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

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

7.1 Basic Principles 7.1.1 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.

7.1.2 Obtaining a voltage amplifier (NMOS and npn amplifiers)

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

7.1.4 Obtaining Linear Amplification by Biasing the Transistor A dc voltage VGS 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 vgs(t)

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

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

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

Example 7.1 Solution: VGS=0.6V, VOV=0.2V

(b) The max negative swing at the drain is 0. 2V (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 VDD) Max More precise analysis

For BJT:

Example 7.2 (Read it by yourself)

7.1.6 Determining the VTC by Graphical Analysis

7.2 Small-Signal Operation and Models 7.2.1 The MOSFET Case

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

If the small-signal condition is satisfied: Voltage gain

Fig. 7.12

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

With MOSFET channel modulation

The Transconductance gm

Example 7.3: small-signal voltage gain?

IG = 0 

Modeling the Body effect

7.2.2 The BJT Case

Collector current and Transcoductance If:

If:

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

Example 7.5: determine vo/vi . Known β=100

At the quiescent operating point Since VC > VB  CBJ is reverse biased, the device is operating in the active mode

2. Determine the small-signal model 3. Determine signal vbe and vo

Small signal at output Voltage gain * The voltage gain is small because RBB is much larger than rπ

7. 3 Basic Configurations

7.3.2 Characterizing Amplifiers Output resistance Overall voltage gain

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

Common-emitter amplifier

7.3.4 CS and CE amplifier with a Rs or Re With load resistance:

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

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

7.4 Biasing 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; To locate the dc operating point in the active region and allow required output signal swing without the transistor leaving the active region.

The MOSFET case - E.g., biasing by fixing VG and connecting a Rs

Example 7.11 Solution: design the resistance by distributing VDD into 3 equal part on RD, transistor VDS and RS (each part = 5 V)

When Vt = 1.5 V

7.5 Discrete-Circuit Amplifiers (self-reading) A. A common-source (CS) amplifier

A common-source (CS) amplifier

B. A Common-Emitter (CE) Amplifier

C. A CE amplifier with an emitter resistance Re

C. A CE amplifier with an emitter resistance Re With Re must use T Model

D. A common-base (CB) amplifier

E. An emitter follower (output at emitter) T model

F. The amplifier frequency response Use CS or CE amplifier as example