VCA610

11

SBOS013A

FIGURE 10. Signal Drive of the VCA610 Gain Control Pin

Produces an Exponential Response, Re-expand-

ing Signal Companded by Figure 9.

R

2

330

Ω

VCA610

R

1

470

Ω

V

–1.925 [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.

= –

R

2

R

1

1

OPA680

VCA610

V

C

V

O

V

OA

R

2

330

Ω

R

1

330

Ω

C

0.047

µF

V

I

Examination of this result illustrates several circuit charac-

a DC reference voltage. Optionally, making this voltage a

second signal produces log-ratio operation. Either way, the

These two voltages must be of opposite polarities to ensure

a positive argument. This polarity combination results when

removes the minus sign of the log term’s argument. Then,

both voltages must be of the same polarity to produce a

positive argument. In either case, the positive polarity re-

scaling control to the circuit. However, a practical matter

sets a minimum level for this gain. The voltage divider

enough to prevent any possibility of an overload voltage at

gain-control circuitry, reducing the amplifier’s gain. For the

feedback connection of Figure 9, this overload condition

ensure that the op amp can not possibly deliver more than

LOW-DRIFT WIDEBAND EXPONENTIAL AMP

A common use of the log amp above involves signal

companding. The inverse function, signal expanding, re-

quires an exponential transfer function. The VCA610 pro-

duces this latter response directly as shown in Figure 10.

ing the gain control input. Setting these resistors at the same

values as in the preceding log amp produces an exponential

amplifier with the inverse function of the log amp.

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

–1.925 (V

C

components shown, the circuit provides a linear variation of

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

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

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

the range from 0dB to –38.5dB. This directly reduces the net

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

V

O

V

I