Analog Circuits and Systems Prof. K Radhakrishna Rao Lecture 15: Amplifiers 1
Review Negative Feedback Systems were discussed Output variation follows the input variation if loop-gain is very large compared to one. Voltage follower and current follower application as voltage/current buffer and voltage/current regulator Phase follower and frequency follower application as FM detector and FSK detector 2
Review (contd.,) Lock range as the range over which loop gain is much greater than one. Dynamic operation of feedback systems in terms of first order and second order systems Gain bandwidth product as a measure of quality of the feedback system Q=1 for high speed feedback systems 3
Feedback amplifier design Earlier lecture (Lecture 8) used nullator-norator (as an active device) in the synthesis of ideal amplifiers: Voltage Amplifiers Current Amplifiers Trans-conductance Amplifiers Trans-resistance Amplifiers Feedback in systems has also been presented in the previous lecture (Lecture 14). It used ideal amplifiers (unilateral) and feedback which was also unilateral. 4
Amplification Signal sources cannot deliver directly required power to the load Amplification is required to enhance the signal power Amplification is one of the major analog signal processing function 5
Types of Amplifiers Based on the output power levels Preamplifiers (tuned or wide band amplifiers) Power amplifiers (tuned or wide band amplifiers) Based on input and output variables Voltage Controlled Voltage Source (VCVS) (Voltage Amplifiers) Voltage Controlled Current Source (VCCS) (Trans-conductance Amplifiers) Current Controlled Voltage Source (CCVS) (Trans-resistance Amplifiers) Current Controlled Current Source (CCCS) (Current Amplifiers) 6
Types of Amplifiers (contd.,) Preamplifiers Signal levels are very low (pico/micro/milli volts and pico/micro/milli amperes) Enhancing signal-to-noise ratio is the primary requirement Power Amplifiers Signal levels are high in terms of voltage and current Efficiency is the primary requirement 7
Types of Amplifiers (contd.,) Ideal Amplifiers have zero input power and deliver finite output power providing infinite power gain have input power zero if the input current to the amplifier is zero (I i = 0) or the input voltage to the amplifier is zero (V i = 0) 8
Types of Amplifiers (contd.,) Active devices for amplification Op amps MOSFETs (available as power devices and only in ICs) JFETs (require several discrete passive devices increasing the foot print of the amplifier) BJTs (require several discrete passive devices increasing the foot print of the amplifier) 9
Types of Amplifiers (contd.,) Op Amps Operational Voltage Amplifiers are available with a very wide range of specifications Numbers of available operational trans-resistance, current amplifiers and trans-conductance amplifiers are limited 10
Major manufacturers of Op Amps Texas Instruments National Semiconductors Analog Devices Linear Technologies Maxim Intersil Fairchild 11
Non-idealities of Op Amps Finite Gain, Finite Bandwidth, Finite Gain-Bandwidth product Offset voltages and currents Offset drifts Finite input impedance and output impedance Slew rate Current and voltage limitations Finite Common Mode Rejection Ratio (CMRR) Parameter dependence on temperature and supply voltage 12
Some popular Op Amps Compensated Op amps include 741 Single bipolar Op Amp 747 Dual bipolar Op Amp TL081 Single BIFET Op Amp TL082 Dual BIFET Op Amp TL084 Quad BIFET Op Amp LF351 Single BIFET Op Amp LF353 Dual Op Amp Uncompensated Op Amps LM748 Single bipolar Op Amp THS4011 Single bipolar high bandwidth Op Amp THS4012 Dual bipolar high bandwidth Op Amp 741 Single bipolar Op Amp 13
Parameters of TL081 All the parameters are defined for +15V TL081 1. Total Supply Voltage 7 to 36 V 2. Gain-Bandwidth Product at 25 O C 3 MHz 3. Slew Rate 13 V/msec 4. CMRR 70 db 5. Input Offset Voltage 20mV (max) 6. Input Offset Voltage Temperature Coefficient 18mV/ O C 7. Input Offset Current 2 na (max) 8. Input Bias Current 10 na (max) 9. Input Resistance 10 12 W 10. Output Resistance 200 W 14
Feedback in two-port active networks A general two-port network in Y-parameters I Y Y V i = ia ra i I Y Y V o fa oa o 15
Feedback in two-port active networks (contd.,) If the two-port network is an active device, which is assumed to be unilateral the reverse transfer parameter Y ra =0 Y ia and Y oa are finite and small compared to Y fa All the Y-parameters of the active device are sensitive to temperature, time and bias supply voltage and have poor manufacturing tolerances Using a suitable passive twoport work with the active device it is possible to make resultant system close to the ideal amplifier 16
