANALYSIS Single-Ended C B E AC equivalent of differential amplifier circuit 38 1.5 Operation Differential Amplifier Circuit AC Analysis - Single ended Scan figure 10.11 & 10.15 KVL Vi1 I b ri I b ri Ib Vi1 2ri Note : ri r Hence re I c I b Partial circuit for calculating Ib 1 2 ri1 ri2 ri I b1 I b2 I b I CQ ri Vi Vo I c Rc Av VT 1 2ri Vi Rc Vo Rc Vi1 2re 1 2ri Rc Vi 2re 1 39 1.5 Operation Differential Amplifier Circuit Example Solution IE VEE 0.7V 9V 0.7V 193A RE 43k IC IE 96.5A 2 Vc Vcc I c Rc Vc 9 (96.5 )( 47k ) 4.5V 9V (96.5A)( 47k) 4.5V Calculate the single-ended output voltage Vo1 V 26 mV T ri1 ri2 20k 1 2 75 VT 26 re 269 I CQ 0.0965 Av Rc 47 k 87.4 2re 2(269) Vo AvVi (87.4)(2m) 0.175V 40 1.5 Operation Differential Amplifier Circuit AC Analysis - Double ended A similar analysis can be used to show that for the condition of signals applied to both inputs, the differential voltage gain magnitude is Ad Vo Rc Vd 2ri where Vd Vi1 Vi 2 41 1.5 Operation Differential Amplifier Circuit AC Analysis - Common-mode Common-mode connection 42 1.5 Operation Differential Amplifier Circuit AC Analysis - Common-mode V 2( 1) I b RE Ib i ri Ib Vi ri 2( 1) RE Vi Rc Vo I c Rc I b Rc ri 2( 1) RE 1 Vo Rc Av Vi ri 2( 1) RE 43