Electron Devices and Circuits

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1 Electron Devices and Circuits (EC 8353) Prepared by Mr.R.Suresh, AP/EEE Ms.S.KARKUZHALI,A.P/EEE

2 BJT small signal model Analysis of CE, CB, CC amplifiers- Gain and frequency response MOSFET small signal model Analysis of CS and Source follower Gain and frequency response- High frequency analysis.

3 BJT: Two port network, Transistor hybrid model, determination of h- parameters, conversion of h-parameters, generalized analysis of transistor amplifier model using h-parameters, Analysis of CB, CE and CC amplifiers using exact and approximate analysis, Comparison of transistor amplifiers. FET: Generalized analysis of small signal model, Analysis of CG, CS and CD amplifiers, comparison of FET amplifiers.

4 Agenda Small Signal Analysis Hybrid h-parameter model for an amplifier Hybrid Parameters or h-parameters Transistor Hybrid Model Analysis of Transistor Amplifier using Complete h- Parameter Model Analysis of Transistor Amplifier using simplified h-parameter Model

5 We represent the transistor amplifier circuit in the form of a two port network as shown in fig. This two port network represent the transistor in any one of its three configurations (CE,CB,CC).

6 Small Signal Analysis of Amplifiers Small signal response is analyzed using the h-parameter model Response of an amplifier depends on frequency considerations. Frequency response curves of RC Coupled amplifier, DC amplifier is shown. There are 3 regions of frequency : low, mid and high The difference between high and low frequency is the bandwidth

7 RC Coupled Amplifier

8 DC Amplifier

9 Hybrid h-parameter model for an amplifier The equivalent circuit of a transistor can be dram using simple approximation by retaining its essential features. These equivalent circuits will aid in analyzing transistor circuits easily and rapidly. A transistor can be treated as a two part network. The terminal behavior of any two part network can be specified by the terminal voltages V 1 & V 2 at parts 1 & 2 respectively and current i 1 and i 2, entering parts 1 & 2, respectively, as shown in figure.

10 Two Port Network

11 Of these four variables V 1, V 2, i 1 and i 2, two can be selected as independent variables and the remaining two can be expressed in terms of these independent variables. This leads to various two part parameters out of which the following three are more important.

12 Hybrid Parameters or h-parameters If the input current i 1 and output Voltage V 2 are takes as independent variables, the input voltage V 1 and output current i 2 can be written as V 1 = h 11 i 1 + h 12 V 2 i 2 = h 21 i 1 + h 22 V 2 The four hybrid parameters h 11, h 12, h 21 and h 22 are defined as follows. h 11 = [V 1 / i 1 ] with V 2 = 0 = Input Impedance with output part short circuited.

13 h 22 = [i 2 / V 2 ] with i 1 = 0 = Output admittance with input part open circuited. h 12 = [V 1 / V 2 ] with i 1 = 0 = reverse voltage transfer ratio with input part open circuited. h 21 = [i 2 / i 1 ] with V 2 = 0 = Forward current gain with output part short circuited.

14 The dimensions of h parameters are as follows: h 11 - Ω h 22 mhos h12, h21 dimension less. as the dimensions are not alike, (i.e) they are hybrid in nature, and these parameters are called as hybrid parameters.

15 The Hybrid Model for Two-port Network:- V 1 = h 11 i 1 + h 12 V 2 I 2 = h 1 i 1 + h 22 V 2 V 1 = h 1 i 1 + h r V 2 I 2 = h f i 1 + h 0 V 2

16 The Hybrid Model for Two-port Network:-

17 Transistor Hybrid Model Use of h parameters to describe a transistor have the following advantages: h parameters are real numbers up to radio frequencies. They are easy to measure They can be determined from the transistor static characteristics curves. They are convenient to use in circuit analysis and design. Easily convert able from one configuration to other. Readily supplied by manufactories.

