®

VCA610

10

STABILIZED WEIN-BRIDGE OSCILLATOR

Adding Wein-bridge feedback to the above AGC amplifier

produces an amplitude-stabilized oscillator. Shown in

Figure 8, this alternative requires the addition of just two

Connecting the feedback network to the amplifier’s

noninverting input introduces positive feedback to induce

oscillation. The feedback factor displays a frequency depen-

dence due to the changing impedances of the C

As frequency increases, the decreasing impedance of the

C

Analysis shows that the maximum factor occurs at f =

1/2

πR

oscillation. At this frequency the impedance magnitude of

condition produces a feedback factor of 1/3. Thus, self-

sustaining oscillation requires a gain of three through the

amplifier. The AGC circuitry establishes this gain level.

negative, increasing the amplifier gain from its minimum.

When this gain reaches three, oscillation begins at f =

1/2

πR

oscillation amplitude grow. This growth continues until that

amplitude reaches a peak value equal to V

tude at V

produces amplitude modulation of the oscillator output.

LOW-DRIFT WIDEBAND LOG AMP

The VCA610 can be used to provide a 250kHz (–3dB) log

amp with low offset voltage and low gain drift.

The exponential gain control characteristic of the VCA610

permits simple generation of a temperature-compensated

logarithmic response. Enclosing the exponential function in

an op amp feedback path inverts this function, producing the

log response. Figure 9 shows the practical implementation

VCA610 inverting input voltage. This makes the amplifier’s

A second input voltage also influences V

input voltage V

producing a feedback current that charges C

V

amp forces this equality by supplying the gain control

voltage V

mic response.

V

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-

The above V

scaling control to the circuit. However, a practical matter

sets a minimum level for this gain. The voltage divider

formed by R

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

permits a circuit latch. To prevent this, choose R

ensure that the op amp can not possibly deliver more than

2.5V to the V

FIGURE 9. Driving the Gain Control Pin of the VCA610 with

a Feedback Amplifier Produces a Temperature-

Compensated Log Response.

R

1

470

Ω

VCA610

R

2

330

Ω

V

OL

V

R

–10mV

OPA620

V

IN

V

C

C

50pF

R

3

100

Ω

V

1 + 0.5 Log (–V

R

1

R

2

(

)

V

C

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. DC

gain control input. Setting these resistors at the same values

as in the preceding Log amp produces an exponential ampli-

fier with the inverse function of the Log amp.