Analog Circuits and Systems

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1 Analog Circuits and Systems Prof. K Radhakrishna Rao Lecture 10: Electronic Devices for Analog Circuits 1

2 Multipliers Multipliers provide multiplication of two input voltages or currents Multipliers can perform voltage controlled amplification filtering mixing phase detection signal generation frequency synthesis modulating demodulating 2

3 Multipliers are available commercially as voltage controlled amplifiers current controlled amplifiers digitally controlled amplifiers Input - Output Characteristic V = V + K V + K V V o offset X X Y Y offset + KVV 0 + non - linear terms is the DC offset voltage through components X Y K V and K V X X Y Y are feed K 0 is the multiplier constant and has the dimension per volts 3

4 Multipliers 4

5 Methods of Multiplication X Y X Y X Y (V V ) (V V ) V V V = = O V V R R Using squaring devices and adders ( ) V = antilog log V + log V log V O X Y Z VV VV = antilog X Y X Y log = V V Z Z Using log and antilog devices and adders 5

6 MPY634 (precision Multiplier) Log-Antilog Multiplier ( V )( ) X1 -VX2 VY1 -VY2 V ( ) o=a - VZ1-V Z2 volts 10 with -10V < V X, V Y < 10V 6

7 MPY634 The dynamic ranges of inputs and output are compatible with one another Parameter Value 1 Bandwidth 6 MHz (min) 2 Slew rate 20 V/msec 3 Output Offset Voltage +100 mv(max) 4 Output Short Circuit Current 30 ma 7

8 Example 1: Mixer Double Side Band generator/ Balanced Modulator V V V = V sinω t V = p1 p2 sinωt sinω t X p1 1 o V V V = V sin ω t = p1 p2 cos ω ω t cos ω + ω t Y p ( ( ) ( ) ) 8

9 Simulation of a Mixer For f 1 = 10 khz, f 2 = 1 khz, V p1 = 2V and V p2 = 1V 9

10 Simulation of a Mixer (contd.,) Output when DC voltage of 1 V is added to V Y 10

11 Example 2: Linear Delay Detector V X is a square wave of 10 khz V Y is a delayed (by t) square wave of 10 khz 11

12 Example 2: Linear Delay Detector (contd.,) T -10τ τ 2 VaV of Vo = = 10 1 T 2 4τ T 12

13 Example 3: Phase Detector V = V sinωt X p1 ( ) V = V sin ω t + φ Y p2 Vp1Vp2 V = o cosφ cos 2ω t + φ 20 with f=10 khz, φ =90 V =V =10 V p1 p2 π π Vo = 5 cos cos 2ω t = 5sin2ωt V = Ave.of V = 0 av o ( ) with φ = 60 π π Vo = 5 cos cos 2ω + 3 t 3 1 π = 5 cos 2ω + 2 t 3 V = Ave.of V = 25. av o Phase detector is used measuring reactive power, active power and power factor 13

14 Simulation with f = 10 khz, f = 90 O, V p1 = V p2 = 10 V 14

15 Simulation (contd.,) with f = 10 khz, f = 60 O, V p1 = V p2 = 10 V 15

16 Example 4: Sine wave generation from a triangular wave Taylor series expansion of a sine function sin x = n= 0 n ( 1) ( ) ( x ) 2n + 1! 2n ( x) ( x) = x ! 5! If x is a triangular wave it is possible to create a sine wave using three or more multipliers and an adder 16

17 Simulation 17

18 Devices for Op Amps, Comparators and Multipliers Input Output relationship of an ideal device 0 0 Y= X pf 0 where p f would be a constant Physical devices have limited dynamic range of operation resulting in saturation nonlinearities in the dynamic range Nonlinearity (first order) p = f δ δ Y X ( X,Y) = KX or KY 18

19 Trans-conducting Devices Ideal Trans-conductance amplifier (VCCS) Ii 0 0 Vi = Io gm 0 Vo g of a Trans-conducting device m Io g = ( V,I ) = KV or KI V m i o i o i The first relationship gives I = KV V; I = K(V V ) o i i o i T where V is known as Threshold Voltage T gives a square law relationship between output current and input voltage 2 19

20 Trans-conducting Devices The second relationship gives Io = I o ln I o o = S K V KV i i ( KV ) i I = I ε 1 where I S is known as Reverse Saturation Current Gives an exponential relationship between the output current and input voltage 20

21 Semiconductor devices exhibiting these relationships Field Effect Transistors (FETs) Bipolar Junction Transistors (BJTs) FET exhibits a square law relationship in the region above threshold voltage exponential relationship in sub-threshold region (V i < V T ) BJT exhibits exponential relationship 21

22 FET Patented by Julius Edgar Lilienfeld in 1926 first and by Oskar Heil in 1934 Practical semiconducting devices (JFETs) were developed only much later MOSFET (Metal Oxide on Semiconductor Field Effect Transistor) was invented by Dawon Kahng and Martin Atalla in MOSFETs largely superseded the JFETs and had a more profound effect on electronic development 22

23 BJT Bipolar point-contact transistor was invented in December 1947 at the Bell Telephone Laboratories by John Bardeen and Walter Brattain under the direction of William Shockley Junction version known as the bipolar junction transistor was invented by Shockley in 1948 BJTs enjoyed three decades as the device of choice in the design of discrete and integrated circuits 23

