MAT02FH 查看數據表（PDF） - Analog Devices

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MAT02FH

Low Noise, Matched Dual Monolithic Transistor

Analog Devices 

MAT02

more troublesome because they vary with signal levels and are

multiplied by absolute temperature. At 25°C, kT/q is

approximately 26 mV and the error due to an rBEIC term will be

rBEIC/26 mV. Using an rBE of 0.4 Ω for the MAT02 and assum-

ing a collector current range of up to 200 µA, then a peak error

of 0.3% could be expected for an rBEIC error term when using

the MAT02. Total error is dependent on the specific application

configuration (multiply, divide, square, etc.) and the required

dynamic range. An obvious way to reduce ICrBE error is to re-

duce the maximum collector current, but then op amp offsets

and leakage currents become a limiting factor at low input lev-

els. A design range of no greater than 10 µA to 1 mA is generally

recommended for most nonlinear function circuits.

A powerful technique for reducing error due to ICrBE is shown in

Figure 4. A small voltage equal to ICrBE is applied to the transis-

tor base. For this circuit:

VB =

RC

R2

V1 and ICrBE =

r BE

R1

V1

(10)

The error from rBEIC is cancelled if RC/R2 is made equal to rOUT R1.

Since the MAT02 bulk resistance is approximately 0.39 Ω, an

RC of 3.9 Ω and R2 of 10 R1 will give good error cancellation.

In more complex circuits, such as the circuit in Figure 3, it may

be inconvenient to apply a compensation voltage to each indi-

vidual base. A better approach is to sum all compensation to the

bases of Q1. The “A” side needs a base voltage of (VO/RO + VZ/

R3) rBE, and the “B” side needs a base voltage of (VX/R1+VY/R2)

rBE. Linearity of better than ± 0.1% is readily achievable with

this compensation technique.

Operational amplifier offsets are another source of error. In

Figure 4, the input offset voltage and input bias current will

cause an error in collector current of (VOS/R1) + IB. A low

offset op amp, such as the OP07 with less than 75 µV of VOS

and IB of less than ± 3 nA, is recommended. The OP193,

micropower op amp, should be considered if low power con-

sumption or single-supply operation is needed. The value of

frequency-compensating capacitor (CO) is dependent on the

op amp frequency response and peak collector current. Typi-

cal values for CO range from 30 pF to 300 pF.

Figure 4. Compensation of Bulk Resistance Error

FOUR-QUADRANT MULTIPLIER

A simplified schematic for a four-quadrant log-antilog multiplier

is shown in Figure 5. Similar to the previously discussed one-

quadrant multiplier, the circuit makes IO = I1 I2/I3. The two

input currents, I1 and I2, are each offset in the positive direction.

This positive offset is then subtracted out at the output stage.

Assuming ideal op amps, the currents are:

I1

=

VX

R1

+

VR

R2

,

I2

=

VY

R1

+ VR

R2

(11)

IO

= VX

R1

+ VY

R1

+ VR

R2

+

VO

RO

,

I3

=

VR

R2

From IO = I1 I2/I3, the output voltage will be:

RO R2 V XVY

VO = R12 V R

(12)

Figure 5. Four-Quadrant Multiplier

–8–

REV. E

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