®

VCA610

11

FIGURE 10. Signal Drive of the VCA610 Gain Control Pin

Produces and Exponential Response, Re-ex-

panding Signal Companded by Figure 9.

R

2

330

Ω

VCA610

R

1

470

Ω

V

–2 [R1 VIN /(R1 + R2) + 1]

V

IN

V

R

–10mV

V

C

FIGURE 11. This Voltage-Tuneable Low-Pass Filter Pro-

duces a Variable Cutoff Frequency with a

3,000:1 Range.

Finite loop gain and a signal swing limitation set perfor-

mance boundaries for the circuit. Both limitations occur

when the VCA610 attenuates rather than amplifies the

feedback signal. These two limitations reduce the circuit’s

utility at the lower extreme of the VCA610’s gain range. For

–1

≤ V

range from 0dB to –40dB. This directly reduces the net gain

in the circuit’s feedback loop, increasing gain error effects.

Also, this attenuation transfers an output swing limitation

from the OPA620 output to the overall circuit’s output. Note

that OPA620 output voltage, V

expression V

V

However, the circuit shown provides greater output swing

than the more common multiplier implementation. The latter

replaces the VCA610 of the figure with an analog multiplier

having a response of V

V

the multiplier implementation amplifies V

10. Circuit constraints require that V

≤ 1. Thus, the multiplier performs only as a variable attenu-

ator and never provides amplification. As a result, the

voltage swing limitation of V

OA

restricts the V

O

swing

throughout most of the circuit’s control range. Replacing the

multiplier with the VCA610 shown permits equivalent gains

greater > 1. Then, operating the VCA610 with gains in the

range of one to 100 avoids the reduction in output swing

capability.

VOLTAGE-CONTROLLED HIGH-PASS FILTER

A circuit analogous to the above low-pass filter produces a

voltage-controlled high-pass response. The gain control pro-

vided by the VCA610 of Figure 12 varies this circuit’s

response zero from 1Hz to 10kHz according to the relation-

ship F

–2 (V

C

+ 1)

.

f

V

O

V

I

= –

R

2

R

1

1

1 + R

VOLTAGE-CONTROLLED LOW-PASS FILTER

In the circuit of Figure 11, the VCA610 serves as the

variable gain element of a voltage-controlled low-pass filter.

As will be described, this implementation expands the circuit’s

voltage swing capability over that normally achieved with

the equivalent multiplier implementation. The circuit’s re-

sponse pole responds to control voltage V

relationship f

–2 (V

C

+ 1)

. With the

components shown, the circuit provides a linear variation of

the low-pass cutoff from 300Hz to 1MHz.

FIGURE 12. A Voltage-Tunable High-Pass Filter Pro-

duces a Response Zero Variable from 1Hz to

10kHz.

The response control results from amplification of the feed-

back voltage applied to R

VCA610 produces G = 1. Then, the circuit performs as if

this amplifier were replaced by a short circuit. Visually

doing so leaves a simple voltage amplifier with a feedback

resistor bypassed by a capacitor. This basic circuit produces

a response pole at f

For G > 1, the circuit applies a greater voltage to R

increasing the feedback current this resistor supplies to the

summing junction of the OPA620. The increased feedback

current produces the same result as if R

in value in the basic circuit described above. Decreasing the

effective R

frequency, producing the f

OPA620

VCA610

V

C

V

O

V

OA

0.047µF

330

Ω

R

2

330

Ω

R

1

C

V

I

OPA620

VCA610

V

C

0.047µF

V

OA

CR

3

33

Ω

V

O

R

2

33k

Ω

R

1

33k

Ω

V

I

For R

V

O

V

I

R

2

R

1

(1 + GR

= –