Bipolar Junction Transistor

BIPOLAR JUNCTION TRANSISTORUNIT 3

Transistors• Two main categories of transistors:

– bipolar junction transistors (BJTs) and – field effect transistors (FETs).

• Transistors have 3 terminals where the application of current (BJT) or voltage (FET) to the input terminal increases the amount of charge in the active region.

• The physics of "transistor action" is quite different for the BJT and FET.

• In analog circuits, transistors are used in amplifiers and linear regulated power supplies.

• In digital circuits they function as electrical switches, including logic gates, random access memory (RAM), and microprocessors.

Bipolar Junction Transistors (BJT)• A bipolar transistor

essentially consists of a pair of PN Junction diodes that are joined back-to-back.

• There are therefore two kinds of BJT, the NPN and PNP varieties.

• The three layers of the sandwich are conventionally called the Collector, Base, and Emitter.

BJT Structure - Planar

• In the planar process, all steps are performed from the surface of the wafer

The “Planar Structure” developed by Fairchild in the late 50s shaped the basic structure of the BJT, even up to the present day.

• BJTs are usually constructed vertically– Controlling depth of the emitter’s n doping sets the base

width

np

n

E B C

Terminals & Operations

• Three terminals:– Base (B): very thin and lightly doped central region (little

recombination).– Emitter (E) and collector (C) are two outer regions sandwiching

B.• Normal operation (linear or active region):

– B-E junction forward biased; B-C junction reverse biased.– The emitter emits (injects) majority charge into base region and

because the base very thin, most will ultimately reach the collector.

– The emitter is highly doped while the collector is lightly doped.– The collector is usually at higher voltage than the emitter.

Terminals & Operations

Operation Mode

Operation Mode

• Active: – Most importance mode, e.g. for amplifier operation.– The region where current curves are practically flat.

• Saturation:– Barrier potential of the junctions cancel each other out

causing a virtual short.– Ideal transistor behaves like a closed switch.

• Cutoff:– Current reduced to zero– Ideal transistor behaves like an open switch.

Operation Mode

Circuit Symbols

Circuit Configuration

I-V Characteristics

• Collector current vs. vCB shows the BJT looks like a current source (ideally)– Plot only shows values where BCJ is reverse biased and so BJT in active

region

• However, real BJTs have non-ideal effects

VCE

IC

VBE1

VBE2

VBE3

VBE3 > VBE2 > VBE1

VBE

IC

VCE

I-V Characteristics

Base-emitter junction looks like a forward biased diode

Collector-emitter is a family ofcurves which are a function ofbase current.

I-V Characteristics

Common-emitter

It is called the common-emitter configuration because (ignoring the power supply battery) both the signal source and the load share the emitter lead as a common connection point.

Common-collector

It is called the common-collector configuration because both the signal source and the load share the collector lead as a common connection point. Also called an emitter follower since its output is taken from the emitter resistor, is useful as an impedance matching device since its input impedance is much higher than its output impedance.

Common-base

This configuration is more complex than the other two, and is less common due to its strange operating characteristics.

Used for high frequency applications because the base separates the input and output, minimizing oscillations at high frequency. It has a high voltage gain, relatively low input impedance and high output impedance compared to the common collector.

BJT Analysis

• Here is a common emitter BJT amplifier:

• What are the steps?

Input & Output

• We would want to know the collector current (iC), collector-emitter voltage (VCE), and the voltage across RC.

• To get this we need to fine the base current (iB) and the base-emitter voltage (VBE).

Input Equation

• To start, let’s write Kirchoff’s voltage law (KVL) around the base circuit.

Output Equation

Likewise, we can write KVL around the collector circuit.

Use Superposition: DC & AC sources

• Note that both equations are written so as to calculate the transistor parameters (i.e., base current, base-emitter voltage, collector current, and the collector-emitter voltage) for both the DC signal and the AC signal sources.

• Use superposition, calculate the parameters for each separately, and add up the results: – First, the DC analysis to calculate the DC Q-point

• Short Circuit any AC voltage sources• Open Circuit any AC current sources

– Next, the AC analysis to calculate gains of the amplifier.• Depends on how we perform AC analysis

– Graphical Method– Equivalent circuit method for small AC signals

BJT - DC Analysis

• Using KVL for the input and output circuits and the transistor characteristics, the following steps apply:1. Draw the load lines on the transistor characteristics2. For the input characteristics determine the Q point for

the input circuit from the intersection of the load line and the characteristic curve (Note that some transistor do not need an input characteristic curve.)

