1. Analog Electronics
Lecture 4:Transistors

2. Semiconductor material and pn-junction diode
P-type semiconductor
N-type semiconductor
PN- Junction
Diode

3. Diode and its resistive behavior
R = inf

4. Transformative Resistor
Signal controlled transformative resistor R
Control signal

5. BJT Structure and Construction
The BJT has three regions called the emitter, base, and collector. Between the regions are junctions as indicated.
Base-emitter junction.
Base-collector junction
pnp
npn
The base is a thin lightly doped region compared to the heavily doped emitter and moderately doped collector regions.

6. BJT Operation
The heavily doped n-type emitter region has a very high density of conduction-band (free) electrons.
These free electrons easily diffuse through the forward biased BE junction into the lightly doped and very thin p-type base region.
The base has a low density of holes, which are the majority carriers.

7. BJT Operation
A very little free electrons recombine with holes in base and move as valence electrons through the base region and into the emitter region as hole current.
The valence electrons leave the crystalline structure of the base and become free electrons in the metallic base lead and produce the external base current.
Majority of free electrons move toward the reverse-biased BC junction and swept across into the collector region by the attraction of the positive collector supply voltage.
The free electrons move through the collector region, into the external circuit, and then return into the emitter region along with the base current.

8. BJT Voltages and Currents
npn
pnp
The voltage drop between base and emitter is VBE whereas the voltage drop between collector and base is called VCE.
The conventional current flows in the direction of the arrow on the emitter terminal. The emitter current IE is the sum of the collector current IC and the small base current IB.
That is, IE = IC + IB.

9. BJT Parameters Two important parameters, βDC (dc current gain) and
αDC are introduced and used to analyze a BJT circuit.
DC Beta (βDC ) Ratio of DC collector current and DC base current.
βDC == IC/IB
DC Alpha (αDC )
Ratio of DC collector current to the DC emitter current.
αDC == IC/IE

10. BJT Circuit Analysis Currents and voltages in BJT IB : dc base current
IE: dc emitter current
IC: dc source current
VBE: dc voltage at base wrt. emitter
VCE: dc voltage at collector wrt. emitter.
VCB: dc voltage at collector wrt. Base.
VBE = 0.7V
VCE = VCC – IC RC
IB = (VBB – VBE ) / RB
VCB = VCE – VBE

11. BJT Biasing
In order for a BJT to operate properly , the two pn junctions must be correctly biased with external dc voltages.
For the npn type shown, the collector is more positive than the base, which is more positive than the emitter.
For the pnp type, the voltages are reversed to maintain the forward-reverse bias.
npn
pnp

12. BJT Collector Characteristics Curve
The collector characteristic curves shows three mode of operations of transistor with the variation of collector current IC w.r.t VCE for a specified value of base current IB.
VBB is set to produce a certain value of IB and VCC is zero and VCE is zero.
As VCE is increased, IC increases until B.
Saturation region
Both BE and BC junctions are forward biased and the transistor is in Saturation region.

13. BJT Collector Characteristics Curve - Saturation
In saturation, an increase of base current has no effect on the collector circuit and the relation IC = bDCIB is no longer valid.
IC(SAT) =VCC –VCE(SAT) /RC
At this point, the transistor current is maximum and voltage across collector is minimum, for a given load.

14. BJT Collector Characteristics Curve - Linear
As VCE is increased furthers and exceeds 0.7V the base-collector junction becomes reverse-biased and the transistor goes into the active, or linear, region of its operation.
IC levels off and remains essentially constant for a
given value of IB as VCE continues to increase.
the value of IC is determined
only by the relationship expressed as
Active region

15. BJT Collector Characteristics Curve –Cut off
The collector characteristic curves illustrate the relationship of the three transistor currents.
By setting up other values of base current, a family of collector curves is developed.
bDC is the ratio of collector current to base current.
It can be read from the curves. The value of bDC is nearly the same wherever it is read in active region.

16. BJT Collector Characteristics Curve –Cut off
In a BJT, cutoff is the condition in which there is no base current, which results in only an extremely small leakage current (ICEO) in the collector circuit. For practical work, this current is assumed to be zero.
In cutoff, neither the base-emitter junction, nor the base-collector junction are forward-biased.

