–11–

REV. B

Figure 46. “Inverted-Multiplier” Configuration for

Analog Division

Figure 47. Signal-Dependent Feedback Makes Variables

Out of Amplifier Bandwidth and Stability

Although this technique works well with almost any operational

amplifier, there is one caveat: for best circuit stability, the unity-

gain crossover frequency of the operational amplifier should be

equal to or less than the MLT04’s 8 MHz bandwidth.

Connection for Square Rooting

Another application of the “inverted multiplier” configuration is the

square-root function. As shown in Figure 48, both inputs of the

MLT04 are wired together and are used as the output of the

circuit. Because the circuit configuration exhibits the following

generalized transfer function:

V

O

=−2.5 ×

R2

R1









×V

IN

the input signal voltage is limited to the range –2.5 V

≤ V

To

prevent circuit latchup due to positive feedback or input signal

polarity reversal, a 1N4148-type junction diode is used in series

with the output of the multiplier.

Figure 48. Connections for Square Rooting

Voltage-Controlled Low-Pass Filter

The circuit in Figure 49 illustrates how to construct a voltage-

controlled low-pass filter with an analog multiplier. The advantage

with this approach over conventional active-filter configurations is

that the overall characteristic cut-off frequency,

ω

proportional to a multiplying input voltage. This permits the

construction of filters in which the capacitors are adjustable

(directly or inversely) by a control voltage. Hence, the frequency

scale of a filter can be manipulated by means of a single voltage

without affecting any other parameters. The general form of the

circuit’s transfer function is given by:

V

O

V

IN

=−

R2

R1









1

s

R2

+ R1

R1









2. 5RC

V

X









+ 1





























In this circuit, the ratio of R2 to R1 sets the passband gain, and the

break frequency of the filter,

ω

ω

LP

=

R1

R1

+ R2









V

X

2. 5RC









Figure 49. A Voltage-Controlled Low-Pass Filter

For example, if R1 = R2 = 10 k

Ω , R = 10 kΩ , and C = 80 pF,

MLT04

V

O

V

IN

R1

10k

R2

10k

1/4 MLT04

0.4

3

2

4

1

GND1

Y1

X1

W1

2

6

3

OP113

V

X

V

V

IN

V

X

+

+

+

+

90

40

100

1k

10M

1M

100k

10k

50

60

70

80

0

10

20

30

FREQUENCY – Hz

OP113

V

X

= 0.025V

V

X

= 0.25V

V

X

= 2.5V

V

O

V

IN

R1

10k

R2

10k

1/4 MLT04

0.4

3

2

4

1

Y1

X1

W1

6

OP113

D1

1N4148

V

–2.5V • V

IN

2

3

+

C

80pF

R2

10k

R

10k

R1

10k

1/4 MLT04

0.4

3

2

1

V

O

Y1

X1

W1

2

3

1

A1

4

A1 = 1/2 OP285

V

IN

GND1

V

X

f

V

X

= –

1

1 + S

5RC

V

X

V

O

V

IN

+

+

+

+