UNIT II Darlingto amplifier and Bootstraping technique.ppt
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Transcript of UNIT II Darlingto amplifier and Bootstraping technique.ppt
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Differential Amplifiers
Differential amps take two input signals and amplify the differences (good signal) while rejecting their common levels (noise)Normal-mode input: differential changes in the input signalsCommon-mode input: both inputs change levels togetherA good differential amp has a high common-mode rejection ratio (CMRR) of about 106 (120 dB)Ratio of response for normal-mode signal to response for common-mode signal of the same amplitudeDifferential amps help us to understand operational amplifiers (coming in Lab 8) -
Differential Amplifiers in Electrocardiography
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Differential Amplifier Construction
(+ or non-inverting input)
( or inverting input)
(single-ended output)
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Differential Amplifier Construction
Long-tailed pair configuration:(The Art of Electronics, Horowitz and Hill, 2nd Ed.)
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Differential Amplifier of Lab 61
+ input
input
output
Q1
Q2
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Differential Amplifier Performance
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Differential Amplifier Performance
(Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)
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Differential Amplifier Performance:
Improving CMRR(Lab 61)
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Single-Ended Input Differential Amplifier
output (not inverted)
(Lab 61)
+ input
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Example Problem 2.13
Solution details given in class.
Verify that and
. Then design a differential amplifier to your own specifications.
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Bootstrapping
Standard emitter follower biasing scheme: -
Bootstrapping
Bootstrapping increases Zin at signal frequencies without disturbing the DC bias:(Lab 62)
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Bootstrap Design
Want Thvenin resistance of bootstrap network at DC to be same as Thvenin resistance of bias voltage divider in original circuit (10k)Choose R3 = 4.7kThen R3 + R1R2 = 10k R1R2 = 5.3k 5kChoose R1 / R2 = 1 (same as original circuit)Solve for R1 and R2 from the above R1 = R2 = 10kChoose f3dB and calculate C2 or choose C2 and calculate f3dB using C2 = 10 mF, f3dB = 3.2 HzWe do the latter since we dont know choice of f3dBSimilarly, choose C1 and calculate f3dB,inFor C1 = 0.1 mF, f3dB,in = 16.9 Hz -
Transistor Junction and Circuit Capacitance
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Miller Effect
Consider the following amplifier with voltage gain G, with a capacitor connected between input and output:The effective input capacitance becomes Ceff = C(1 + G)According to the Miller model, the equivalent input circuit is:Ceff
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Miller Effect
Source impedance (Rsource) and Ceff form a low-pass filter with an f3dB smaller than without Miller Effect(CMiller = Ceff)
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Defeating Miller Effect
Reduce Rsource (Rsource = 0 eliminates Miller Effect)Arrange things so that base and collector of any one transistor do not head in opposite directions at the same time -
Defeating Miller Effect
Cascode circuit(Lab 63)
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Beating Miler Effect
Single-ended input differential amplifier -
Darlington Connection
VC
VE = 0 V
VB 1.2 V
0.6 V
(Lab 64)
IC
IB
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Superbeta Transistor
Superbeta transistor used in Lab 65
(Lab 65)
(
)
E
e
C
R
r
R
G
+
=
2
diff
tail
CM
2
R
R
r
R
G
E
e
C
+
+
-
=
e
E
r
R
R
G
G
+
=
tail
CM
diff
CMRR
C
R
V
G
20
max
diff,
=
1
20
CMRR
max
R
V
=
B
C
Q
Q
I
I
=
=
2
1
Darlington
b
b
b
V
6
.
0
sat
,
CE
V