ECE 333 Linear Electronics Chapter 7 Transistor Amplifiers How a MOSFET or BJT can be used to make an amplifier  linear amplification  model the linear operation  Three basic ways  Practical circuits by discrete components 1

Introduction The basic principles of using MOSFET and BJT in amplifier design are the same. Active region - MOSFET: saturation or pinch-off region) - BJT: active mode 2

7.1 Basic Principles The basis for amplifier operation – The basic application of a transistor in amplifier design is that when the device is operated in the active region, a voltage-controlled current source is realized. 3

4

7.1.2 Obtaining a voltage amplifier (NMOS and npn amplifiers) 5

6

7.1.3 The voltage-transfer characteristics – VTC is non-linear: 7 For BJT:

7.1.4 Obtaining Linear Amplification by Biasing the Transistor – A dc voltage V GS is selected to obtain operation at a point Q on the segment AB of the VTC – Q: bias point or dc operation point, or quiescent point – The signal to be amplified is v gs (t) 8

9 Figure 7.3 Biasing the MOSFET amplifier at a point Q located on the segment AB of the VTC.

10 Fig. 7.4 The MOSFET amplifier with a small time-varying signal v gs (t)

7.1.5 The Small-Signal Voltage Gain 11 (* because at B point, V GS is largest for saturation)

Example 7.1 Solution: V GS =0.6V, V OV =0.2V 12

(b) The max negative swing at the drain is 0.2V. The positive side is fine with 0.2V (0.6V is still less than V DD ) Max More precise analysis 13

For BJT: 14

Example 7.2 (Read it by yourself) 15

7.1.6 Determining the VTC by Graphical Analysis 16

17

7.2 Small-Signal Operation and Models The MOSFET Case 18

The signal current in the drain terminal 19 Small-signal condition:

If the small-signal condition is satisfied: 20 Voltage gain

21 Fig. 7.12

Separating the DC analysis and the signal analysis Small-signal equivalent circuit models 22

With MOSFET channel modulation 23

The Transconductance g m 24

Example 7.3: small-signal voltage gain? 25

26 I G = 0 

27

28

Modeling the Body effect 29

7.2.2 The BJT Case 30

Collector current and Transcoductance 31 If:

32 If:

33

The base current and the input resistance at the base The emitter current and the input resistance at the emitter 34

Example 7.5: determine v o /v i. Known β=100 35

1.At the quiescent operating point Since V C > V B  CBJ is reverse biased, the device is operating in the active mode 36

2. Determine the small-signal model 3. Determine signal v be and v o 37

38 Small signal at output Voltage gain * The voltage gain is small because R BB is much larger than r π

7. 3 Basic Configurations 39

7.3.2 Characterizing Amplifiers 40 Output resistance Overall voltage gain

7.3.3 The common-source (CS) and common- emitter amplifiers 41 common-source

Common-emitter amplifier 42

7.3.4 CS and CE amplifier with a R s or R e 43 With load resistance:

7.3.5 The common-Gate (CG) and the Common-Base (CB) Amplifiers 44

The source and emitter followers (common- drain or common-collector amplifiers) 45 (* because infinite R in )

7.4 Biasing 1.To establish in the drain (collector) a dc current that is predictable, and insensitive to variations in temperature and to large variations in parameter values between devices of the same type; 2.To locate the dc operating point in the active region and allow required output signal swing without the transistor leaving the active region. 46

The MOSFET case - E.g., biasing by fixing V G and connecting a R s 47

Example 7.11 Solution: design the resistance by distributing V DD into 3 equal part on R D, transistor V DS and R S (each part = 5 V) 48

49 When V t = 1.5 V

7.5 Discrete-Circuit Amplifiers (self-reading) 50