17. Variable IB

18. BJT Switches
A BJT can be used as a switching device in logic circuits to turn on or off current to a load. As a switch, the transistor is normally in either cutoff (load is OFF) or saturation (load is ON).
In cutoff, the transistor looks like an open switch.
In saturation, the transistor looks like a closed switch.

19. DC Load Line
The DC load line represents the circuit that is external to
the transistor. It is drawn by connecting the saturation and cutoff points.
The transistor characteristic curves are shown superimposed on the load line. The region between the saturation and cutoff points is called the active region.
Here VCE = 0 and
IC = IC-Sat = VCC -VCE(Sat) / RC
Here IB = 0 and VCE = VCC

20. The DC Operating Point
Bias establishes the operating point (Q-point) of a transistor amplifier; the ac signal moves above and below this point.
Improper biasing can cause distortion in the output signal as the transistor may go into the saturation and cutoff region.

21. The DC Operating Point
The point at which the load line intersects a characteristic curve represents the Q-point for that particular value of IB.
Point A,Q,B represents the Q-point for IB 400mA. 300 mA and 200 mA respectively.
Load line
Assume a sinusoidal Ib is superimposed on VBB varying between 200uA to 400uA. It makes the collector current varies between 20 mA and 40 mA.

22. The DC Operating Point
A signal that swings outside the active region will be clipped.
For example, the bias has established a low Q- point. As a result, the signal is will be clipped because it is too close to cutoff.

23. Summary Voltage-Divider Bias
A practical way to establish a Q-point is to form a voltage-divider from VCC.
R1 and R2 are selected to establish VB. If the divider is stiff, IB is small compared to I2. Then,
Example: I2 IB
Determine the base voltage for the circuit.
Solution:
4.62 V

24. DC Load Line Example: Solution: Follow-up:
What is the saturation current and the cutoff voltage for the circuit? Assume VCE = 0.2 V in saturation.
Solution:
4.48 mA
15 V
Follow-up:
Is the transistor saturated?
IC = b IB = 200 (10.45 mA) =
2.09 mA
Since IC < ISAT, it is not saturated.

25. Data Sheets
Data sheets give manufacturer’s specifications for maximum operating conditions, thermal, and electrical characteristics. For example, an electrical characteristic is bDC, which is given as hFE. The 2N3904 shows a range of b’s on the data sheet from 100 to 300 for IC = 10 mA.

26. Data Sheets

27. DC and AC Quantities
The text uses capital letters for both AC and DC currents and voltages with rms values assumed unless stated otherwise.
DC Quantities use upper case roman subscripts. Example: VCE. (The second letter in the subscript indicates the reference point.)
AC Quantities and time varying signals use lower case italic subscripts. Example: Vce.
Internal transistor resistances are indicated as lower case quantities with a prime and an appropriate subscript. Example: re’.
External resistances are indicated as capital R with either a capital or lower case subscript depending on if it is a DC or ac resistance. Examples: RC and Rc.

28. 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.

29. The FET
The idea for a field-effect transistor (FET) was first proposed by Julius Lilienthal, a physicist and inventor. In 1930 he was granted a U.S. patent for the device.
His ideas were later refined and developed into the FET. Materials were not available at the time to build his device. A practical FET was not constructed until the 1950’s. Today FETs are the most widely used components in integrated circuits.

30. The JFET
The JFET (or Junction Field Effect Transistor) is a normally ON device. For the n-channel device illustrated, when the drain is positive with respect to the source and there is no gate-source voltage, there is current in the channel.
When a negative gate voltage is applied to the FET, the electric field causes the channel to narrow, which in turn causes current to decrease. D

31. The JFET
As in the base of bipolar transistors, there are two types of JFETs: n-channel and p-channel. The dc voltages are opposite polarities for each type.
The symbol for an n-channel JFET is shown, along with the proper polarities of the applied dc voltages. For an n-channel device, the gate is always operated with a negative (or zero) voltage with respect to the source.
Drain D
Gate
Source

