SSM2220 Data Sheet
Rev. C | Page 10 of 12
e
n
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
1kΩ 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 =
e
n
10,000 × 2
03096-019
Figure 19. Voltage Noise Measurement Circuit
NOISE MEASUREMENT
All resistive components and semiconductor junctions contribute
to the system input noise. Resistive components produce Johnson
noise (e
n
2
= 4kTBR, or e
n
= 0.13√R nV/√Hz, where R is in kΩ). At
semiconductor junctions, shot noise is caused by current flowing
through a junction, producing voltage noise in series impedances
such as transistor collector load resistors (I
n
= 0.556√I pA/√Hz,
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
to allow the noise spectral density,
× 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
OUT
= I
I =
+V – 2V
BE
R
Q4
Q3
Q1 Q2
03096-020
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
V
BE
matching (the voltage difference between one V
BE
and another,
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
the current source because V
CE
stays relatively constant. High fre-
quency characteristics are improved due to a reduction of Miller
capacitance. The small signal output impedance can be determined
