SSM2220

Data Sheet

Rev. C | Page 8 of 12

APPLICATIONS INFORMATION

5V

20µs

PULSE RESPONSE

LOW FREQUENCY NOISE

10µF

AD8671

2

3

6

7

4

0.01µF

0.01µF

–15V

+15V

Q5

Q2

Q3

Q7

Q4

Q1

Q6

+

–

83Ω

1.5kΩ

0.01%

1.5kΩ

0.01%

27kΩ

–15V

RED

LED

+15V

–15V

+

0.001µF

SSM2220 PAIRS:

Q1 – Q2

Q3 – Q4

Q5 – Q6

150Ω

0.01µF

+

10µF

0.001µF

Figure 16. Super Low Noise Amplifier

SUPER LOW NOISE AMPLIFIER

The circuit in Figure 16 is a super low noise amplifier, with equiv-

alent input voltage noise of 0.32 nV/√Hz. By paralleling SSM2220

matched pairs, a reduction of the base spreading resistance by a

factor of 3 results in a further reduction of amplifier noise by a fac-

tor of √3. Additionally, the shot noise contribution is reduced by

maintaining a high collector current (2 mA/device), which reduces

the dynamic emitter resistance and decreases voltage noise. The

voltage noise is inversely proportional to the square root of the

stage current, whereas current noise increases proportionally.

Accordingly, this amplifier capitalizes on voltage noise reduction

techniques at the expense of increasing the current noise. However,

high current noise is not usually important when dealing with

low impedance sources.

This amplifier exhibits excellent full power ac performance,

0.08% THD into a 600 Ω load, making it suitable for exacting

audio applications (see Figure 17).

0.1

0.001

0.01

10

100k

10k

1k

100

FREQUENCY (Hz)

600Ω LOAD

NO LOAD

Figure 17. Total Harmonic Distortion vs. Frequency of Circuit in Figure 16