BJT as Amplifiers, Lecture notes of Analog Electronics

Download BJT as Amplifiers and more Lecture notes Analog Electronics in PDF only on Docsity!

ANALOGUE ELECTRONICS

AENG001- 4 - 2

CHAPTER 5

BJT as Amplifiers

Chapter 5 BJT as Amplifiers

 5.1 General Considerations  5.2 Operating Point Analysis and Design  5.3 Bipolar Amplifier Topologies  5.4 Summary and Additional Examples

Voltage Amplifier

 In an ideal voltage amplifier, the input impedance is infinite and the output impedance zero.  But in reality, input or output impedances depart from their ideal values.

Input/Output Impedances

 The figure above shows the techniques of measuring input and output impedances. x x x i V R 

Impedance at a Node

 When calculating I/O impedances at a port, we usually ground one terminal while applying the test source to the other terminal of interest.

Impedance at Collector

 With Early effect, the impedance seen at the collector is equal to the intrinsic output impedance of the transistor (if emitter is grounded). out o R  r

Three Master Rules of Transistor Impedances

 Rule # 1: looking into the base, the impedance is r if emitter is (ac) grounded.  Rule # 2: looking into the collector, the impedance is ro if emitter is (ac) grounded.  Rule # 3: looking into the emitter, the impedance is 1/gm if base is (ac) grounded and Early effect is neglected.

Biasing of BJT

 Transistors and circuits must be biased because (1) transistors must operate in the active region, (2) their small- signal parameters depend on the bias conditions.

Notation Simplification

 Hereafter, the battery that supplies power to the circuit is replaced by a horizontal bar labeled Vcc, and input signal is simplified as one node called Vin.

Example of Bad Biasing

 The microphone is connected to the amplifier in an attempt to amplify the small output signal of the microphone.  Unfortunately, there’s no DC bias current running thru the transistor to set the transconductance.

Biasing with Base Resistor

 Assuming a constant value for VBE, one can solve for both IB and IC and determine the terminal voltages of the transistor.  However, bias point is sensitive to  variations. B CC BE C B CC BE B R V V I R V V I     , 

Improved Biasing: Resistive Divider

 Using resistor divider to set VBE, it is possible to produce an IC that is relatively independent of  if base current is small. exp( ) 1 2 2 1 2 2 T CC C S X CC V V R R R I I V R R R V    

Emitter Degeneration Biasing

 The presence of RE helps to absorb the error in VX so VBE stays relatively constant.  This bias technique is less sensitive to  (I 1 >> IB) and VBE variations.

Design Procedure

 Choose an IC to provide the necessary small signal parameters, gm, r, etc.  Considering the variations of R 1 , R 2 , and VBE, choose a value for VRE.  With VRE chosen, and VBE calculated, Vx can be determined.  Select R 1 and R 2 to provide Vx.