1. BJT Amplifiers

2. Objectives Understand the concept of amplifiers
Identify and apply internal transistor parameters
Understand and analyze common-emitter

3. Introduction
One of the primary uses of a transistor is to amplify ac 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.

4. Amplifier Operation
Recall from the previous chapter that the purpose of dc biasing was to establish the Q-point for operation. The collector curves and load lines help us to relate the Q-point and its proximity to cutoff and saturation. The Q-point is best established where the signal variations do not cause the transistor to go into saturation or cutoff.
What we are most interested in is the ac signal itself. Since the dc part of the overall signal is filtered out in most cases, we can view a transistor circuit in terms of just its ac component.

5. Amplifier Operation
For the analysis of transistor circuits from both dc and ac perspectives, the ac subscripts are lower case and italicized. Instantaneous values use both italicized lower case letters and subscripts.
Fig 6-1 V vs. t graph

6. Amplifier Operation
The boundary between cutoff and saturation is called the linear region. A transistor which operates in the linear region is called a linear amplifier. Note that only the ac component reaches the load because of the capacitive coupling and that the output is 180º out of phase with input.
Fig 6-2 amplifier circuit

7. Transistor Equivalent Circuits
We can view transistor circuits by use of resistance or r parameters for better understanding. Since the base resistance, rb is small it normally is not considered and since the collector resistance, rc is fairly high we consider it as an open. The emitter resistance, rc is the main parameter that is viewed.
You can determine rc from this simplified equation. (Appendix B)
re = 25 mV/IE
Fig 6-6 r-parameter comparison

8. Transistor Equivalent Circuits
The two graphs best illustrate the difference between DC and ac. The two only differ slightly.
Fig 6-7 IC vs IB curves

9. Transistor Equivalent Circuits
Since r parameters are used throughout the rest of the textbook we will not go into deep discussion about h parameters. However, since some data sheets include or exclusively provide h parameters these formulas can be used to convert them to r parameters.
r’e = hre/hoe
r’c = hre + 1/hoe
r’b = hie – hre/hoe(1+ hfe)

10. The Common-Emitter Amplifier
The common-emitter amplifier exhibits high voltage and current gain. The output signal is 180º out of phase with the input.
Now let’s use our dc and ac analysis methods to view this type of transistor circuit.
Fig. 6-8 ce amp

11. The Common Emitter Amplifier DC Analysis
The dc component of the circuit “sees” only the part of the circuit that is within the boundaries of C1, C2, and C3 as the dc will not pass through these components. The equivalent circuit for dc analysis is shown.
The methods for dc analysis are just are the same as dealing with a voltage-divider circuit.
Fig 6-9 dc eq. ce amp

12. Common Emitter Amplifier AC Equivalent Circuit
The ac equivalent circuit basically replaces the capacitors with shorts, being that ac passes through easily through them. The power supplies are also effectively shorts to ground for ac analysis.
Fig 6-10&6-11 ac eq. ce circuit

13. Common Emitter Amplifier AC Equivalent Circuit
We can look at the input voltage in terms of the equivalent base circuit (ignore the other components from the previous diagram). Note the use of simple series-parallel analysis skills for determining Vin.

14. Common Emitter Amplifier AC Equivalent Circuit
The input resistance as seen by the input voltage can be illustrated by the r parameter equivalent circuit. The simplified formula below is used.
Rin(base) = acr’e
Example 6-3
The output resistance is for all practical purposes the value of Rc=RC||RL.
Fig 6-12 r parameter ce amp

15. Common Emitter Amplifier AC Equivalent Circuit
Voltage gain can be easily determined by dividing the ac output voltage by the ac input voltage.
Av = Vout/Vin = Vc/Vb
Voltage gain can also be determined by the simplified formula below.
Av = RC/r’e
Fig 6-14

16. Common Emitter Amplifier AC Equivalent Circuit
Taking the attenuation from the ac supply internal resistance and input resistance into consideration is included in the overall gain.
A’v = (Vb/Vs)Av or
Fig 6-19

17. Summary
Most transistors amplifiers are designed to operate in the linear region.
Transistor circuits can be view in terms of its ac equivalent for better understanding.
The common-emitter amplifier has high voltage and current gain.
The common-collector has a high current gain and voltage gain of 1. It has a high input impedance and low output impedance.

18. Summary
The common-base has a high voltage gain and a current gain of 1. It has a low input impedance and high output impedance