MAT02 データシートの表示（PDF） - Analog Devices

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MAT02

Low Noise, Matched Dual Monolithic Transistor

Analog Devices 

MAT02

Figure 18. Log Conformance Test Circuit

LOG CONFORMANCE TESTING

The log conformance of the MAT02 is tested using the circuit

shown above. The circuit employs a dual transdiode logarith-

mic 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 AMP01 instrumentation amplifier. The differen-

tial emitter-base voltage (∆VBE) consists of a temperature-

dependent dc level plus an ac error voltage which is the devia-

tion from true log conformity as the collector currents vary.

The output of the transdiode logarithmic converter comes

from the idealized intrinsic transistor equation (for silicon):

VBE =

kT In IC

q IS

where

(1)

k = Boltzmann’s Constant (1.38062 × 10-23 J/°K)

q = Unit Electron Charge (1.60219 × 10-19 °C)

T = Absolute Temperature, °K (= °C + 273.2)

IS = Extrapolated Current for VBE→0

IC = Collector Current

An error term must be added to this equation to allow for the

bulk resistance (rBE) of the transistor. Error due to the op amp

input current is limited by use of the OP15 BiFET-input op

amp. The resulting AMP01 input is:

∆VBE =

kT

q

In

IC1

IC2

+

IC1

rBE1

– IC2 rBE2

(2)

A ramp function which 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,

the previous equation 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 then displayed as shown below:

REV. C

–7–

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