REV. B

AD633

–5–

the basis of voltage controlled integrators and oscillators as will

be shown later in this Applications section. The transfer func-

tion of this circuit has the form

I

R

XX

Y

Y

V

−

(

(

)

1

10

12

1

2

(Equation 7)

Linear Amplitude Modulator

The AD633 can be used as a linear amplitude modulator with

no external components. Figure 10 shows the circuit. The car-

rier and modulation inputs to the AD633 are multiplied to

produce a double-sideband signal. The carrier signal is fed

forward to the AD633’s Z input where it is summed with the

double-sideband signal to produce a double-sideband with carrier

output.

Voltage Controlled Low-Pass and High-Pass Filters

Figure 11 shows a single multiplier used to build a voltage con-

trolled low-pass filter. The voltage at output A is a result of

f

E

VRC

C

2

20

(Equation 8)

and the rolloff is 6 dB per octave. This output, which is at a

high impedance point, may need to be buffered.

The voltage at output B, the direct output of the AD633, has

filter,

f

RC

1

1

2

=

π

(Equation 9)

0.1 F

+15V

MODULATION

INPUT

W = 1+

10V

8

7

6

5

1

2

3

4

AD633JN

0.1 F

X1

X2

Y1

Y2

W

Z

–15V

CARRIER

INPUT

Figure 10. Linear Amplitude Modulator

For example, if R = 8 k

Ω and C = 0.002 µF, then output A has

from 100 mV to 10 V. Output B has an additional zero at 10 kHz

(and can be loaded because it is the multiplier’s low impedance

output). The circuit can be changed to a high-pass filter Z inter-

changing the resistor and capacitor as shown in Figure 12 below.

0.1 F

+15V

OUTPUT B =

8

7

6

5

1

2

3

4

AD633JN

0.1 F

X1

X2

Y1

Y2

W

Z

–15V

SIGNAL

CONTROL

R

C

OUTPUT A =

1

1

1

10

0

dB

f

f2 f1

OUTPUTB

–6dB/OCTAVE

OUTPUTA

Figure 11. Voltage Controlled Low-Pass Filter

0.1 F

+15V

OUTPUT B

8

7

6

5

1

2

3

4

AD633JN

0.1 F

X1

X2

Y1

Y2

W

Z

–15V

SIGNAL

CONTROL

R

C

OUTPUT A

0

dB

f

f2

f1

OUTPUTB

+6dB/OCTAVE

OUTPUTA

Figure 12. Voltage Controlled High-Pass Filter

Voltage Controlled Quadrature Oscillator

Figure 13 shows two multipliers being used to form integrators

with controllable time constants in a 2nd order differential

equation feedback loop. R2 and R5 provide controlled current

output operation. The currents are integrated in capacitors C1

and C2, and the resulting voltages at high impedance are applied

to the X inputs of the “next” AD633. The frequency control

with a calibration of 100 Hz/V. The accuracy is limited by the

Y-input offsets. The practical tuning range of this circuit is

100:1. C2 (proportional to C1 and C3), R3, and R4 provide

regenerative feedback to start and maintain oscillation. The

diode bridge, D1 through D4 (1N914s), and Zener diode D5

provide economical temperature stabilization and amplitude

stabilization at

±8.5 V by degenerative damping. The out-

put from the second integrator (10 V sin

ωt) has the lowest

distortion.

AGC AMPLIFIERS

Figure 14 shows an AGC circuit that uses an rms-dc converter

to measure the amplitude of the output waveform. The AD633

and A1, 1/2 of an AD712 dual op amp, form a voltage con-

trolled amplifier. The rms dc converter, an AD736, measures

the rms value of the output signal. Its output drives A2, an

integrator/comparator, whose output controls the gain of the

voltage controlled amplifier. The 1N4148 diode prevents the

output of A2 from going negative. R8, a 50 k

Ω variable resistor,

sets the circuit’s output level. Feedback around the loop forces

the voltages at the inverting and noninverting inputs of A2 to be

equal, thus the AGC.