3. ENT 162 Analog Electronics
Chapter 4
Bipolar Junction Transistors

4. Chapter Outline BJT Structure Basic BJT Operation
BJT Characteristics and Parameters
BJT as Amplifier and Switch

5. BJT Structure
Transistor can be used as amplifier or switch. It’s act as a current controlling device.
Transistor is to amplify 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.

6. A Sample of Common Transistor Packages
SOT-23
TO-18
TO-220AB
TO-225AA
TO-3

7. BJT Structure
BJT is constructed with three doped semiconductor regions called emitter, base and collector. These three regions are separated by two PN junctions.
There are two types of BJT, NPN where two N regions separated by a thin P region and PNP, consist of two P regions separated by a thin N region. Each region is called emitter, base and collector.
Emitter function as current supplier or carrier.
Collector role is to collect charge for circuit operation
Base act as junction that control the current flow.

8. There is one PN junction in diodes.
While in bipolar junction transistors (BJT), there are three layers and two PN junctions, base-emitter junction and base-collector junction.
Base region is lightly doped, emitter heavily and collector moderate.
Fig 4-1 b & c
Basic BJT Construction

9. Basic BJT Operation
PN junction must be supplied with external dc bias voltage and for both NPN and PNP BE junction is forward-bias and BC junction is reverse-bias, but the bias voltage polarities and current direction are reversed between two types.

10. For NPN transistor, electron at emitter is being pushed by negative terminal power supply toward junction into P type base. Base is lightly doped and very narrow, so only small percentage of electron flow from BE junction can combines with hole, flow out of base lead as valence electron forming small base current, IB.
Most electron will diffuse into BC depletion region. They are pulled through reverse-bias BC junction by attraction between positive and negative ions i.e. by positive collector voltage and forming collector current, IC which is larger than base current.

11. Illustration of BJT action

12. Basic BJT Operation - Transistor Currents
Emitter current is sum of collector current and base current.
IE = IC + IB
IB is small compare to IE and IC, that we assume IE ≈ IC.

13. BJT Characteristics and Parameters
When transistor connected to DC bias voltages (as shown in figures below), VBB forward-bias base-emitter junction and VCC reverse-bias the base-collector junction. Normally VCC direct from supply and VBB (which is smaller) from voltage divider.
When transistor operate within it’s linear limit, IC = ßIB, where dc beta, ßDC is current gain of transistor and usually designated as hFE. The value typically from 20 to 200.
Fig 4-6a
ßDC = hFE with range 20 to 200

14. Ratio of IC and IE is alpha dc, αDC = IC / IE
Ratio of IC and IE is alpha dc, αDC = IC / IE . Typically, αDC range from 0.95 to 0.99 and always less than 1. For proper operation the base-emitter junction is forward biased by VBB and conducts just like a diode. 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.

15. BJT Characteristics & Parameters: Current and Voltage Analysis
There are three key dc voltages and three key DC currents to be considered.
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
Fig 4-7

16. Analysis of this transistor circuit to predict the DC voltages and currents requires use of Ohm’s, Kirchhoff’s and transistor β.
VBE = 0.7V
VBB – VBE = VRB
VRB/RB = IB
VCE = VCC – VRC
= VCC – ICRC
IC = βDC IB
VCB = VCE - VBE
Fig 4-7

17. VBE = 0.7V
β =?
VBB – VBE = VRB
VCE = VCC – VRC
BJT
VRB = IBRB
VRC= ICRC
IE = IC+ IB
VCB = VCE - VBE
IC = β IB

18. BJT Characteristics & Parameters - Collector Characteristic Curves
Recall that the IC is proportional to the base current (IC=ßDCIB). IF IB=0 so IC=0. In order to plot the collector characteristic, a base current (IB) must be selected and held constant. A circuit in figure below can be used to generate a set collector IV curves to show how IC varies with VCE for a given base current (IB).
VBB and VCC are adjustable!!

19. IF VBB is set to produce a specific value of IB and VCC is zero, then IC= 0 and VCE= 0. NOW, as VCC is gradually increased, VCE will increase and so will IC, as indicated between points A and B on the curve in figure below.
When VCE ≈ 0.7 or exceed, the base-collector junction becomes reversed biased and IC reached its full value determined by the relationship (IC=ßDCIB). Ideally, IC levels off to an almost constant value as VCE continues to increases. This action apperas between point B ad C.

20. By setting IB to other constant values, a additional IC versus VCE curves can be produced as shown in figure below.
These curve constitute a ‘family’ of collector curves for a given transistor. A family of curves allows you to visualize the complex situation when the three variable interact.

21. Cutoff
With IB = 0, transistor is in cutoff region and just as the name implies there is practically no current flow in the collector part of the circuit. Very small collector leakage current, ICEO due to thermal and usually neglected so VCC = VCE.Here base-emitter and base-collector are reverse-bias.
Fig 4-12 cutoff ex.

22. Saturation
When base-emitter become forward-bias, IB will increase so will IC and VCE will decrease as result of VRC. (VCE = VCC – ICRC).
When VCE saturated, base-collector become forward-bias and IC cannot increase even IB increase.
Once IC maximum , transistor is said to be in saturation. By Ohm’s law, IC(sat)=VCC/RC . Voltage across this now seemingly “shorted” collector and emitter is 0V.
Fig 4-13 Saturation ex.

