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 10 6 (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 “inverting” input) (“+” or “non-inverting” input) (

The Art of Electronics

, Horowitz and Hill, 2 nd Ed.)

Differential Amplifier Construction

• “Long-tailed” pair configuration: (

The Art of Electronics

, Horowitz and Hill, 2 nd Ed.)

+ input

Differential Amplifier of Lab 6 –1

Q

1

Q

2 output – input (

Student Manual for The Art of Electronics

, Hayes and Horowitz, 2 nd Ed.)

Differential Amplifier Performance

G

diff  2 

r e R C



R E

 (

Student Manual for The Art of Electronics

, Hayes and Horowitz, 2 nd Ed.)

Differential Amplifier Performance

G

CM  

r e



R E R C

 2

R

tail CMRR 

G

diff

G

CM 

R E R

tail 

r e

(

Student Manual for The Art of Electronics

, Hayes and Horowitz, 2 nd Ed.)

Differential Amplifier Performance: Improving CMRR (Lab 6 –1) (

The Art of Electronics

, Horowitz and Hill, 2 nd Ed.)

Single-Ended Input Differential Amplifier (Lab 6 –1) output (not inverted) + input (

The Art of Electronics

, Horowitz and Hill, 2 nd Ed.)

Example Problem 2.13

CMRR

G

 20

V

 20

V C R

1 . Then design max a differential amplifier to your own specifications.

(

The Art of Electronics

, Horowitz and Hill, 2 nd Ed.) Solution details given in class.

Bootstrapping

• “Standard” emitter follower biasing scheme: (

The Art of Electronics

, Horowitz and Hill, 2 nd Ed.)

Bootstrapping

• “Bootstrapping” increases

Z

in at signal frequencies without disturbing the DC bias: (Lab 6 –2) (

The Art of Electronics

, Horowitz and Hill, 2 nd Ed.)

Bootstrap Design

• Want Thévenin resistance of bootstrap network at DC to be same as Th évenin resistance of bias voltage divider in original circuit (10k) • Choose • Then

R

3

R

3 = 4.7k

+

R

1 

R

2 = 10k 

R

1 

R

2 = 5.3k ≈ 5k • Choose

R

1 /

R

2 – Solve for

R

1 = 1 and

R

2 (same as original circuit) from the above 

R

1 =

R

2 = 10k • Choose

f

3dB calculate

f

3dB and calculate  using

C

2

C

2 or choose

C

2 = 10 m F,

f

3dB and = 3.2 Hz – We do the latter since we don’t know choice of

f

3dB • Similarly, choose

C

1 – For

C

1 = 0.1 m F,

f

3dB,in and calculate = 16.9 Hz

f

3dB,in

Transistor Junction and Circuit Capacitance (

The Art of Electronics

, Horowitz and Hill, 2 nd Ed.)

Miller Effect

• Consider the following amplifier with voltage gain –

G

, with a capacitor connected between input and output: – The effective input capacitance becomes

C

eff =

C

(1 +

G

) • According to the Miller model, the equivalent input circuit is:

C

eff (Graphics from www.rfic.co.uk)

Miller Effect

• Source impedance (

R

source ) and

C

eff filter with an

f

3dB form a low-pass smaller than without Miller Effect (

C

Miller =

C

eff ) (

Student Manual for The Art of Electronics

, Hayes and Horowitz, 2 nd Ed.)

Defeating Miller Effect

• Reduce

R

source (

R

source = 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, 2 nd Ed.)

Defeating Miller Effect

• Cascode circuit (Lab 6 –3) (

Student Manual for The Art of Electronics

, Hayes and Horowitz, 2 nd Ed.)

Beating Miler Effect

• Single-ended input differential amplifier (

Student Manual for The Art of Electronics

, Hayes and Horowitz, 2 nd Ed.)

Darlington Connection

(Lab 6 –4)  Darlington  

Q

1 

Q

2 

I C I B V CE

, sat  0 .

6 V

I C V B

≈ 1.2 V

V C I B

≈ 0.6 V

V E

= 0 V (

Student Manual for The Art of Electronics

, Hayes and Horowitz, 2 nd Ed.)

Superbeta Transistor

Superbeta transistor used in Lab 6 –5 (Lab 6 –5) (

The Art of Electronics

, Horowitz and Hill, 2 nd Ed.)