Analogue ELECTRONICS

Cascade and Power Amplifiers

• Define cascade and cascade connection

• Explain Darlington connection and transistor.

• Describe and Analyze the operation of class A power amplifiers.

• Describe and Analyze the operation of class B and class AB power amplifiers.

• Discuss and Analyze the operation of class C amplifiers.

• Explain amplifier distortion.

Define cascade and cascade connection

Two or more amplifiers can be connected in a cascaded arrangement with the output of one amplifier driving the input of the next.

Each amplifier in a cascaded arrangement is known as a stage.

The basic purpose is to increase the overall voltage gain.

MULTISTAGE AMPLIFIERS

•Multistage voltage gain

•Capacitively-coupled Multistage Amplifier

•Direct-coupled Multistage Amplifier

Multistage voltage gain

•Amplifier voltage gain is often expressed in decibels (dB) as follows:

•Av(dB) = 20 log Av

•Useful in multistage systems because the overall gain is the sum of the individual voltage gains in dB

•A’v(dB)= Av1(dB) + Av2(dB)+…+Avn(dB)

CaPACITIVELY-COUPLED MULTISTAGE AMPLIFIER

• Capacitive coupling prevents the DC bias of one stage from affecting that of the other BUT allows the AC signal to pass without attenuation because Xc≈0 at the frequency of operation.

Attenuation: gradual loss in

intensity of any kind of flux through

a medium

Common emitter amplifier at both

stages

1st stage output capacitively

coupled 2nd stage input

Direct-COUPLED MULTISTAGE AMPLIFIER

• Has a better low frequency response that the capacitively-coupled in which the reactance of coupling and bypass capacitors at very low frequency may become exessice(Xc=1/2πfC). • The increased reactance of capacitors at lower

frequencies produces gain reduction in capacitively- coupled amplifiers

• Can be used to amplify low frequencies without loss of voltage gain because there are no capacitive reactances in the circuit.

• Small changes in the DC bias voltages from temperature effects or power supply variation are amplified by succeeding stages • result in significant drift in the DC levels

Differential Amplifiers

• An amplifier that produces outputs that are a function of the difference between two input voltages

• Has 2 basic mode of operation:• Differential (different two inputs)

• Common mode (same two inputs)

DIFFERENTIAL AMPLIFIER

• Basic Operation

• Modes of Signal Operation

• Single-Ended Differential Input

• Double-Ended Differential Input

• Common-Mode Rejection Ratio

Basic operation

• Amplifier has 2 inputs and 2 outputs

Basic analysis of differential amplifier operation

• Both inputs are grounded (0 V), emitters are -0.7 V

• Assuming when there are no input signal, DC emitter currents are the same

• IE1=IE2=IRE/2

• IRE=(VE-VEE)/RE

• IC=IE

• IC1=IC2=IRE/2

• When input voltage is 0, both collector current and both collector resistors are equal

• VC1=VC2=VCC-IC1RC1

• Input 2 is grounded, input 1 positive biased

• Q1 conduct more, IC1 increase

• IC1 increase, VC1 decrease

• VBE of Q2 reduces, thus IC2 decrease

• IC2 decrease, VC2 increase

• Input 1 is grounded, input 2 positive biased

• Q2 conducts more, IC2 increase

• IC2 increase, VC2 decrease

• VBE of Q1 reduces, thus IC1 decrease

• IC1 decrease, VC1 increase

Modes of signal operation:single-ended differential input

• Input 2 is grounded, signal voltage applied to input 1

• At output 1, an inverted, amplified signal voltage appear

• At emitter Q1 an in phase signal voltage appears

• Emitters Q1 and Q2 are common, this becomes input to Q2 (function as common-base amplifier)

• Signal amplified by Q2 appears non inverted at output 2

Modes of signal operation:double-ended differential inputs

• Two opposite-polarity (out of phase) signals are applied to the inputs

Signal on input 2 acting alone, see output 1 and output 2Signal on input 1 acting alone, see output 1 and output 2

Signals of output 1 same polarity.Signals of out put 2 same polaritySuperimpose these signals and get total output signals

Modes of signal operation:common-mode inputs

Common-mode condition:Two signal voltage of the same phase, frequency and amplitudeare applied to the two inputs

Common-mode rejection ratio (cmrr)

• The measure of an amplifiers’ ability to reject common-mode signals parameter

• CMRR = Av(d) / Acm = 20 log (Av(d)/Acm)

• The higher CMRR the better

• A CMRR of 10000 means that the desired input signal (differential) is amplified 10000 times more than the unwanted noise (common-mode)