Electronic-1 course.

1. ‫الرحمي‬ ‫الرمحن‬ ‫هللا‬ ‫ِسم‬‫ب‬
INTERNATIONAL UNIVERSITY OF AFRICA (I.U.A)
FACULTY OF ENGINEERING
ELECTRONIC & ELECTRICAL DEPARTMENT
ELECTRONIC 2
‫إلكترونيات‬
2
SEMESTER -5
LECTURE TWO (MULTISTAGE AMPLIFIER)
MUSSAAB I. NIASS
2022 1

2. AGENDA
1. MULTISTAGE AMPLIFIERS
2. MULTISTAGE VOLTAGE GAIN
3. CAPACITIVELY-COUPLED MULTISTAGE AMPLIFIER
4. LOADING EFFECTS
5. VOLTAGE GAIN OF THE FIRST STAGE
6. VOLTAGE GAIN OF THE SECOND STAGE
7. OVERALL VOLTAGE GAIN
8. DC VOLTAGES IN THE CAPACITIVELY COUPLED MULTISTAGE
AMPLIFIER
9. DIRECT-COUPLED MULTISTAGE AMPLIFIERS

3. 1. MULTISTAGE AMPLIFIERS
• Two or more amplifiers can be connected in a
cascaded arrangement with the output of one
amplifier driving the input of the next.
• Each amplifier in a cascaded arrangement is
known as a stage.
• The basic purpose of a multistage arrangement is
to increase the overall voltage gain.

4. 2. MULTISTAGE VOLTAGE GAIN
• The overall voltage gain, 𝑨𝒗
′ , of cascaded amplifiers, as
shown in figure 6–33, is the product of the individual voltage
gains.
Figure 6–33

5. • Amplifier voltage gain is often expressed in decibels(dB) as
follows:
• this is particularly useful in multistage systems because the
overall voltage gain in dB is the sum of the individual voltage
gains in db.
2. MULTISTAGE VOLTAGE GAIN

6. EXAMPLE-1
Answer:

7. 3. CAPACITIVELY-COUPLED
MULTISTAGE AMPLIFIER
• For purposes of illustration, we will use the two-stage
capacitively coupled amplifier in figure 6–34. notice that
both stages are identical common-emitter amplifiers with
the output of the first stage capacitively coupled to the
input of the second stage.
• Capacitive coupling prevents the dc bias of one stage
from affecting that of the other but allows the ac signal to
pass without attenuation because 𝑋𝐶 ≈ 0 Ω at the
frequency of operation. notice, also, that the transistors are
labeled Q1 and Q2.

8. Figure 6-34: a two-stage common-emitter
amplifier
3. CAPACITIVELY-COUPLED
MULTISTAGE AMPLIFIER

9. 4. LOADING EFFECTS
• In determining the voltage gain of the first stage, you must
consider the loading effect of the second stage.
• because the coupling capacitor 𝐶3 effectively appears as a
short at the signal frequency, the total input resistance of
the second stage presents an ac load to the first stage.

10. • looking from the collector of Q1, the two biasing resistors in
the second stage, R5 and R6 , appear in parallel with the
input resistance at the base of Q2.
• In other words, the signal at the collector of Q1 “sees” R3 ,
R5, R6, and 𝑅𝑖𝑛(𝑏𝑎𝑠𝑒2) of the second stage all in parallel to
ac ground.
• Thus, the effective ac collector resistance of Q1 is the
total of all these resistances in parallel, as figure 6–35
illustrates.
Figure 6-35:
AC equivalent of
first stage in
Figure
6–34, showing
loading from
second
stage input
resistance.
4. LOADING EFFECTS

11. • The voltage gain of the first stage is reduced by the
loading of the second stage because the effective ac
collector resistance of the first stage is less than the actual
value of its collector resistor, R3. remember that:
4. LOADING EFFECTS

12. 5. VOLTAGE GAIN OF THE FIRST
STAGE
• The ac collector resistance of the first stage is
• Remember that lowercase italic subscripts denote ac
quantities such as for Rc. H.W-1 (Verify IE, re` and
Rin(base2)
Imagine
the
loading
effect !!
Imagine
If there
is no C2
!!

13. 6. VOLTAGE GAIN OF THE
SECOND STAGE
• The second stage has no load resistor, so the ac collector
resistance is 𝑅7, and the gain is:
• Compare this to the gain of the first stage, and notice how
much the loading from the second stage reduced the gain.
Imagine If
there is
third stage
!!

14. 7. OVERALL VOLTAGE GAIN
• The overall amplifier gain with no load on the output is:

15. 8. DC VOLTAGES IN THE
CAPACITIVELY COUPLED
MULTISTAGE AMPLIFIER
• Since both stages in figure 6–34 are identical, the dc
voltages for Q1 and Q2 are the same. Since 𝛽𝐷𝐶𝑅4 ≫ 𝑅2
and 𝛽𝐷𝐶𝑅8 ≫ 𝑅6, the dc base voltage for Q1 and Q2 is:

16. 9. DIRECT-COUPLED
MULTISTAGE AMPLIFIERS
• A basic two-stage, direct-
coupled amplifier is shown in
figure 6–36. notice that there
are no coupling or bypass
capacitors in this circuit.
• The dc collector voltage of
the first stage provides the
base-bias voltage for the
second stage.
Figure 6-36: A basic two-stage direct-
coupled
amplifier.
H.W2: please
assume your
own values and
find the Vc?

17. 9. DIRECT-COUPLED
MULTISTAGE AMPLIFIERS
• Because of the direct coupling, this type of amplifier has a
better low-frequency response than the capacitively
coupled type in which the reactance (Xc) of coupling and
bypass capacitors at very low frequencies may become
excessive ((two much)).
• The increased reactance of capacitors at lower
frequencies produces gain reduction in capacitively
coupled amplifiers.

18. Thank you,,,,