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
signals
• Common-mode input: both inputs change levels
together
• A 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 amplitude
• Differential amps help us to understand operational
amplifiers (coming in Lab 8)
Differential Amplifiers in Electrocardiography
(Analog Electronics for Scientific
Application, D. Barnaal, Waveland
Press, 1989)
Differential Amplifier Construction
(“single-ended” output)
(“+” or “non-inverting” input)
(“–” or “inverting” input)
(The Art of Electronics, Horowitz and Hill, 2nd Ed.)
Differential Amplifier Construction
• “Long-tailed” pair configuration:
(The Art of Electronics, Horowitz and Hill, 2nd Ed.)
Differential Amplifier of Lab 6–1
output
+ input
(Student Manual for The Art
of Electronics, Hayes and
Horowitz, 2nd Ed.)
Q1
Q2
– input
Differential Amplifier Performance
Gdiff
RC

2re  RE 
(Student Manual for The Art of Electronics,
Hayes and Horowitz, 2nd Ed.)
Differential Amplifier Performance
GCM
RC

re  RE  2Rtail
Gdiff
Rtail
CMRR 

GCM RE  re
(Student Manual for The Art
of Electronics, Hayes and
Horowitz, 2nd Ed.)
Differential Amplifier Performance:
Improving CMRR (Lab 6–1)
(The Art of Electronics, Horowitz and Hill, 2nd Ed.)
Single-Ended Input Differential Amplifier
(Lab 6–1)
output
(not inverted)
+ input
(The Art of Electronics, Horowitz and Hill, 2nd Ed.)
Example Problem 2.13
Verify that Gdiff,max  20VRC and
CMRRmax  20VR1. Then design
a differential amplifier to your own
specifications.
(The Art of Electronics, Horowitz and Hill, 2nd Ed.)
Solution details given in class.
Bootstrapping
• “Standard” emitter follower biasing scheme:
(The Art of Electronics, Horowitz and Hill, 2nd Ed.)
Bootstrapping
• “Bootstrapping” increases Zin at signal frequencies
without disturbing the DC bias: (Lab 6–2)
(The Art of Electronics,
Horowitz and Hill, 2nd Ed.)
Bootstrap Design
• Want Thévenin resistance of bootstrap network at
DC to be same as Thévenin resistance of bias
voltage divider in original circuit (10k)
• Choose R3 = 4.7k
• Then R3 + R1R2 = 10k  R1R2 = 5.3k ≈ 5k
• Choose R1 / R2 = 1 (same as original circuit)
– Solve for R1 and R2 from the above  R1 = R2 = 10k
• Choose f3dB and calculate C2 or choose C2 and
calculate f3dB  using C2 = 10 mF, f3dB = 3.2 Hz
– We do the latter since we don’t know choice of f3dB
• Similarly, choose C1 and calculate f3dB,in
– For C1 = 0.1 mF, f3dB,in = 16.9 Hz
Transistor Junction and Circuit Capacitance
(The Art of Electronics, Horowitz and Hill, 2nd Ed.)
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
(Graphics from www.rfic.co.uk)
Miller Effect
• Source impedance (Rsource) and Ceff form a low-pass
filter with an f3dB smaller than without Miller Effect
(CMiller = Ceff)
(Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)
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
(Student Manual for The Art of Electronics,
Hayes and Horowitz, 2nd Ed.)
Defeating Miller Effect
• Cascode circuit (Lab 6–3)
(Student Manual for The Art of Electronics,
Hayes and Horowitz, 2nd Ed.)
Beating Miler Effect
• Single-ended input differential amplifier
(Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)
Darlington Connection
 Darlington   Q  Q
1
2
IC

IB
VCE ,sat  0.6 V
IC
VB ≈ 1.2 V
VC
IB
≈ 0.6 V
(Student Manual for The Art
of Electronics, Hayes and
Horowitz, 2nd Ed.)
VE = 0 V
(Lab 6–4)
Superbeta Transistor
(Lab 6–5)
Superbeta transistor used in Lab 6–5
(The Art of Electronics,
Horowitz and Hill, 2nd Ed.)