23. DC load line
Cutoff and saturation can illustrated in relation to collector curve by a load line. Ideal cutoff is IC = 0 and VCE = VCC and saturation when IC = IC(sat) = and VCE = VCE(sat). In between cutoff and saturation along the load line is active region of transsistor operation.
Active Region
Fig 4-14

24. Beta, ß
Beta for a transistor is not always constant. Temperature and collector current both affect beta.
When temperature constant and IC increase cause ß to increase until maximum.

25. Further increase IC beyond this max point cause ß to decrease
Further increase IC beyond this max point cause ß to decrease. If IC is held constant and temperature is varied, ß change directly with temperature. If temperature inc, ß inc and vice versa.
Data sheet usually specifies ß at specific IC value and it varies from device to device due to inconsistencies in the manufacturing process.

26. Maximum Transistor Rating
There are also maximum power ratings to consider. The data sheet provides information on these characteristics.
Typically, max rating are VCE, VCB, VEB, IC and power dissipation. Power dissipation, PD(max) = IC x VCE
PD(max) = IC x VCE

27. Transistor Applications

28. BJT as an Amplifier
Amplification is the process of linearity increasing the amplitude of an electrical signal.
dc current
- VBE, VCB, VCE are dc voltages from one transistor terminal to another.
- VB, VC, VE are dc voltages from transistor terminal to ground.
- IB, IC and IE are the dc transistor currents.
- RE is an external dc emitter resistance
ac current
- Vbe, Vcb, Vce are ac voltages from one transistor terminal to another.
- Vb, Vc, Ve are ac voltages from transistor terminal to ground.
- Ib, Ic and Ie are the ac transistor currents.
- Internal ac resistances, r’
- Internal ac emitter resistances, r’e
- External ac emitter resistance, Re

29. BJT Amplifiers
A BJT amplifies AC signals by converting some of the DC power from the power supplies to AC signal power. An ac signal at the input is superimposed in the dc bias by the capacitive coupling. The output ac signal is inverted and rides on a dc level of VCE.

30. Amplification of a relatively small ac voltage can be had by placing the ac signal source in the base circuit.
Recall that small changes in the base current (Ib) circuit causes large changes in collector current (Ic) circuit.
The small ac voltage causes the Ib to increase and decrease accordingly and with this small change in current the Ic will mimic the input only with greater amplitude.

31. Forward bias b-e junction present low r’e (internal ac emitter resistance) and appears in series with RB, so;
Vb = Ier’e
Ic = Vb/r’e
Vc = IcRC
= IeRC
Av = Vc/Vb
= IeRC/(Ier’e)
= RC/r’e
Vb is considered as transistor ac input voltage.
Vc is considered as transistor ac output voltage.
Therefore, ac voltage gain , Av, is:

32. BJT as Switch
A transistor when used as a switch is simply being biased so that it is in cutoff (switched off) or saturation (switched on).

33. In cutoff condition, BE junction is not forward-bias, all current is zero and
VCE(cutoff) = VCC
In saturation condition, BE junction is forward-bias, so enough IB to produce max IC, and transistor is saturated.
IC(sat) = (VCC – VCE(sat)) /RC
VCE(sat) << VCC, so neglected.
IB(min) for saturation = IC(sat) / ß

34. A simple application of a transistor switch
A square wave input voltage with a period of 2 s is applied to the input.
When 0 V, the transistor is cutoff, no Ic, then the LED does not emit light.
When input is high level, the transistor saturates, LED is forward biased, Ic flow though the LED and causes it to emit light.

35. Selected Key Terms BJT (bipolar junction transistor) Emitter Base
Collector
a transistor constructed with three doped semiconductor regions separated by two pn junctions.
the most heavily doped of the three semiconductor regions of a BJT.
one of the three semiconductor regions of a BJT. The base is thin and lightly doped compared to the other regions.
the largest of the three semiconductor regions of a BJT.

36. Selected Key Terms Beta Saturation Cutoff
the ratio of dc collector current to the dc base current in a BJT; current gain from base to collector.
the state of a BJT in which the collector current has reached a maximum and is independent of the base current.
the nonconducting state of a transistor.

37. Test yourself..
1. The region on the characteristic curve in which the current changes only slightly with an increase in VCE is called the a. saturation region b. cutoff region c. breakdown region d. active region

38. Test yourself..
2. bDC is defined as the ratio of a. collector current to base current b. collector current to emitter current c. emitter current to base current d. emitter current to collector current

39. Test yourself.. 3. When a BJT is in saturation, the
a. collector current does not change with an increase in base current
b. base current cannot increase
c. collector to emitter voltage is maximum
d. all of the above

40. Test yourself.. 4. When a BJT is cutoff, the
a. voltage from collector to emitter is near zero
b. collector current is near zero
c. base-emitter junction is forward-biased
d. none of the above

41. Test yourself..
5. The lower end of the dc load line touches the x-axis at a. saturation b. cutoff c. breakdown d. 0.7 V

42. Test yourself.. 6. For the circuit shown, the base current is
a. 1.0 mA
b mA
c. 10 mA
d mA

43. Test yourself..
7. Assume VCE (sat) = 0.2V. For the circuit shown, the saturation current is
a. 200 mA
b. 2.0 mA
c mA
d mA