Data Sheet MAT12
Rev. A | Page 9 of 12
LOG CONFORMANCE TESTING
The log conformance of the MAT12 is tested using the circuit
shown in Figure 18. The circuit employs a dual transdiode
logarithmic converter operating at a fixed ratio of collector
currents that are swept over a 10:1 range. The output of each
transdiode converter is the V
BE
of the transistor plus an error
term, which is the product of the collector current and r
BE
, the
bulk emitter resistance. The difference of the V
BE
is amplified at
a gain of ×100 by the AMP02 instrumentation amplifier. The
differential emitter base voltage (∆V
BE
) consists of a temperature-
dependent dc level plus an ac error voltage, which is the deviation
from true log conformity as the collector currents vary.
The output of the transdiode logarithmic converter comes from
the following idealized intrinsic transistor equation (for silicon)
(1)
where:
k is Boltzmann’s constant (1.38062 × 10
–23
J/K).
q is the unit electron charge (1.60219 × 10
–19
°C).
T is the absolute temperature, K (= °C + 273.2).
I
S
is the extrapolated current for V
BE
→ 0 (V
BE
tending to zero).
I
C
is the collector current.
An error term must be added to Equation 1 to allow for the
bulk resistance (r
BE
) of the transistor. Error due to the op amp
input current is limited by use of the AD8512 dual op amp. The
resulting AMP02 input is:
BE2
C2
BE1
C1
C2
C1
BE
rI
rI
I
I
q
kT
V −
+=
=∆ ln
(2)
A ramp function that sweeps from 1 V to 10 V is converted by
the op amps to a collector current ramp through each transistor.
Because I
C1
is made equal to 10 I
C2
, and assuming T
A
= 25°C,
Equation 2 becomes
∆V
BE
= 59 mV + 0.9 I
C1
r
BE
(∆r
BE
~ 0)
As viewed on an oscilloscope, the change in ∆V
BE
for a 10:1
change in I
C
is shown in Figure 17.
09044-016
61.5
61.0
60.5
60.0
59.5
59.0
58.5
1
10
100
COLLECTOR CURRENT (mA)
LOGGING ERROR, ΔV
BE
(mV)
Figure 17. Emitter Base, Log Conformity
With the oscilloscope ac-coupled, the temperature dependent
term becomes a dc offset and the trace represents the deviation
from true log conformity. The bulk resistance can be calculated
from the voltage deviation, ∆V
O
, and the change in collector
current (9 mA):
(3)
This procedure solves for r
BE
for Side A. Switching R
1
and R
2
provides the r
BE
for Side B. Differential r
BE
is found by making
R
1
= R
2
.
–15V
+15V
AMP02
V
OUT
= 100ΔV
BE
A
V
= 100
–15V
100pF
+15V
500Ω
1N914
V
BE
V
BE
1kΩ
I
C1
I
C2
SIDE A DUT
Q1
V
CC
–15V
100pF
+15V
+
–
500Ω
1N914
1/2
AD8512
1/2
AD8512
1kΩ
SIDE B DUT
Q2
V
CC
+
–
09044-017
Figure 18. Log Conformance Circuit
