1. Unit II
BJT Amplifiers

2. Outline Small signal analysis of common Emitter
Small signal analysis of common Base
Small signal analysis of common Collector
Differential Amplifiers-CMRR
Darlington Amplifier
Bootstrap Technique
cascaded Stages,Cascode stage

3. Linear analog amplifier

4. Notation

5. Basic characteristics of an amplifier

6. Basic BJT amplifier

7. Analysis of BJT amplifiers

8. Dc analysis and equivalent circuit

9. Ac analysis and equivalent circuit

10. BJT Small-Signal Models
h-parameter model
More complex
Better for ac operation
Common Emitter model
hie = input impedance (Ω)
hre = reverse voltage transfer ratio (unitless)
hfe = forward current transfer ratio (unitless)
hoe = output admittance (S) ib iC B
hie
hfeib
1/hoe
hreVce ie E

11. Calculating Av, zin, zout, and Ai of a Transistor Amplifier
Voltage Gain, Av
Output voltage divided by input voltage
Input Impedance, zin
Input voltage divided by input current

12. Calculating Av, zin, zout, and Ai of a Transistor Amplifier
Output Impedance, zout
Current Gain, Ai
Power Gain, Ap

13. The Hybrid Equivalent Model
Hybrid parameters are developed and used for modeling the transistor. These parameters can be found on a transistor’s specification sheet:
hi = input resistance
hr = reverse transfer voltage ratio (Vi/Vo)  0
hf = forward transfer current ratio (Io/Ii)
ho = output conductance

14. Simplified General h-Parameter Model
hi = input resistance
hf = forward transfer current ratio (Io/Ii)

15. Common-Emitter General BJT circuit analysis Find operating point
Determine ac parameters (T- or h- models)
Remove dc Voltage sources & replace with short circuits
Replace coupling & bypass capacitors with short circuits
Replace BJT with circuit model
Solve resulting circuit

16. Common-Emitter Amplifier
ac equivalent of fixed-bias CE amplifier using h-parameter model

17. Common-Emitter Amplifier-contd…
Equations for h-parameter model for fixed-bias CE amplifier
Circuit voltage gain a function of
Model forward current transfer ratio, hfe
Model input impedance, hie
Circuit collector resistance, RC
Circuit load resistance, RL

18. Common-Emitter Amplifier-contd…
Circuit current gain a function of
Same parameters, plus Fixed bias resistance, RB

19. Common-Emitter Amplifier-contd…
Equations for h-parameter model for fixed-bias CE amplifier
Circuit input impedance a function of
Model forward current transfer ratio, hfe
Model input impedance, hie

20. Common-Emitter Amplifier-contd…
Circuit output impedance a function of
Collector resistance (model output admittance), hoe very low

21. Common-Emitter Fixed-Bias Configuration
The input is applied to the base
The output is taken from the collector
High input impedance
Low output impedance
High voltage and current gain
Phase shift between input and output is 180

22. Fixed-Bias-contd… Input impedance: Output impedance: Voltage gain:
Current gain:

23. Emitter-Follower Configuration
Input impedance:
Output impedance:
Voltage gain:
Current gain:

24. Common Base Configuration

25. Common-Base Configuration
Input impedance:
Output impedance:
Voltage gain:
Current gain:

26. Hybrid pi model
The hybrid pi model is most useful for analysis of high-frequency transistor applications.
At lower frequencies the hybrid pi model closely approximate the re parameters, and can be replaced by them.

27. Small-signal hybrid-π equivalent circuit

28. Small-signal hybrid-π equivalent circuit (Cont’d)

29. Small-signal voltage gain

30. Input and output resistances

31. Common-emitter amplifiers (with voltage-divider biasing & coupling capacitor)

32. Common-emitter amplifiers (with voltage-divider biasing & coupling capacitor)- Cont’d

33. Common-emitter amplifiers (with voltage-divider biasing & coupling capacitor & emitter resistor)

34. Dc & Ac load lines Dc load line is used to find Q-point
Ac load line is used to determine graphically the operation of a BJT amplifier
Dc and ac load lines are essentially different since capacitors appear as an open circuit for a de operation but a short circuit for an ac operation

35. Ac load line

36. Maximum output symmetrical swing

37. Common-Collector Amplifier
Circuit gains and impedances
Av ≈ 1
zin = RB||zin(Q)
close to hfe
very small

38. BJT Transistor Modeling
A model is an equivalent circuit that represents the AC characteristics of the transistor.
A model uses circuit elements that approximate the behavior of the transistor.
There are two models commonly used in small signal AC analysis of a transistor:
re model
Hybrid equivalent model

39. The re Transistor Model
BJTs are basically current-controlled devices.
The re model uses a diode and a current source to duplicate the behavior of the transistor.
One disadvantage to this model is its sensitivity to the DC level. This model is designed for specific circuit conditions.

