MAT02F Просмотр технического описания (PDF) - Analog Devices

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MAT02F

Low Noise, Matched Dual Monolithic Transistor

Analog Devices 

MAT02

Figure 1. 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 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 prod-

uct 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 differential emitter-base

voltage (∆VBE) consists of a temperature-dependent dc level plus

an ac error voltage, which is the deviation from true log confor-

mity as the collector currents vary.

The output of the transdiode logarithmic converter comes from

the idealized intrinsic transistor equation (for silicon):

∆VBE

=

kT

q

In

IC1

IC2

+

IC1

rBE1

–

IC2

rBE2

(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,

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 in Figure 2 below:

VBE

=

kT

q

In IC

IS

(1)

where

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:

Figure 2.

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):

rBE

=

∆VO × 1

9 mA 100

(3)

This procedure finds rBE for Side A. Switching R1 and R2 will

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

R1 = R2.

–6–

REV. E

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