3. From the output characteristics, find the intersection of the load line and characteristic curve determined from the Q point found in step 2, determine the Q point for the output circuit.

Base-Emitter Circuit Q point

The Load Line intersects the Base-emitter characteristics at VBEQ = 0.6 V and IBQ = 20 µA

Collector-Emitter Circuit Q point

Now that we have the Q-point for the base circuit, let’s proceed to the collector circuit.

The Load Line intersects the Collector-emitter characteristic, iB = 20 µA at VCEQ = 5.9 V and ICQ = 2.5mA, then β = 2.5m/20 µ = 125

BJT DC Analysis - Summary• Calculating the Q-point for BJT is the first step in analyzing the

circuit• To summarize:

– We ignored the AC (variable) source• Short circuit the voltage sources• Open Circuit the current sources

– We applied KVL to the base-emitter circuit and using load line analysis on the base-emitter characteristics, we obtained the base current Q-point

– We then applied KVL to the collector-emitter circuit and using load line analysis on the collector-emitter characteristics, we obtained the collector current and voltage Q-point

• This process is also called DC Analysis• We now proceed to perform AC Analysis

BJT - AC Analysis

• How do we handle the variable source Vin(t) ?

• When the variations of Vin(t) are large we will use the base-emitter and collector-emitter characteristics using a similar graphical technique as we did for obtaining the Q-point.

• When the variations of Vin(t) are small we will shortly use a linear approach using the BJT small signal equivalent circuit.

BJT - AC Analysis

• Let’s assume that Vin(t) = 0.2 sin(ωt).• Then the voltage sources at the base vary from a

maximum of 1.6 + 0.2 = 1.8 V to a minimum of 1.6 -0.2 = 1.4 V

• We can then draw two “load lines” corresponding the maximum and minimum values of the input sources

• The current intercepts then become for the:– Maximum value: 1.8 / 50k = 36 µA– Minimum value: 1.4 / 50k = 28 µA

AC Analysis Base-Emitter Circuit

From this graph, we find:At Maximum Input Voltage: VBE = 0.63 V, iB = 24 µAAt Minimum Input Voltage: VBE = 0.59 V, iB = 15 µARecall: At Q-point: VBE = 0.6 V, iB = 20 µA

Note the asymmetry around the Q-point of the Max and Min Values for the base current and voltage which is due to the non-linearity of the base-emitter characteristics

∆iΒmax = 24-20 = 4 µA; ∆iBmin = 20-15 = 5 µA

AC Analysis Base-Emitter Circuit

AC Characteristics-Collector Circuit

Using these max and min values for the base current on the collect circuit load line, we find:At Max Input Voltage: VCE = 5 V, iC = 2.7mAAt Min Input Voltage: VCE = 7 V, iC = 1.9mARecall: At Q-point: VCE = 5.9 V, iB = 2.5ma

AC Characteristics-Collector Circuit

The pnp Transistor

• Basically, the pnp transistor is similar to the npn except the parameters have the opposite sign.– The collector and base currents flows out of the

transistor; while the emitter current flows into the transistor

– The base-emitter and collector-emitter voltages are negative

• Otherwise the analysis is identical to the npn transistor.

Current flow in a pnp transistor biased to operate in the active mode.

The PNP Transistor

The pnp Transistor• Two junctions

– Collector-Base and Emitter-Base• Biasing

– vBE Forward Biased

– vCB Reverse Biased

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(0)pn

(x )

pn o

W E B W B CW B

np o

np

(0)

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ICIE

I B

xB

V C BV E B

np(x )

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(b)

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(a)

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H o l ed r i f t

R e c o b i n a t i o n

E l e c t r o n s

E lec tronD i ff u s io n

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(d)

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H ole d i ffus ion

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ICIE

I B

V C BV EB

O u tp u tc irc u it

In p u tc irc u it

pnpE C

B(c)

(a) A schematic illustration of pnp BJT with 3 differently doped regions. (b) The pnp bipolar operated under normal and active conditions. (c) The CB configuration with input and output circuits identified. (d) The illustration of various current component under normal and active conditions.

Current flow in an pnp transistor biased to operate in the active mode.