40. Common-Emitter Configuration-re model
The diode re model can be replaced by the resistor re.

41. Input and Output Impedances
An equivalent small signal circuit of a differential amplifier can be drawn as

42. Input Impedance During the small signal analysis, it was shown that:
But,

43. Output Impedance
Applying Kirchoff’s current law:
By Ohm’s law:

44. Coupling and Biasing
Input and output coupling capacitors may be required to remove d.c. bias voltages
If input coupling capacitors are used, a d.c. bias current path to the transistors’ bases must be established
Extra base resistors accomplish this
These will appear in parallel with the input impedance

45. Non-Ideal D.C. Effects
If operation down to d.c is required, the coupling components are omitted
This leads to some effects that are peculiar to d.c. operation:
Offset Voltage
Bias Current

46. Offset Voltage
With zero differential input, the collector currents and, therefore, the collector voltages should be identical
This assumes that:
The transistors are identical
The loads are also identical
In practice, loads will vary and the quiescent conditions will not be perfectly symmetrical
There will be an offset voltage between the actual output and the ideal assumption

47. Bias Current
In order to bring the transistors into the active region, a small d.c. base bias current is required
This d.c. current must be supplied by the signal source
This is a separate issue to the current drawn by the input impedance
Note that bias current and offset voltage effects are identical to those observed with op-amps

48. Differential Amplifier-Common mode

49. Differential Amplifier-Differential mode

50. Differential Amplifier-Transfer Characteristics

51. Differential Amplifier-Emitter Resistor

52. Differential Amplifier-one half Equivalent Circuit

53. Differential Amplifier –active loaded

54. Differential Amplifier –active loaded small signal equivalent

55. Applications Differential inputs and outputs
Useful when negative feedback is required in a multi-stage amplifier
Also useful for balanced signals
Transmitter
Noisy Channel
Noisy received signals
Difference Amp
Output

56. Bootsrap Technique
The field of electronic a bootstrap circuit is one where part of the output of an amplifier stage is applied to the input, so as to alter the input impedance of the amplifier.
When applied deliberately, the intention is usually to increase rather than decrease the impedance.

57. Bootsrap Technique
The effect of a high input impedance is to reduce the input current to the amplifier.
If the input current for a given input voltage is reduced by whatever method, the effect is to increase the input impedance.
The emitter follower has a high input impedance, but this may be reduced to an unacceptable level by the presence of the base bias resistor.

58. Boosted Output Impedances

59. Darlington Amplifier
One emitter follower (Tr1) to drive another (Tr2) the overall current gain becomes the product of the individual gains, hfe1 x hfe2 and can be typically 1000 or more.
This greatly reduces the signal current required by the base of Tr1 and thereby dramatically increases the input impedance.

60. Darlington Amplifier(cont)
The Darlington circuit provides very high current gain, equal to the product of the individual current gains:
D = 1 2
The practical significance is that the circuit provides a very high input impedance.

61. DC Bias of Darlington Circuits
Base current:
Emitter current:
Emitter voltage:
Base voltage:

62. Feedback Pair
This is a two-transistor circuit that operates like a Darlington pair, but it is not a Darlington pair.
It has similar characteristics:
High current gain
Voltage gain near unity
Low output impedance
High input impedance
The difference is that a Darlington uses a pair of like transistors, whereas the feedback-pair configuration uses complementary transistors.

63. Cascaded Systems
The output of one amplifier is the input to the next amplifier
The overall voltage gain is determined by the product of gains of the individual stages
The DC bias circuits are isolated from each other by the coupling capacitors
The DC calculations are independent of the cascading
The AC calculations for gain and impedance are interdependent

64. Cascaded Systems CE-CC
The cascade of a Common Emitter amplifier stage followed by a Common Collector amplifier stage can provide a good overall voltage amplifier 

65. Cascaded Systems CE-CC
The Common Emitter input resistance is relatively high and Common Collector output resistance is relatively low.
The voltage follower second stage, Q2, contributes no increase in voltage gain but provides a near voltage-source (low resistance) output so that the gain is nearly independent of load resistance. 

66. Cascaded Systems CE-CC
The high input resistance of the Common Emitter stage, Q1, makes the input voltage nearly independent of input-source resistance.
Multiple Common Emitter stages can be cascaded with emitter follower stages inserted between them to reduce the attenuation due to inter-stage loading.

67. Cascaded Systems CE-CE
Each stage is separately biased and coupled to adjacent stages via DC blocking capacitors.
Inserting coupling capacitors between stages blocks the DC operating bias level of one stage from affecting the DC operating point of the next.

68. R-C Coupled BJT Amplifiers
Cascaded Systems
R-C Coupled BJT Amplifiers
Voltage gain:
Input impedance,
first stage:
Output impedance,
second stage:

69. Bipolar Cascode Stage

70. Maximum Bipolar Cascode Output Impedance
The maximum output impedance of a bipolar cascode is bounded by the ever-present r between emitter and ground of Q1.

71. Example: Output Impedance
Typically r is smaller than rO, so in general it is impossible to double the output impedance by degenerating Q2 with a resistor.

72. PNP Cascode Stage

73. Improved Cascode Stage
In order to preserve the high output impedance, a cascode PNP current source is used.

74. Cascode Connection
This example is a CE–CB combination. This arrangement provides high input impedance but a low voltage gain.
The low voltage gain of the input stage reduces the Miller input capacitance, making this combination suitable for high-frequency applications.

75. MOS Cascode Stage

76. Improved MOS Cascode Amplifier
Similar to its bipolar counterpart, the output impedance of a MOS cascode amplifier can be improved by using a PMOS cascode current source.