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 VBE of the transistor plus an error

term, which is the product of the collector current and rBE, the

bulk emitter resistance. The difference of the VBE is amplified at

a gain of ×100 by the AMP02 instrumentation amplifier. The

differential emitter base voltage (∆VBE) 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)

S

C

BE

I

I

q

kT

V

ln

=

(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).

IS is the extrapolated current for VBE → 0 (VBE tending to zero).

IC is the collector current.

An error term must be added to Equation 1 to allow for the

bulk resistance (rBE) 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

r

I

r

I

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 IC1 is made equal to 10 IC2, and assuming TA = 25°C,

Equation 2 becomes

∆VBE = 59 mV + 0.9 IC1 rBE (∆rBE ~ 0)

As viewed on an oscilloscope, the change in ∆VBE for a 10:1

change in IC is shown in Figure 17.

61.5

61.0

60.5

60.0

59.5

59.0

58.5

1

10

100

COLLECTOR CURRENT (mA)

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, ∆VO, and the change in collector

current (9 mA):

100

1

mA

9

×

∆

=

O

BE

V

r

(3)

This procedure solves for rBE for Side A. Switching R1 and R2

provides the rBE for Side B. Differential rBE is found by making

R1 = R2.

–15V

+15V

AMP02

VOUT = 100ΔVBE

AV = 100

–15V

100pF

+15V

500

Ω

1N914

VBE

VBE

1k

Ω IC1

IC2

SIDE A DUT

Q1

VCC

–15V

100pF

+15V

+

–

500

Ω

1N914

1/2

AD8512

1/2

AD8512

1k

Ω

SIDE B DUT

Q2

VCC

+

–

Figure 18. Log Conformance Circuit