The pnp Transistor

Frequency Response Curve

The frequency response curves of a circuit is commonly described in terms of the effect that a change in frequency has on the ratio of its output amplitudethe ratio of a circuit’s output amplitude to its input amplitude is referred as gain.a frequency response curve is a graph that show the effect that frequency has on circuit gain.A frequency-response curve is a graphical representation of the relationship between amplifier gain and operating frequency

There are four basic types of filters. The low-pass filter is designed to pass all frequencies below its

cutoff frequency. The high-pass filter is designed to pass all frequencies above its

cutoff frequency. The bandpass filter is one designed to pass the band of

frequencies that lies between two cutoff frequencies. The band-stop (or notch) filter is designed to block the band of

frequencies that lies between two cutoff frequencies.

DesiBell (dB)The ratio of circuit output amplitude to input amplitude is normally expressed using decibels (dB). Decibels are used because they allow us to easily represent very large and very small values.

Gain (Av)is a value that indicates the magnitude relationship between the circuit’s input and output signals. If the gain of an amplifier is 100, then the output is 100 times greater than the input signal

The ratio of circuit output voltage to input voltage is generally referred to as voltage gain (Av).

The voltage gain of a circuit equals 70.7% of its maximum value at the cutoff frequencies. Expressed mathematically: Av = 0.707 Av(max) when ƒ = ƒC. This relationship is based on the fact that power gain equals 50% of its maximum value when voltage gain equals 70.7% of its maximum value.The dB voltage gain of a circuit is found as twenty times the common log of Av. By formula:

When dB voltage gain drops to 70.7% of its maximum value, the change in dB voltage gain is –3 dB.

Bandwidth is the difference between the upper and lower frequencies in a contiguous set of frequencies. It is typically measured in hertz, and may sometimes refer to passband bandwidth, sometimes to baseband bandwidth, depending on context.

Passband bandwidth is the difference between the upper and lower cutoff frequencies

baseband bandwidth always refers to the upper cutoff frequency, regardless of whether the filter is bandpass or low-pass.

The bandwidth of an amplifier is the range of frequencies for which the amplifier gives "satisfactory performance". The definition of "satisfactory performance" may be different for different applications. However, a common and well-accepted metric is the half power points (i.e. frequency where the power goes down by half its peak value) on the output vs. frequency curve. Therefore bandwidth can be defined as the difference between the lower and upper half power points. This is therefore also known as the −3 dB bandwidth.The gain of a good quality full-range audio amplifier will be essentially flat between 20 Hz to about 20 kHz (the range of normal human hearing). In ultra high fidelity amplifier design, the amp's frequency response should extend considerably beyond this (one or more octaves either side) and might have −3 dB points < 10 and > 65 kHz.

DefinitionsIn small-signal amplifiers the main factors are:

• Amplification• Linearity• Gain

Since large-signal, or power, amplifiers handle relatively largevoltage signals and current levels, the main factors are:

• Efficiency• Maximum power capability• Impedance matching to the output device

Amplifier TypesClass A

The amplifier conducts through the full 360° of the input. The Q-point isset near the middle of the load line.

Class BThe amplifier conducts through 180° of the input. The Q-point is set atthe cutoff point.

Class ABThis is a compromise between the class A and B amplifiers. The amplifier conducts somewhere between 180° and 360° . The Q-point is located between the mid-point and cutoff.

Class CThe amplifier conducts less than 180 of the input. The Q-point is located below the cutoff level.

Class A Amplifier

The output of a class A amplifier conducts for the full 360° of the cycle.The Q-point is set at the middle of the load line so that the AC signal can swing a full cycle.

Remember that the DC load lineindicates the maximum and minimumlimits set by the DC power supply.

Class B Amplifier

A class B amplifier output only conducts for 180° or one-half of the AC input signal.

The Q-point is at 0V on the load line, so that the AC signal can only swing for one-half cycle.

Class AB Amplifier

This amplifier is a compromise between the class A and class B amplifier—the Q-point is above that of the Class B but below the class A.

The output conducts between 180° and 360° of the AC input signal.

Class C

The output of the class Cconducts for less than 180° of theAC cycle. The Q-point is belowcutoff.

Efficiency refers to the ratio of output to input power. The lower the amount

of conduction of the amplifier the higher the efficiency.

Class C usually not used for delivering large amount of power, thus the efficiency is not given here.