Feedback in two-port active networks (contd.,) We choose the passive twoport network with Y-matrix Y Y ip rp Y Y rp op andwith Y fa? Y rp 17
Feedback in two-port active networks (contd.,) The passive two-port network is connected to the active device in shunt at both the input and output Iia + Iip Ii Yia + Yip Yra + Yrp V(V i ia = V ip) = = Ioa + Iop Io Yfa + Yfp Yoa + Yop V o(voa = V op) where Y + Y = Y and Y + Y Y ra rp rp fa fp fa Admittances are added at the input and output 18
Feedback in two-port active networks (contd.,) When admittances increase at the input the resultant input impedance is decreasing leading to the system becoming near ideal current controlled (CC) When admittances increase at thee output the resultant controlled source becomes a near ideal voltage source (VS) The resultant ideal CCVS should have an impedance matrix 0 0 Z 0 with Z independent of parameters of the active device 19
Feedback in two-port active networks (contd.,) Y +Y + Y -Y = Yfa Y oa +Yop + YL -Y Y fa ia +Yip + YS Δ Δ 1 oa op L rp Y ia +Yip + YS Y rp Δ Δ fa rp ( Y )( ) ia +Y ip +YS Y oa +Y op +YL ( )( ) where the detrminant Δ= Y +Y + Y Y +Y + Y Y Y If Y Y ia ip S oa op L fa rp? 1then Δ=-Y Y YfaY rp where is the loop gain ( Y ) ia +Y ip +Y ( ) S Y oa +Y op +YL If negative then, negative feedback, otherwise it is positive feedback. fa rp 20
Z-Matrix CCVS ( + ) Y oa +Yop YL Y rp 0 0 Y Y Y Y fa rp fa rp 1 asy ( ) fa islarge 0 Y Y fa ia +Yip + YS Yrp YfaYrp YfaY rp The use of feedback passive network around an active device made the input-output relationship independent of parameters of the active device as well as source and load admittances 21
Resultant Z-matrix - CCVS Macromodel 22
Z-matrix of the composite CCVS L L f ia S f oa f -1 1 1 1 1 Rf Roa Roa 1 + + 1 Ria Rf RS R + + f A RL Rf ARoa = A 1 1 1 1 Rf Roa Roa Roa + + R R f oa Rf Roa Rf R + + L A Ria RS Rf A gl = Roa Roa Roa Roa R 1 oa + + + + R R R R R If g? 1then Δ= R A R 23
Example Design trans-resistance amplifier with a Z-matrix 0 0 1kΩ 0 for a source resistance of 10 kw and a load resistance of 1kW Consider an Op Amp with input impedance of 1 MW, voltage gain of 10 6, and output impedance of 100W The Op Amp is represented by Y-matrix 1S µ 0 4 10 S 10mS The feedback passive network is chosen to have an admittance matrix 1mS 1mS 1mS 1mS 24
Resultant Z-matrix - CCVS 1.2mΩ 0.1mΩ 0 0 1kΩ 0.11mΩ 1kΩ 0 25
Feedback network The feedback passive network is chosen to have an admittance matrix The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert again. 1mS 1mS 1mS 1mS R f = 1kΩ 26
The composite Y-matrix 1µ S+ 1mS + 0.1 ms 1 ms 1.1mS 1mS 10 4 S 1mS 10mS 1mS 1mS 10 4 + + S 12mS ( )( ) Δ= Y + Y + Y Y + Y + Y Y Y ia ip S oa op L fa rp 7 fa rp 2 Y Y 10 =? 1then Δ= 10S 13.2 ( Y + Y + Y )( Y + Y + Y ) ia ip S oa op L 27
CCVS R s =10k R L =1k R f =1k 28
General Amplifier (VCVS, CCVS, VCCS and CCCS) An ideal general amplifier should have an immittance matrix 0 0 p 0 f p f is finite and is the chosen design parameter should be independent of parameters of active device used 29
Available active devices p 0 ia p p fa oa P fa is very large All the three elements of immittance matrix have poor manufacturing tolerances can vary widely with temperature, time and bias supply voltages 30
Available active devices (contd.,) This can be achieved by adding two matrices and inverting the resultant matrix p + p p oa op rp 1 p + p p ia ip rp Δ Δ = p p p p ( p p ) p p + + fa fp oa op + + fa fp ia ip Δ Δ 31
Available active devices (contd.,) 1 prp Composite ( ) ( ) ( ) ( ) p ( ) ia + pip 1 gl pia + pip poa + pop 1 g L Inverted = ( p ) fa pfp Matrix + 1 ( p ) ( ) ( ) ( ) ( ) ia pip poa pop 1 gl poa pop 1 g + + + L L ( ) ( ) ( ) where Δ= p + p p + p 1 g p p when g? 1 where g = ia ip oa op L fa rp L ( p ) fa + pfp prp ( p + p ) ( p + p ) ia ip oa op 0 0 1 pia + pip prp 1 p 0 fa pfp poa p + + op p rp isloop gainwhich has to be negative for negative feedback 32
Available active devices (contd.,) Choose a passive linear two-port network p p ip fp p p rp op We need to generate a matrix similar to from the two matrices 0 0 pf 0 p p p 0 ia ip rp and p p p p fa oa fp op 33
Conclusion Design of feedback amplifiers Feedback Matrix Voltage h g Current g h Trans-conductance Z Y Trans-resistance Y Z In the next lecture we shall discuss the design of voltage and current amplifiers and trans-conductance amplifiers 34