Chapter 4 Bipolar Junction Transistors Outline Basic operation of the npn Bipolar Junction Tr ansistor Load-line analysis of a common-emitter amplifi er The pnp bipolar junction transistor Small-signal equivalent circuits The common-emitter amplifier The emitter follower (common-collector amplifi er) The common-base amplifier
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Chapter 4 Bipolar Junction TransistorsOutline Basic operation of the npn Bipolar Junction Transistor Load-line analysis of a common-emitter amplifier The pnp bipolar junction transistor Small-signal equivalent circuits The common-emitter amplifier The emitter follower (common-collector amplifier) The common-base amplifier
4.1 Basic operation of the npn transistor
Figure 4.1 The npn BJT
A npn BJT consists of a thin layer of p-type material between two layers of n-type material. Two pn junctions are formed in the device. The current flowing across one junction affects the current in the other junction. It is this interaction that makes the BJT useful as an amplifier.
Basic operation in the active region The common-emitter configuration Active region (normal operation)
Base-emitter junction is forward biased The base-collector junction is reverse
biased
Figure 4.2 An npn transistor with variable biasing sources (CE configuration)
4.1 Basic operation of the npn transistor
The emitter region is doped heavily. The base region is very thin.
Amplification by the BJT
4.1 Basic operation of the npn transistor
Figure4.4 CE characteristics of a typical npn BJT
A small change in vbe can result in an appreciable change in ib, this causes a much larger change in ic, and in suitable circuits, it is converted into a much larger voltage change than the initial change in vbe.
The common-emitter current gain
Device equations
4.1 Basic operation of the npn transistor
B
C
ii
10~1000
CBE iii
The common-base current gain
E
C
ii
Exercise:1. Find the relationship between
and 2. Estimate and for Figure4.4
4.2 Load-line analysis of a CE amplifier
The power-supply voltages VBB and VCC bias the device at an Q point for which the amplification of the input signal is possible.Exercise:1. Write KVL for input
loop.2. Write KVL for output
loop.
P144 Example 4.2Figure4.6 Common emitter amplifier
4.2 Load-line analysis of a CE amplifier
Pay attention to Phase relationship between vin and vout
Figure 4.11 Amplification in the active region
4.2 Load-line analysis of a CE amplifier (ref 4.4)
Active region model — amplification (1) Saturation region model (2) Cutoff region model (3) Inverted (reverse) model (4) Model B-E junction B-C junction
The overall voltage gain of cascaded amplifier stages is the product of the voltage gains of the individual stages.
Think over: If Avo1=100, Avo2=200, what is the overall open circuit voltage gain of cascaded amplifier?
Avo=Avo1 Avo2 ? (refer to p17)
Review 1.5 Cascaded amplifiers
Applications calling for high or low input impedance Applications calling for high or low output impedance Application calling for a particular impedance Refer to examples shown on page 26-27
AC coupling — The DC voltages of the amplifier circuits do not affect the signal source, adjacent stages, or the load.Amplifiers that are realized as integrated circuits are almost always DC coupled because the capacitors or transformers needed for ac coupling cannot be fabricated in integrated form.
BACK
Example
Frequency response (P30) The complex gain: The ratio of the phasor for the output signal to
the input signal Bode plot (P271)
How circuit functions can be quickly and easily plotted against frequency? (straight line approximation & smart scale)
4.10 Frequency response for an amplifier
Review
Logarithmic Frequency Scale
A decade is a range of frequencies for which the ratio of the highest frequency to the lowest is 10.An octave is a two-to-one change in frequency.
Review: Passive low-pass filter
Figure 8.1 Low-pass RC filter.
=
Review 2.8 Active Filter-High pass filter
4.10 Frequency response for an amplifier
Question:1. What is the voltage
gain of the circuit?2. Can you draw the
frequency response of amplifier?
3. What kind of filter is it?
4. Do you remember the small signal equivalent circuit of the NPN transistor?
5. Any changes to (4) if the frequency is high?
4.10 Frequency response for an amplifier
The small signal equivalent circuit for the NPN transistor
Mid-frequencyCbe
Cbc
Usually, Cbe is about 10PF ~ 1000PF
Cbc <10PF
At high frequencies
Miller effect
Question: (P296)If Zf=-j/C for a CE amplifier, how do you find Zin,miller
and Zout,miller
Cbc
ibrbe
Cbe
(1+gmRL’)Cbc
Find the Thevenin equivalent resistance Rs’?
Figure 8.34 Simplified equivalent circuit for the common-emitter amplifier.
1. What kind of filter is it?2. What is the break (cut-off) frequency? 3. What factors affect the value of the break
frequency?4. What do we desire about an amplifier?
4.10 Frequency response for an amplifier
Figure 8.36 High-frequency behavior of the common-emitter amplifier.
Conclusion: The common-base amplifier achieves wide bandwidth, but its input impedance is very low. Consequently, the mid-band gain can be quite small due to loading of source. CE-CC cascade amplifier combines the advantages of both.