18 Transistor Hybrid Model CE Configuration In common emitter transistor configuration, the input signal is applied between the base and emitter terminals of the transistor and output appears between the collector and emitter terminals. The input voltage (V be ) and the output current (i c ) are given by the following equations: V be = h ie.i b + h re.v c i e = h fe.i b + h oe.v c

19 Transistor Hybrid Model CE Configuration

20 Transistor Hybrid Model CB Configuration Where h ie =( f 1 / i B )V c = ( v B / i B )V c = (Δv B /Δi B )V c = (v b / i b )V c h re =( f 1 / v c )I B = ( v B / v c ) I B = (Δv B /Δv c ) I B = (v b /v c ) I B h fe =( f 2 / i B )V c = ( i c / i B )V c = (Δ i c /Δi B )V c = (i c / i b )V c h oe = ( f 2 / v c )I B = ( i c / v c ) I B = (Δ i c /Δv c ) I B = (i c /v c ) I B The same theory is extended to other configurations including CB and CC

21 Hybrid Model and Equations for the transistor in three different configurations are are given below.

22 Analysis of Transistor Amplifier using Complete h- Parameter Model In the h-parameter model consider the load Resistance R L and input signal V s. The expressions for Current gain, Voltage gain,input and output impedance are: 1. Current Gain: A i =-h f /(1+h o R L ) Where A i is the current amplification or current gain The overall current gain taking source resistance is given by: A is =A i * (R s /Z i + R s ) where Z i input impedance R s source resistance

23 Analysis of Transistor Amplifier using Complete h-parameter Model 2)Input Impedance(Zi) Z i = h i +h r A i R L 3) Voltage Gain(A v ): A v =(A i * R L )/ Z i Voltage gain taking source resistance is given by A vs =(A v * Z i )/(Z i +R s ) 4) Output Admittance(Y o ) Y o =h o -h f * h r /(h i +R s )

24 Analysis of Transistor Amplifier using simplified h- Parameter Model Common Emitter Configuration Fixed Bias configuration: Input Impedance Z i = R B h ie Output Impedance Z o =R C (1/h oe ) Voltage gain A v =-h fe * (R C (1/h oe ) /h ie Current Gain A i =h fe * R B /(R B + h ie ) Voltage Divider Configuration: Input impedance Z i =(R B1 R B2 ) h ie Output Impedance Z o =R C (1/h oe ) Voltage gain Av=-h fe * [R C (1/h oe )]/h ie Current gain A i =h fe * (R B1 R B2 )/(R B1 R B2 ) + h ie

25 Hybrid Equivalent Model The hybrid parameters: hie, hre, hfe, hoe are developed and used to model the transistor. These parameters can be found in a specification sheet for a transistor. 25

26 26 0V V o i 12 0V V i i 11 o 12 i 11 i o o V V h I V h V h I h V 0A I o o 22 0V V o i 21 o o 22 i 21 O o o V I h I I h 0V, Solving V V h I h I H 22 is a conductance!

27 General h-parameters for any Transistor Configuration hi = input resistance hr = reverse transfer voltage ratio (Vi/Vo) hf = forward transfer current ratio (Io/Ii) ho = output conductance 27

28 28

29 29

30 Simplified General h-parameter Model The model can be simplified based on these approximations: hr 0 therefore hrvo = 0 and ho (high resistance on the output) Simplified 30

31 Common-Emitter re vs. h-parameter Model hie = re hfe = hoe = 1/ro 31

32 Common-Emitter h-parameters h ie r e [Formula 7.28] h fe ac [Formula 7.29] 32

33 Common-Base re vs. h-parameter Model hib = re hfb = - 33

34 Common-Base h-parameters h ib r e [Formula 7.30] h fb 1 [Formula 7.31] 34

SMALL-SIGNAL LOW-FREQUENCY OPERATION OF TRANSISTORS Hybrid Parameters and Two-Port Network For the hybrid equivalent model to be described, the parameters are defined at an operating point that may

35 SMALL-SIGNAL LOW-FREQUENCY OPERATION OF TRANSISTORS Hybrid Parameters and Two-Port Network For the hybrid equivalent model to be described, the parameters are defined at an operating point that may or may not give an actual picture of the operating condition of the amplifier. The quantities h ie, h re, h fe and h oe are called the hybrid parameters and are the components of a small-signal equivalent circuit. The description of the hybrid equivalent model begins with the general two-port system. Two-port system representation (Black model realisation)

EQUIVALENT CIRCUITS THROUGH HYBRID PARAMETERS AS A TWO-PORT NETWORK For the transistor, even though it has three basic configurations, they are all fourterminal configurations, and thus, the

