SSM2220

Data Sheet

Rev. C | Page 10 of 12

10µF

+

0.1µF

AD8671

2

3

6

7

4

0.01µF

0.01µF

+15V

–15V

AD8671

3

2

6

7

4

0.01µF

0.01µF

+15V

–15V

1kΩ

3

6

1

2

7

8

SSM2220

3

6

1

2

7

8

SSM2220

+5V

1kΩ

500Ω

ADJUST POT

FOR 2mA

(2V ACROSS

1k

Ω RES)

SSM2220

DUT

1

8

3

6

2

7

2mA

5kΩ

1%

5kΩ

1%

10Ω

10kΩ

100Ω

2.2pF

10µF

+

0.1µF

5kΩ

–15V

SPOT NOISE FOR

EACH TRANSISTOR =

10,000 × 2

Figure 19. Voltage Noise Measurement Circuit

NOISE MEASUREMENT

All resistive components and semiconductor junctions contribute

to the system input noise. Resistive components produce Johnson

semiconductor junctions, shot noise is caused by current flowing

through a junction, producing voltage noise in series impedances

where I is in μA).

Figure 19 illustrates a technique for measuring the equivalent

input noise voltage of the SSM2220. A stage current of 1 mA is

used to bias each side of the differential pair. The 5 kΩ collector

resistors noise contribution is insignificant compared to the voltage

noise of the SSM2220. Because noise in the signal path is referred

back to the input, this voltage noise is attenuated by the gain of the

circuit. Consequently, the noise contribution of the collector load

resistors is only 0.048 nV/√Hz. This is considerably less than the

typical 0.8 nV/√Hz input noise voltage of the SSM2220 transistor.

The noise contribution of the AD8671 gain stages is also negligible,

due to the gain in the signal path. The op amp stages amplify the

input referred noise of the transistors, increasing the signal strength

input

n

e

× 10,000, to be meas-

ured with a spectrum analyzer. Because equal noise contributions

from each transistor in the SSM2220 are assumed, the output is

divided by √2 to determine the input noise of a single transistor.

Air currents cause small temperature changes that can appear as

low frequency noise. To eliminate this noise source, the measure-

ment circuit must be thermally isolated. Effects of extraneous noise

sources must also be eliminated by totally shielding the circuit.

SSM2220

SSM2220

+V

R

I =

R

Q4

Q3

Q1

Q2

Figure 20. Cascode Current Source

CURRENT SOURCES

A fundamental requirement for accurate current mirrors and active

load stages is matched transistor components. Due to the excellent

which is required to equalize collector current) and gain matching,

the SSM2220 can be used to implement a variety of standard cur-

rent mirrors that can source current into a load such as an amplifier

stage. The advantages of current loads in amplifiers vs. resistors

are an increase of voltage gain due to higher impedances, larger

signal range, and in many applications, a wider signal bandwidth.

Figure 20 illustrates a cascode current mirror consisting of two

SSM2220 transistor pairs.

The cascode current source has a common base transistor in series

with the output, which causes an increase in output impedance of

quency characteristics are improved due to a reduction of Miller

capacitance. The small signal output impedance can be determined