Chapter 6BJT Amplifiers

Objectives

Understand the concept of amplifiers Identify and apply internal transistor

parameters Understand and analyze common-emitter

Introduction

One of the primary uses of a transistor is to amplify ac signals. This could be an audio signal or perhaps some high frequency radio signal. It has to be able to do this without distorting the original input.

Amplifier OperationRecall from the previous chapter that the purpose of dc biasing was to establish the Q-point for operation. The collector curves and load lines help us to relate the Q-point and its proximity to cutoff and saturation. The Q-point is best established where the signal variations do not cause the transistor to go into saturation or cutoff. What we are most interested in is the ac signal itself. Since the dc part of the overall signal is filtered out in most cases, we can view a transistor circuit in terms of just its ac component.

Amplifier OperationFor the analysis of transistor circuits from both dc and ac perspectives, the ac subscripts are lower case and italicized. Instantaneous values use both italicized lower case letters and subscripts.

Amplifier OperationThe boundary between cutoff and saturation is called the linear region. A transistor which operates in the linear region is called a linear amplifier. Note that only the ac component reaches the load because of the capacitive coupling and that the output is 180º out of phase with input.

Transistor Equivalent CircuitsWe can view transistor circuits by use of resistance or r parameters for better understanding. Since the base resistance, rb is small it normally is not considered and since the collector resistance, rc is fairly high we consider it as an open. The emitter resistance, rc is the main parameter that is viewed.You can determine rc from this simplified equation. (Appendix B)re = 25 mV/IE

Transistor Equivalent CircuitsThe two graphs best illustrate the difference between DC and ac. The two only differ slightly.

Transistor Equivalent CircuitsSince r parameters are used throughout the rest of the textbook we will not go into deep discussion about h parameters. However, since some data sheets include or exclusively provide h parameters these formulas can be used to convert them to r parameters.

r’e = hre/hoe

r’c = hre + 1/hoe

r’b = hie – hre/hoe(1+ hfe)

The Common-Emitter AmplifierThe common-emitter amplifier exhibits high voltage and current gain. The output signal is 180º out of phase with the input.Now let’s use our dc and ac analysis methods to view this type of transistor circuit.

The Common Emitter AmplifierDC Analysis

The dc component of the circuit “sees” only the part of the circuit that is within the boundaries of C1, C2, and C3 as the dc will not pass through these components. The equivalent circuit for dc analysis is shown. The methods for dc analysis are just are the same as dealing with a voltage-divider circuit.

Common Emitter AmplifierAC Equivalent Circuit

The ac equivalent circuit basically replaces the capacitors with shorts, being that ac passes through easily through them. The power supplies are also effectively shorts to ground for ac analysis.

Common Emitter AmplifierAC Equivalent Circuit

We can look at the input voltage in terms of the equivalent base circuit (ignore the other components from the previous diagram). Note the use of simple series-parallel analysis skills for determining Vin.

SinS

inin V

RRR

V

Common Emitter Amplifier AC Equivalent Circuit

The input resistance as seen by the input voltage can be illustrated by the r parameter equivalent circuit. The simplified formula below is used. Rin(base) = acr’e

Example 6-3The output resistance is for all practical purposes the value of Rc=RC||RL.

Common Emitter AmplifierAC Equivalent Circuit

Voltage gain can be easily determined by dividing the ac output voltage by the ac input voltage.Av = Vout/Vin = Vc/Vb

Voltage gain can also be determined by the simplified formula below.Av = RC/r’e

Common Emitter AmplifierAC Equivalent Circuit

vtotalins

totalinv A

RRR

)(

)('A

Taking the attenuation from the ac supply internal resistance and input resistance into consideration is included in the overall gain.

A’v = (Vb/Vs)Av

or

inS

SS

S

Ci RR

VIII

,A ivp AAA

Summary

Transistor circuits can be view in terms of its ac equivalent for better understanding.

The common-emitter amplifier has high voltage and current gain.

The common-collector has a high current gain and voltage gain of 1. It has a high input impedance and low output impedance.

Most transistors amplifiers are designed to operate in the linear region.

Summary The common-base has a high voltage gain and

a current gain of 1. It has a low input impedance and high output impedance