24 Present Status Discrete MOSFETS are not available commercially because of problems associated with electro static discharge. JFET are available, but their use is not popular in signal processing MOSFET technology is dominant in both digital and analog integrated circuits While discrete BJTs were commercially made available for several decades, their usage at present in signal processing functions has practically reduced considerably 24

25 Present Status (contd.,) This was mainly due to ready availability of Op Amps and the requirement of smaller footprints for the electronic systems Discrete semiconductor devices at present are mainly available as power devices including Power MOSFETs and IGBTs 25

26 Integrated Circuits Predominantly use CMOS (Complementary MOSFET) technology, and BiCMOS (Bipolar and Complementary MOSFET) technology to a limited extent. All digital integrated circuits are manufactured using mostly CMOS technology Some mixed signal circuits are made with BiCMOS technology Some Op Amps based on bipolar devices are still produced today because of their popularity with the users. 26

27 Why study FETs and BJTs? Design of power electronic circuits 27

28 Field Effect Transistors FET is a four terminal device: source, drain, gate and substrate Gate-substrate voltage controls the current between the source and the drain. Channel can exist between source and drain Channel can be created between source and drain by applying voltage between gate and substrate The gate is isolated from the channel by a reverse biased junction or by an insulator known as gate oxide 28

29 Field Effect Transistors JFET can be either n-type channel or p-type channel Gate is isolated from channel by a reverse biased junction Channel is always controlled in depletion mode MOSFET can be either n-type channel or p-type channel Gate is isolated from channel by insulator Channel can be controlled by either polarity of gate voltage, that is, control is achieved through either depletion or enhancement. 29

30 Types of FETs n-channel JFET p-channel JFET n-channel depletion mode FET p-channel depletion mode FET n-channel enhancement mode FET p-channel enhancement mode FET 30

31 Preferred Technology and Devices Enhancement mode FETs are the preferred devices as they are normally off-devices (no channel between source and drain) when no voltage is applied to gate with respect to the substrate. Depletion mode FET technology would have been the natural choice for analog ICs because the devices are in the active region with zero DC bias As the enhancement mode FET technology is used mainly for digital ICs, the same technology is also used for analog ICs in view of higher reliabilities and yields Use of a single technology for mixed signal processing ICs 31

32 FET (n-channel enhancement) When a voltage V higher than V is applied to the gate;current GS K ( ) 2 IDS = VGS VT 2 For V V V current saturation region and DS GS T DS DS GS ( ) T through the channel increases in proportion to square of the voltage for V I K V = ( ) DS V V ( triode r e gion) T DS V

33 Micro Models: Large Signal Model 33

34 Micro Models: Small Signal Model I where g = DS = K ( V V ) = 2I K V m GS T DS GS ( ) in the region where V V V DS GS T 34

35 Micro Models: Higher Order Model ( ) [ ] V V - V is given by I =I 1+λV DS GS T DS DSS OS where λ is known as channel length modulation factor I g DS ds= =λidss VDS 35

36 High Frequency Equivalent Circuit of MOSFET When used in analog integrated circuits 36

37 Bipolar Junction Transistor (BJT) BJT is a three terminal device: emitter, base and collector Transistor is brought into active region by forward biasing the emitter base junction reverse biasing the collector base junction types of BJTs: npn and pnp 37

38 Model of BJT I is the input current. The voltage current relationship E at the base-emitter junction, when it is forward biased E ( V V ) I =αi where α; 1 ; I = I + I ; I = α I + I C E I=I ε E0 BE T The emitter current is very nearly transported to the collector E C B E E B 38

39 Model of BJT If I is considered as the input current and I as the output current B α I I= C I=βI B B V BE=VTln 1-α I Common emitter configuration of the transistor V BE I=αI ε T g E E0 BE I I VT I α ε = = = α V C EO E C EO m VBE VT VT C V 39

40 Higher Order Micro Model Base width modulation effects I with respect to V CE C BE I=αI ε T 1+ C EO V E V V V CE where V is known as "Early Voltage" E α B C β δi = where β= β+1 δi I is the base current and I is the collector current δi δv C CE I = C =g V E ce C B kt V= is 26 mv at T= 300 K T q 40

41 High Frequency Equivalent of BJT used in ICs 41

42 Model of Ideal 3-terminal Trans-conducting Device g and finite output; V 0 and I 0 m i i Represented by a nullator at the input port a norator at the output port to make the collector or drain current the same as the emitter or source current 42

43 Synthesis of Ideal Amplifiers VCVS with gain 1 43

44 Synthesis of Ideal Amplifiers CCCS with gain -1 44

45 Synthesis of Ideal Amplifiers Trans-conductor Amplifier (VCCS) I = G V o f i 45

46 Synthesis of Ideal Amplifiers Trans-resistance Amplifier (CCVS) V o = R I f i 46

47 Conclusion 47

COLLECTOR DRAIN BASE GATE EMITTER. Applying a voltage to the Gate connection allows current to flow between the Drain and Source connections.

COLLECTOR DRAIN BASE GATE EMITTER. Applying a voltage to the Gate connection allows current to flow between the Drain and Source connections. MOSFETS Although the base current in a transistor is usually small (< 0.1 ma), some input devices (e.g. a crystal microphone) may be limited in their output. In order to overcome this, a Field Effect Transistor