36 EQUIVALENT CIRCUITS THROUGH HYBRID PARAMETERS AS A TWO-PORT NETWORK For the transistor, even though it has three basic configurations, they are all fourterminal configurations, and thus, the resulting equivalent circuit will have the same format. The h-parameter will however change with each configuration. To distinguish which parameter has been used or which is available, a second subscript has been added to the h-parameter notation. (i) For the common-base configuration: the lower case letter b (ii) For the common-emitter configuration: the lower case letter e (iii) For the common-collector configuration: the lower case letter c Complete hybrid equivalent model

37 TRANSISTOR AS AMPLIFIER An n p n transistor in the common-base bias mode

38 EXPRESSIONS OF CURRENT GAIN, INPUT RESISTANCE, VOLTAGE GAIN AND OUTPUT RESISTANCE The h-parameter equivalent circuit of a transistor amplifier having a voltage source V g, with its input resistance R g connected to the input terminals and a load resistance R L connected to the output terminals. h-parameter equivalent circuit of a transistor

39 EXPRESSIONS OF CURRENT GAIN, INPUT RESISTANCE, VOLTAGE GAIN AND OUTPUT RESISTANCE Current Gain (A I ) Input Resistance (R I )

40 EXPRESSIONS OF CURRENT GAIN, INPUT RESISTANCE, VOLTAGE GAIN AND OUTPUT RESISTANCE Voltage Gain:- Voltage gain or voltage amplification is defined as the ratio of the output voltage V2 to the input voltage V1. Where, Output Resistance (R O )

41 FREQUENCY RESPONSE FOR CE AMPLIFIER WITH AND WITHOUT SOURCE IMPEDANCE At different frequencies of the input signal, the performance of the device is different. The analysis till now has been limited to the mid-frequency spectrum. Frequency response of an amplifier refers to the variation of the magnitude and phase of the amplifier with frequency. a) Gain vs. frequency for a CE amplifier (b) Phase angle vs. frequency for a CE amplifier

42 EMITTER FOLLOWER The emitter follower transistor is a design which is basically a CC amplifier. Current gain: Input resistance: Voltage gain: Output resistance An emitter follower configuration with biasing The emitter follower is used for impedance matching.

44 Figure Small-signal equivalent circuit for FETs.

45 Figure FET small-signal equivalent circuit that accounts for the dependence of i D on v DS.

46 Figure Determination of g m and r d. See Example 5.5.

47 Figure Common-source amplifier.

48 For drawing an a c equivalent circuit of Amp. Assume all Capacitors C1, C2, Cs as short circuit elements for ac signal Short circuit the d c supply Replace the FET by its small signal model

49 L gs m L o o gs o v R v g R i v v v A Voltage gain, d D L L m gs o v r R R R g v v A, D d D d D d o R r R r R r Z imp., put Out 2 1 imp., Input R R R Z G in A C Equivalent Circuit Simplified A C Equivalent Circuit

50 Av gm(rd RD) This is a CS amplifier configuration therefore the input is on the gate and the output is on the drain. Av Av g g m R m D, (r d r R d D ) 10R D Zi R1 R2 Zo rd RD Zo R D r d 10R D

51 Figure v o (t) and v in (t) versus time for the common-source amplifier of Figure 5.28.

52 An Amplifier Circuit using MOSFET(CS Amp.) Figure Common-source amplifier.

53 A small signal equivalent circuit of CS Amp. Figure Small-signal equivalent circuit for the common-source amplifier.

54 Figure v o (t) and v in (t) versus time for the common-source amplifier of Figure 5.28.

55 Figure Gain magnitude versus frequency for the common-source amplifier of Figure 5.28.

56 Figure Source follower.

57 Figure Small-signal ac equivalent circuit for the source follower.

58 Figure Equivalent circuit used to find the output resistance of the source follower.

59 Figure Common-gate amplifier.

Module-1 BJT AC Analysis: The re Transistor Model. Common-Base Configuration

Module-1 BJT AC Analysis: The re Transistor Model. Common-Base Configuration Module-1 BJT AC Analysis: BJT AC Analysis: BJT AC Analysis: BJT Transistor Modeling, The re transistor model, Common emitter fixed bias, Voltage divider bias, Emitter follower configuration. Darlington