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

Differential Amplifier Construction

(+ or non-inverting input)

( or inverting input)

(single-ended output)

Differential Amplifier Construction

Long-tailed pair configuration:

(The Art of Electronics, Horowitz and Hill, 2nd Ed.)

Differential Amplifier of Lab 61

+ input

input

output

Q1

Q2

Differential Amplifier Performance

Differential Amplifier Performance

(Student Manual for The Art of Electronics, Hayes and Horowitz, 2nd Ed.)

Differential Amplifier Performance:
Improving CMRR

(Lab 61)

Single-Ended Input Differential Amplifier

output (not inverted)

(Lab 61)

+ input

Example Problem 2.13

Solution details given in class.

Verify that and

. Then design a differential amplifier to your own specifications.

Bootstrapping

Standard emitter follower biasing scheme:

Bootstrapping

Bootstrapping increases Zin at signal frequencies without disturbing the DC bias:

(Lab 62)

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

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

Miller Effect

Source impedance (Rsource) and Ceff form a low-pass filter with an f3dB smaller than without Miller Effect

(CMiller = Ceff)

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)

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

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