32. The JFET
There are three regions in the characteristic curve for a JFET as illustrated for the case when VGS = 0 V.
Between A and B is the Ohmic region, where current and voltage are related by Ohm’s law.
Ohmic region
VGS = 0
From B to C is the active (or constant-current) region where current is essentially independent of VDS.
Beyond C is the breakdown region. Operation here can damage the FET.
Active region
(constant current)
Breakdown
(pinch-off voltage)

33. The JFET
When VGS is set to different values, the relationship between VDS and ID develops a family of characteristic curves for the device.
Pinch-off Voltage
IDSS
VGS(off).
Notice that Vp is positive and has the same magnitude as VGS(off). D

34. The JFET
A plot of VGS to ID is called the transfer or transconductance characteristic curve. The transfer curve is a is a plot of the output current (ID) to the input voltage (VGS).
The transfer curve is based on the equation
IDSS 2
By substitution, you can find other points on the curve for plotting the universal curve.
IDSS 4
0.3 VGS(off)
0.5 VGS(off)

35. Example JFET Input Resistance Example: Solution:
The input resistance of a JFET is given by:
where IGSS is the current into the reverse biased gate.
JFETs have very high input resistance, but it drops when the temperature increases.
Example:
Compare the input resistance of a 2N5485 at 25 oC and at 100 oC. The specification sheet shows that for VGS = -20 V, IGSS – 1 nA at 25 oC and 0.2 mA at 100 oC.
Solution:
At 25 oC,
20 GW!
At 100 oC,
100 MW

36. JFET Biasing
Self-bias is simple and effective, so it is the most common biasing method for JFETs. With self bias, the gate is essentially at 0 V.
= +12 V
An n-channel JFET is illustrated. The current in RS develops the necessary reverse bias that forces the gate to be less than the source.
1.5 kW
Example:
Assume the resistors are as shown and the drain current is 3.0 mA. What is VGS?
Solution:
330 W
1.0 MW
VG = 0 V; VS = (3.0 mA)(330 W) = 0.99 V
VGS = 0 – 0.99 V = V

37. JFET Biasing
You can use the transfer curve to obtain a reasonable value for the source resistor in a self-biased circuit.
Example:
What value of RS should you use to set the Q point as shown?
Solution:
The Q point is approximately at ID = 4.0 mA and VGS = V. Q
375 W

38. JFET Biasing
Voltage-divider biasing is a combination of a voltage-divider and a source resistor to keep the source more positive than the gate.
VG is set by the voltage-divider and is independent of VS. VS must be larger than VG in order to maintain the gate at a negative voltage with respect to the source.
Voltage-divider bias helps stabilize the bias for variations between transistors.

39. The MOSFET
The metal oxide semiconductor FET uses an insulated gate to isolate the gate from the channel. Two types are the enhancement mode (E-MOSFET) and the depletion mode (D-MOSFET).
E-MOSFET
An E-MOSFET has no channel until it is induced by a voltage applied to the gate, so it operates only in enhancement mode. An n-channel type is illustrated here; a positive gate voltage induces the channel.

40. The MOSFET
The D-MOSFET has a channel that can is controlled by the gate voltage. For an n-channel type, a negative voltage depletes the channel; and a positive voltage enhances the channel.
D-MOSFET
A D-MOSFET can operate in either mode, depending on the gate voltage.
operating in D-mode
operating in E-mode

41. Summary
The MOSFET
MOSFET symbols are shown. Notice the broken line representing the E-MOSFET that has an induced channel. The n channel has an inward pointing arrow.
E-MOSFETs
n channel p channel D G S
D-MOSFETs D D G G S S
n channel p channel

42. Summary
The MOSFET
The transfer curve for a MOSFET has the same parabolic shape as the JFET but the position is shifted along the x-axis. The transfer curve for an n-channel E-MOSFET is entirely in the first quadrant as shown.
The curve starts at VGS(th), which is a nonzero voltage that is required to have channel conduction. The equation for the drain current is

43. The MOSFET
The D-MOSFET can be operated in either mode. For the n-channel device illustrated, operation to the left of the y-axis means it is in depletion mode; operation to the right means is in enhancement mode.
As with the JFET, ID is zero at VGS(off). When VGS is 0, the drain current is IDSS, which for this device is not the maximum current. The equation for drain current is