Ch(2) Signal Processing Circuits
IDEALAMPLIFIERS
Amplification
most
important
principle
of
electronic circuits
small signals of currents and voltages need to amplify
small signals
large signal
applied to input
available at the output
Ideal Transformers
Both coils wound on
the same iron core
have
the same flux φ
Any changing in the flux φ
V∝N
V = N (dφ/dt)
N F the number of turns
١
induced voltage
Ch(2) Signal Processing Circuits
ac (dφ ) flux
induced voltage ac
the voltage ratio (rms)
V2 N 2
=
=a
V1 N 1
(X)
no loss of power means
Pin = Pout
a step - down in current
a step up in V and vice
versa
then
I 2 V1 N1 1
=
=
=
I 1 V2 N 2 a
the simple model of the real device
٢
(Y)
Ch(2) Signal Processing Circuits
the terminal behavior of the model F the controlled
source model
Eqn (X) F satisfied by using a controlled voltage
source,
V 2 = a V1
Eqn (Y) F satisfied by using a controlled current
source,
I1 = a I2
Transformer
passive device
Pout never excess Pin
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Ch(2) Signal Processing Circuits
(Home work )Practice Problem P. 75
HERE
Real and Ideal Amplifiers
Transistor
active device F amplifies weak
signals
In the circuit model of a real amplifier
Ri determined ii F from Vi
+
tell us F AVi
the controlled voltage source.
V in one part determined V in another part of the
circuit
Vo F consisting F Ro Û RLF controlled by Vi
RL = ∞ F Ιο = 0 F Vo = A Vi
Factor A F the gain of the real amplifier.
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Ch(2) Signal Processing Circuits
In practical case
Ri
Vi =
Vs
R s + Ri
RL
∴ Vo =
AV i
Ro + R L
The gain of the real amplifier
Ar =
Vo AVs [Ri / Rs + R1 ][RL / Ro + RL ]
Ri
RL
=
= A⋅
⋅
Vs
Vs
Rs + Ri RL + Ro
For good performance
Ri F large
Vi ≅ Vs F unaffected by Rs
Ro F small
Vo ≅ AVi F unaffected by RL
For an ideal amplifier
Ri = ∞ Û Ro = 0 F then
Vo = AVi = AVs or
A = Vo / Vs = Vo / Vi
In ideal amplifier A F constant F independent
magnitude or frequency of vs.
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Ch(2) Signal Processing Circuits
Operational Amplifiers (OP amps)
using
in
analog
computers
F
mathematical
operations
Fig. F represent the differential amplifier
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Ch(2) Signal Processing Circuits
Conventional symbol F (OP amps) F all voltages F
measured with respect to ground
It F an integrated circuit F less than dollar
With very high Ri F very low Ro F get A = 105 or
more
it F widely used in
amplification
instrumentation
waveform generation.
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