Electronics/Amplifiers

From Wikibooks, the open-content textbooks collection

Electronics | Foreword | Basic Electronics | Complex Electronics | Electricity | Machines | History of Electronics | Appendix | edit

Contents 1 Types of Amplifiers 2 Gain 3 Transistor amplifiers 3.1 Transistor Amplifier Configurations 3.1.1 BJT Configurations 3.1.1.1 Common Emitter 3.1.1.2 Common Collector 3.1.1.3 Common Base 3.1.2 FET Configurations 3.1.2.1 Common Source 3.1.2.2 Common Drain 3.1.2.3 Common Gate 3.2 Class 3.2.1 Class A 3.2.2 Class AB 3.2.3 Class B 3.2.4 Class C 3.2.5 Class D 3.2.6 Class E 3.2.7 Class F 3.2.8 Class S 4 Amplifier

[edit] Types of Amplifiers

Amplifier are generally put in four categories: voltage, current, transresistance and transconductance. The model for a voltage amplifier is shown in figure 1. Real amplifiers have input and output resistance. This is reflected in the model.

Figure 1: A Voltage Amplifier.

[edit] Gain

Gain is the increase in the strength of a signal and is often expressed in decibels (dB). An increase of 3 dB is about equal to doubling in a linear scale.

A gain of more than 1 is called amplification, while a gain of less than 1 is called attenuation.

Gain is given different symbols depending of the type. No load gains: Voltage gain is A_vo, current gain A_io, transconductance G_m and transresistance R_m.

Using the model, the gain with a load can be calculated.

[edit] Transistor amplifiers

(The rest of this chapter discusses only "transistor amplifiers". Far more common are "operational amplifiers". Op-Amps are covered in a different chapter.)

[edit] Transistor Amplifier Configurations

There are transistor Amplifier Configurations corresponding to each major type of transistor i.e. for FETs and BJTs. Each configuration has a different gain, input and output impedance.

[edit] BJT Configurations

There are three BJT configurations each one named after one of the terminal. These configuration are Common Collector (as called Emitter Follower), Common Emitter and Common Base.

[edit] Common Emitter

QUALITATIVE CHARACTERISTICS

• Current Gain: ..... HIGH
• Voltage Gain: ..... HIGH
• Power Gain: ..... HIGH
• Input Impedance: ... AVERAGE
• Output Impedance:... AVERAGE

QUANTITATIVE CHARACTERISTICS

Input Resistence(base): Zb=β×re'
 
-> β: Current Gain (Ic/Ib), where 'Ic' is Colector DC current and 'Ib' is DC Base current;
-> re': Base-Emitter dynamic resistor (Ut/Ie), where Ut is thermal voltage(≈25mV at 25°C) and
'Ie' is DC emitter current;

Input Resistence(general): Zg= Zb || R1 || R2, where R1 and R2 are the same as the picture above.

[edit] Common Collector

QUALITATIVE CHARACTERISTICS

Current Gain: ..... HIGH
Voltage Gain: ..... ≈1
Power Gain:   ..... LOW
Input Resistence: ... HIGH
Output Resistence:... LOW

[edit] Common Base

QUALITATIVE CHARACTERISTICS

Current Gain: ..... ≈1
Voltage Gain: ..... HIGH
Power Gain:   ..... AVERAGE
Input Resistence: ... LOW
Output Resistence:... HIGH

[edit] FET Configurations

As with BJT configurations, there are three FET configurations, each one correponding to one of the terminals of the transistor.

[edit] Common Source

[edit] Common Drain

[edit] Common Gate

[edit] Class

Transistors may be biased in a variety of classes. A trade off of linearity and power consumption is usually made where a Class A

[edit] Class A

The transistor is "on" all the time. We say 360 degrees of conduction, representing an entire period of the sine waveform. Ideally, this class produces very little distortion, however consumes a lot of power and is also least preferred.

[edit] Class AB

The transistor is "on" for slightly more than half the cycle (>180 degrees) of a sine wave and is the most common configuration used in push-pull audio power amplifiers. In push-pull amplifiers, Class AB produces mostly odd order distortion, however it is far more power efficient than Class A. Odd order distortion is not considered pleasing to hear in audio power amplifiers. This distortion can easily be removed with the addition of a simple negative feedback loop into the system as shown by the diagram below:

This type of amplifier is extremely easy to build and is the industry standard for audio amplifiers.

Issues with this type of amplifier include poor efficiency, size and cost. A typical class ab amplifier will have a power efficiency of 40-80%. Because of this they require large heat sinks to cool the transistors, this also increases the cost of the amplifier due to the extra material to create these heat sinks.

[edit] Class B

The transistor is "on" for only half the cycle (exactly 180 degrees) of a sine wave and is also very typically used in push-pull amplifier circuits. Ideally this class produces mostly odd order distortion. In audio applications it is believed that odd order distortion is not pleasing to hear. It is difficult to build a low distortion Class B amplifier and hence Class AB is almost universal.

[edit] Class C

The transistor is "on" less than half the cycle of a sine wave. We say <180 of conduction. This class produces both even and odd order distortion, however is very efficient.

[edit] Class D

The class D amp has been developed after the short-comings of past generations, including classes A, B, AB, and C. Many people mistake the D as standing for digital. Although it is a “switching” amp, meaning it turns “on” and “off” at a specific frequency, it is a wrong assumption. D was simply the next letter in the alphabet. Consuming the least power out of its previous generations, the class D amps are generally smaller, making them ideal for mobile devices. It is because of their power efficiency, small size, and cheaper costs that the class D amps are quickly becoming the new industry standard for audio electronic. Companies such as Advanced Analog, Texas Instruments as well as other companies have released 50W stereo class d amplifiers that are the size of a penny and did not require any sort of heat sinking, something that was not possible with other types of amplifiers.

The basic design includes two MOSFET transistors in series, one pFET and one nFET being driven by a pulse width modulated (PWM) signal. Because of the properties of MOSFET transistors they are either fully on or fully off. When the transistor is off and the current is zero (so the amount of power wasted heating up the transistor is zero), or the transistor is fully on and the voltage across it is very close to zero (so the amount of power wasted heating up the transistor is again, very close to zero).

Because an analog signal needs to be transformed into a PWM signal a certain amount of distortion can occur, but the amount of distortion can be minimized. Because a PWM signal is very much like a digital signal, the sampling theorem states that if the sampling frequency is more than half the maximum frequency of the source it can be reproduced exactly. For audio signals the maximum frequency heard by humans is roughly 20kHz, so a PWM generator would only need to provide a minimum switching frequency of 40kHz. Because of the availability of faster components many class d amplifier designers will use switching frequencies closer to 400kHz to further reduce the distortion.

Issues of concern with a class d amplifier include electromagnetic emissions. Due to the presence of a medium frequency signal in the circuit, steps must be taken to reduce the emission of these signals that could interfere with other electronic devices.

[edit] Class E

Switching amplifier

[edit] Class F

[edit] Class S

never heard of this

[edit] Amplifier