An overview of bipolar transistors

● Control in a bipolar transistor is generally considered to be due to an electric current
    – Current into one terminal determines the current between two others
    – A bipolar transistor can be used as a ‘control device’



● Relationship between the collector current and the base current in a bipolar transistor
   – characteristic is approximately linear
   – magnitude of collector current is generally many times that of the base current
   – the device provides current gain



● Construction

    – two polarities: npn and pnp



● Notation
   – bipolar transistors are 3 terminal devices
• collector (c)
• base (b)
• emitter (e)
   – the base is the control input
   – diagram illustrates the notation used for labelling voltages and currents



NOTATION
• In transistor circuits, voltages and currents have an AC component superimposed on a DC level.
• DC and AC voltages are analysed separately and independently • The following notation will be used (emitter current as example):

Bipolar transistor operation

● We will consider npn transistors
   – pnp devices are similar but with different polarities of voltage and currents
   – when using npn transistors
   • collector is normally more positive than the emitter
   • VCE might be a few volts
   • device resembles two back-to-back diodes – but has very different characteristics
   • with the base open-circuit negligible current flows from the collector to the emitter

● Now consider what happens when a positive voltage is applied to the base (with respect to the emitter)
   – This forward biases the base-emitter junction
   – The base region is lightly doped and very thin
   – Because it is lightly doped, the current produced is mainly electrons flowing from the emitter to the base
   – Because the base region is thin, most of the electrons entering the base get swept across the base-collector junction into the collector
   – This produces a collector current that is much larger than the base current – this gives current amplification

● Transistor action

● Behaviour can be described by the current gain, hfe or by the transconductance, gm of the device

A simple amplifier

Watch the Video  📹

● The circuit shows a simple amplifier
   – RB is used to ‘bias’ the transistor by injecting an appropriate base current
   – C is a coupling capacitor and is used to couple the AC signal while preventing external circuits from affecting the bias
   – This is an AC-coupled amplifier

Bipolar transistor characteristics

● Transistor configurations
   – Transistors can be used in a number of configurations
   – Most common is as shown
   – Emitter terminal is common to input and output circuits
   – This is a common-emitter configuration
   – We will look at the characteristics of the device in this configuration

● Input characteristics
   – The input takes the form of a forward- biased pn junction
   – The input characteristics are therefore similar to those of a semiconductor diode

● Output characteristics
   – region near to the origin is the saturation region
   – this is normally avoided in linear circuits
   – slope of lines represents the output resistance



● Transfer characteristics
   – can be described by either the current gain or by the transconductance
   – DC current gain hFE or β is given by IC / IB
   – AC current gain hfe is given by ic / ib
   – transconductance gm is given approximately by
            gm ≈ 40IC ≈ 40 IE siemens

   – the units of gm are those of admittance
   – therefore 1/gm has the units of resistance
   – the quantity 1/gm is termed the emitter resistance re
   – therefore


● Simple equivalent circuits for a bipolar transistor



● Limitations of the simple models
   – While the simple models shown above give a reasonable representation of the behaviour of devices they do not show the effects of the output voltage on the input (as shown here)
   – This can be modelled by thereverse transfer ratio hre


● The hybrid-parameter model

● The hybrid-Π model
    – gives a better representation at high frequencies


● Bipolar transistors at high frequencies

Bipolar amplifier circuits

Watch the Video  📹

● Analysis of a simple amplifier
    – Earlier we looked at a simple amplifier
    – This is an AC-coupled amplifier
    – It is convenient to look at its DC (or quiescent) behaviour separately from its AC (or small signal) behaviour
    – We will begin by looking at its DC analysis

Small signal analysis of an amplifier

● To determine the small-signal behaviour of the circuit, we first construct a small-signal equivalent circuit
   – We start with our model of the transistor
– Then add the other components


● From the equivalent circuit, if we ignore the effects of C
   Vbe = Vi
and therefore

● Example – see Example 19.2 from course text
Determine the small signal voltage gain, input resistance and output resistance of the following circuit, given that hfe = 100 and hoe = 100 μS



The first step is to construct the small signal equivalent circuit

● We first need to establish gm and hie. From the earlier example IE ≈ IC = 1.02 mA Therefore


● Voltage gain


or, using the approximation: voltage gain ≈ -gm RC = -40.8x10-34700 ≈ -192

Given the inaccuracies involved, this seems a reasonable approximation

● Input resistance
– from the equivalent circuit the input resistance is simply RB // hie

– Since RC << 1/hoe then RC // (1/hoe) ≈ RC and therefore ro ≈ RC

Amplifier configurations

● So far we have concentrated on circuits where the input is applied to the base and the output is taken from the collector – the common-emitter amplifier is the simplest form of these
● Transistors are also used in other configurations



Watch the VIdeo  📹


● A common-collector amplifier


● A common-base amplifier

Darlington transistors

● The darlington connection


● A high input resistance buffer amplifier

● The complementary darlington connection

Bipolar transistor applications

● A bipolar transistor as a constant current source



● A bipolar transistor as a current mirror


● Bipolar transistors as differential amplifiers


● A long-tailed pair amplifier using a current mirror