ELTX II 3rd Sem

POWER POINT PRESENTATION

SUBMITTED BY:-

POONAM SHARMA

Important terms about power amplifier

1. Collector Efficiency - The ration of A.C. output at zero signal power to average D.C. power input to transistor .

collector efficiency = average a.c. power output

average d.c. power input

2. Distortion - When the output of an amplifier is not similar to the input signal we say distortion is there. This is due to non-linear characteristic of transistor .

Types of distortion - a. Harmonic Distortion

b. Phase distortion c. Frequency distortion

3. Power dissipation capacity Is defined as the ability of a transistor to dissipate the heat developed in it.

5

TUNED AMPLIFIER

Feedback

• Consists of returning part of the output of a system

to the input

• Negative Feedback: a portion of the output signal is

returned to the input in opposition to the original

input signal

• Positive Feedback: the feedback signal aids the

original input signal

TOPOLOGIES OF THE FEEDBACK AMPLIFIER:

There are four basic amplifier types. Each of these is being approximated by the characteristics of an ideal controlled source. The four feedback topologies are as follows:

1. Series-shunt feedback

2. Series-series feedback

3. Shunt-series feedback

4. Shunt-shunt feedback

The alternative nomenclature used is as follows:

1. Voltage-series or series-shunt feedback

2. Current-series or series-series feedback

3. Current-shunt or shunt-series feedback

4. Voltage-shunt or shunt-shunt feedback

Voltage amplifiers with voltage-series feedback:

• The input voltage Vi of the basic amplifier is the algebraic sum of input signal Vs and the feedback signal Vo , where Vo is the output voltage.

Current-Series or Series-Series Feedback:

• Trans-conductance feedback amplifier provides an output current Io which is proportional to the input voltage Vs . The feedback signal is the voltage Vf , which is added to Vs at the input of the basic amplifier.

Current amplifiers with current-shunt feedback

• The current-shunt feedback amplifier, supplies an output current Io which is proportional to the input current Ii . This makes it a current amplifier.

• The feedback signal is the current if the input current of the basic amplifier is Ii = Is + If and the output current is Io = I L.

Voltage-Shunt or Shunt-Shunt Feedback

• The voltage-shunt or shunt-shunt feedback amplifier provides an output voltage Vo in proportion to the input current Is. The input current Ii of the basic amplifier is the algebraic sum of Is and the feedback current If .

EFFECT OF FEEDBACK ON GAIN, INPUT AND OUTPUT IMPEDANCES:

• Feedback is applied with the objective of improving the performance of an amplifier. The operation of an amplifier is regulated by controlling the gain and impedance.

• The effect of feedback on gain and impedance for the different topologies—voltage-series, current-series, current-shunt, voltage-shunt—are discussed in the following sections.

Barkhausen Criterion:

POINTS TO REMEMBER:

IMPORTANT FORMULAE:

Negative Feedback Advantages &

Disadvantages • Negative Feedback Effects:

– Reduces gain

– Stabilizes gain

– Reduces non linear distortion

– Reduces certain types of noise

– Controls input and output impedances

– Extends bandwidth

• The disadvantage of reducing the gain can be overcome by adding few more stages of amplification

Design of Feedback Amplifiers

• Series feedback try to select small resistance values, so that the network does not insert significant resistance into the input circuit

• Parallel feedback

try to select large resistance values so that the feedback network does not tend to short out the input terminals

• Voltage feedback try to select large feedback resistance to do not load the amplifier

• Current feedback try to select small feedback resistances because the input of the feedback network is in series with the load

Common-Collector Amplifier

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Fig. (a)

Fig. (a) shows a CC amplifier circuit which is also called an emitter follower.

• Remember that for Common Collector Amplifier,

– the output is measured at the emitter terminal.

– the gain is a positive value

AC Analysis of an Emitter Follower

• In an emitter follower circuit, the input is applied to the base while the output is taken from the emitter.

• Because the collector is tied directly to the collector supply voltage, VCC, no ac signal appears there.

• Because the emitter follower takes its output from the emitter, an emitter bypass capacitor is not used.

• The emitter is typically unbypassed, therefore, the swamping is heavy and the distortion in the output signal is extremely small.

Emitter Follower Applications

• An emitter follower has high input impedance and low output impedance.

• This makes the emitter follower ideal for impedance matching applications.

OSCILLATOR

Objectives

Describe the basic concept of an oscillator

Discuss the basic principles of operation of an oscillator

Describe the operation of Phase-Shift Oscillator, Wien Bridge Oscillator, Crystal Oscillator and Relaxation Oscillator

Introduction

Oscillators are circuits that produce a continuous signal of some type without the need of an input.

These signals serve a variety of purposes such as communications systems, digital systems (including computers), and test equipment

The Oscillator

An oscillator is a circuit that produces a repetitive signal from a dc voltage.

The feedback oscillator relies on a positive feedback of the output to maintain the oscillations.

The relaxation oscillator makes use of an RC timing circuit to generate a non-sinusoidal signal such as square wave.

Types of Oscillator

1. RC Oscillator - Wien Bridge Oscillator

- Phase-Shift Oscillator

2. LC Oscillator - Crystal Oscillator

3. Crystal Oscillator

Feedback Oscillator Principles

An oscillator is an amplifier with positive feedback.

Ve = Vi + Vf (1) Vo = AVe (2) Vf = (AVe)=Vo (3)

From (1), (2) and (3), we get

where A is loop gain Aβ

A

V

VA

s

of

1

In general A and are functions of frequency and thus may be written as;

sβsA1

sAs

V

VsA

s

of

sβsA is known as loop gain

Feedback Oscillator Principles

Design Criteria for oscillators

1) |A| equal to unity or slightly larger at the desired oscillation frequency.

- Barkhaussen criterion, |A|=1

2) Total phase shift, of the loop gain must be 0° or 360°.

Factors that determine the frequency of oscillation

Oscillators can be classified into many types depending on the feedback components, amplifiers and circuit topologies used.

RC components generate a sinusoidal waveform at a few Hz to kHz range.

LC components generate a sine wave at frequencies of 100 kHz to 100 MHz.

Crystals generate a square or sine wave over a wide range,i.e. about 10 kHz to 30 MHz.

1. RC Oscillators

1. RC Oscillators

RC feedback oscillators are generally limited to frequencies of 1MHz or less

The types of RC oscillators that we will discuss are the Wien-Bridge and the Phase Shift

Wien-Bridge Oscillator

• It is a low frequency oscillator which ranges from a few kHz to 1 MHz.

• Structure of this oscillator is

Phase-Shift Oscillator

.

C C C

R R

R

R f

+

V o0 V

Phase-shift oscillator

The phase shift oscillator utilizes three RC circuits to provide 180º phase shift that when coupled with the 180º of the op-amp itself provides the necessary feedback to sustain oscillations.

2. LC Oscillators

Oscillators With LC Feedback Circuits

For frequencies above 1 MHz, LC feedback oscillators are used.

We will discuss the crystal-controlled oscillators.

Transistors are used as the active device in these types.

The crystal appears as a resonant circuit (tuned circuit oscillator). The crystal has two resonant frequencies: Series resonant condition

• RLC determine the resonant frequency • The crystal has a low impedance

Parallel resonant condition

• RLC and CM determine the resonant frequency • The crystal has a high impedance

The series and parallel resonant frequencies are very close, within 1% of each other.

Crystal Oscillator

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