Chapter 4 Bipolar Junction Transistors. Objectives. Describe the basic structure of the bipolar junction transistor (BJT). Explain and analyze basic transistor bias and operation. Discuss the parameters and characteristics of a transistor and how they apply to transistor circuits.
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A transistor is a device that can be used to either switch flow on/offor “control flow” device.
Let’s first consider its operation in a simpler view as a flow switching device.
1 - open
2 - closed
Let’s first consider its operation in a simpler view as a current controlling
With diodes there is one p-n junction. With bipolar junction transistors (BJT), there are three layers forming two p-n junctions. Transistors can be either pnp or npn type.
Physical construction of a BJT is deposited layers of “doped “ N and P semiconductor materials.BJT Construction
Note: Epitaxial films may be grown from gaseous or liquid precursors. Because the substrate acts as a seedcrystal, the deposited film takes on a lattice structure and orientation identical to those of the substrate. This is different from other thin-film deposition methods which deposit polycrystalline or amorphous films, even on single-crystal substrates. If a film is deposited on a substrate of the same composition, the process is called homoepitaxy; otherwise it is called heteroepitaxy.
IE = IC + IB
Below, is the proper bias configuration for the npn and the pnp transistor. Note that base-emitter junction is forward-biased, while the base-collector junction is reverse-biased (known as forward-reverse bias). For the npn, the forward-bias BE, narrows the BE junction depletion zone, while the reverse-bias BC depletion zone grows larger. Let’s examine this more closely (next slide).
IC = ß IB
Forward-Reverse bias of a bipolar transistor.
“valence electron” current
IE = IC + IB Note relative current sizes.
For proper operation, the base-emitter junction is forward-biased by VBBand conducts just like a diode. IB is set.
The collector-base junction is reverse biased by VCC and blocks current flow through it’s junction just like a diode.
Current flow through the base-emitter junction will help establish the path for current flow from the collector to emitter.
IC + IB results in IE
Analysis of this transistor circuit to predict the dc voltages and currents requires use of Ohm’s law, Kirchhoff’s voltage law and the ß for the transistor.
Determine the base BIAS current.
Using Kirchhoff’s voltage law, subtract the 0.7VBE from VBB and the remaining voltage is dropped across RB. Determining the current for the base with this information is a matter of applying of Ohm’s law. VRB/RB = IB (ohm’s law)
The collector DC current is determined by multiplying the base current by beta.ICdc = IBdc
orDC = IC/IB
0.7 VBE will be used in most analysis examples.
What we ultimately determine by use of Kirchhoff’s voltage law for series circuits is that; In the base circuit, VBB is distributed across the base-emitter junction and RB in the base circuit.
In the collector circuit we determine that VCC is distributed proportionally across RCand the transistor (VCE).
DC = IC/IB (Current Gain)
Data sheets will refer to hybrid (h) parameters hFE = DC (typical hFEvalues 20 – 200)
See Ex. 4-1
There are three key dc voltages and three key dc currents to be considered. Note that these measurements are important for troubleshooting.
Relationships of Key Parameters:
VBE= 0.7 V.
IB = (VBB – VBE)/RB
VCE =VCC –ICRC
VCB = VCE -VBE
IB: dc base current
IE: dc emitter current
IC: dc collector current
VBE: dc voltage across base-emitter junction
VCB: dc voltage across collector-base junction
VCE: dc voltage from collector to emitter
See Ex 4-2
Collector characteristic curves give a graphical illustration of the relationship of: Ic (collector current), and VCE (Voltage-Collector to Emitter) with specified amounts of IB(base current).
With greater increases of VCC , VCE continues to increase until it reaches breakdown, but the current remains about the same in the linearregion from .7V to the breakdown voltage.
See Ex. 4-2
1.Vcc = 0
IB is thru the base & emitter.
Transistor is in Saturation
2. VCC , VCE and Ic with increase Vcc
3. VCE exceeds 0.7V
Base/Collector junction is
in Linear region
4. “breakdown” or
See Ex. 4-3
With no IB this transistor is in the cutoff region and just as the name implies there is practically no current flow in the collector part of the circuit. With the transistor in a cutoff state the full VCC can be measured across the collector and emitter (VCE). Rc≈0V.
Current flow in the collector part of the circuit is, determined by IB multiplied by .
However, there is a limit to how much current can flow in the collector circuit regardless of additional increases in IB.
Once this maximum is reached, the transistor is said to be in saturation. Note that saturation can be determined by application of Ohm’s law. IC(sat)=VCC/RC
The measured voltage across the now “shorted” collector and emitter is VCE = 0V.
The dc load line graphically illustrates IC(sat)and cutoff for a transistor.
This will be the area of operation for all out transistor applications.
The beta for a transistor is not always constant. Temperature and collector current both affect β. Variances in the manufacture of the transistor may also result in variable β’s.
There are also maximum power ratings to consider.
The data sheet below, provides information on these characteristics.
Note:ΔβDC for various temperatures.
Note: ΔβDCfor ΔIC
See End of Chap. Q.4-20 & 21
Assuming PD(MAX) is 500mW:
VCEmax is 20V, IC(MAX) is 50mA.
The graph shows the transistor can’t be operated in the “shaded“ areas.
See Ex. 4-5 & 4-6
Power Derating – P base–collector junction' means Vbc < 0 for NPN, opposite for PNP)Dmax
3 factors decide Power Deratings: see the previous slide.
ICmax , VCEmax & PDmax -------- IC = PD(max) / VCE
See the Datasheet for DeratingsTransistor Characteristics and ParametersPower Derating/Data Sheets
See Ex. 4-7
See Ex. 4-8
Amplification is the process of linearly increasing the amplitude of an electrical signal.
Some designation standards and conventions:
DC Currents:IC IEIB
DC Voltages: VBE VCB VCE VB VC VE
External DC Resistance: RERCRB
AC Currents: Ic Ie Ib
AC Voltages: Vbe Vcb Vce Vb Vc Ve
ExternalAC Resistances: ReRcRb
Internal Transistor Resistances: r ’ r ’e
A transistor when used as a switch is simply being biased so that it is in cutoff (switched off) or saturation (switched on). Remember that the VCE in cutoff is VCC and 0 V in saturation.
VCE(cutoff) = VCC
IC(sat) = VCC – VCE(sat)
IB(min) = Ic (sat)
See Ex 4-10
Application: Transistor Switch
Base of transistor is “pulsed” with a “squarewave”.
“Off” periods provide no base biasing….transistor remains OFF.
“On” periods provide DC bias, switching the transistor “ON”
IB is max. Ic is max.
See Ex 4-11
Same transistor type in metal packaging
High-density, multi-transistor packaging
Troubleshooting a live transistor circuit requires us to be familiar with known good voltages, but some general rules do apply. Certainly a solid fundamental understanding of Ohm’s law and Kirchhoff’s voltage and current laws is imperative. With live circuits it is most practical to troubleshoot with voltage measurements.
Opens in the external resistors or connections of the base or the circuit collector circuit would cause current to cease in the collector and the voltage measurements would indicate this.
Internal opens within the transistor itself could also cause transistor operation to cease.
Erroneous voltage measurements that are typically low are a result of point that is not “solidly connected”. This called a floating point. This is typically indicative of an open.
More in-depth discussion of typical failures are discussed within the textbook.
Testing a transistor can be viewed more simply if you view it as testing two diode junctions. Forward bias having low resistance and reverse bias having infinite resistance.
The diode test function of a multimeter is more reliable than using an ohmmeter. Make sure to note whether it is an npn or pnp and polarize the test leads accordingly.