“Class A Power Amplifiers”

ITT Technical Institute• 3rd Quarter/December2014 •3-6-2015

Introduction
The common-emitter (CE) amplifier provides high voltage gain with moderate input resistance whereas the common collector (CC) amplifier provides current gain and low output resistance. Combining the two amplifiers gives the advantages of each, allowing the amplifier to drive a relatively low resistance load such as a speaker. In all amplifiers, some power from the supply is wasted – that is, it does not show up as signal power in the load. In class A power amplifiers, the transistor is biased on at all times, causing power to be dissipated in the transistor, even when no signal is present. Because of this, class A amplifiers are not as efficient as class B designs. For low power applications, this reduced efficiency is not a major problem. Further, the power dissipated in the transistor is highest when no signal is present, so it is simple to compute the worst case power dissipated in the transistor − it is simply VCEQICQ. In this experiment, you will combine a CE and CC amplifier to form an amplifier that will be used for driving a small speaker. Most speakers are low resistance devices, requiring the driving amplifier to have a low output resistance. Because the CE amplifier typically has relatively high output resistance, a Darlington CC amplifier is selected to minimize the loading effect. In the For Further Investigation section, you can complete the amplifier by adding a common base (CB) driver and microphone for a small intercom system. As in earlier experiments, you should analyze the operation of the amplifier by first computing the dc parameters. After analyzing the dc conditions, the ac parameters for the amplifier are evaluated and power input, power output, and power gain are calculated.
Calculations1. Measure and record the values of the resistors listed in Table 16-1. Resistor Listed
Value Measured
Value
R1 56 k! (4.7)
R2 10 k! .075
RE1 100 ! (6.7)
RE2 560 ! (.652)
RC 4.7 k! (.326)
R3 10 k! (.265)
R4 22 k! (.250)
RE3 22 ! (8.90)

Schematic Lists

Results

Class-A designs are simpler than other classes; for example class -AB and -B designs require two connected devices in the circuit (push–pull output), each to handle one half of the waveform; class A can use a single device single-ended .The amplifying element is biased so the device is always conducting, the quiescent (small-signal) collector current (for transistors; drain current for FETs or anode/plate current for vacuum tubes) is close to the most linear portion of its transconductance curve .Because the device is never 'off' there is no "turn on" time, no problems with charge storage, and generally better high frequency performance and feedback loop stability (and usually fewer high-order harmonics).The point at which the device comes closest to being 'off' is not at 'zero signal', so the problems of crossover distortion associated with class-AB and -B designs is avoided. Best for low signal levels of radio receivers due to low distortion.

Discussion Questions

1. What changes would you make on your current design to improve amplifier performance and efficiency? I wouldn’t make any changes in my current design because the circuit I built was a success and operated as accorded. 2. What changes would you make on your current design to reduce the cost of production?
The changes I would make in my current design the reduce the cost of production in my design would be reduce the number of resistors in the design. 3. What are some real-world industry applications for which your design can be used? My design can be used in the field of audio installation.

Conclusion In the class A Power Amplifier design I used Resistors: one 22 ! (2 W), one 100 !, one 560 !, one 4.7 k!, two 10 k!, one 22 k!, one 56 k! Capacitors: one 0.22 µF, one 1.0 µF, one 10 µF, two 100 µF Transistors: two 2N3904, one SK3024 (or equivalent) with heat sink One small 8 ! speaker. To Bulid a design where I built a circuit to produce power through a circuit board to get sound out a speaker.