MC33110

9

MOTOROLA RF/IF DEVICE DATA

Figure 24. Compressor

+

–

2.2

µF

12

10

Input

10 k

Iref

VCC

Rectifier

IControl

10 k

∆ Gain

VB

8

11

9

20 k

R2

20 k

R1

1.0

µF

C1

Output

2.0

µF

4.7 k

Compressor

The compressor is an operational amplifier with a fixed

input resistor and a variable gain cell in its feedback path as

shown in Figure 24.

The amplifier output is sampled by the precision rectifier

which, in turn, supplies a DC signal (IControl), representative

of the rectifier’s AC signal, to the variable gain cell. The

reference current (Iref) is an internally generated precision

current. The effective impedance of the variable gain cell

varies with the ratio of the two currents, and decreases as

IControl increases, thereby providingcompression.Theoutput

is related to the input by the following equation:

Vout = 0.3162 x √Vin

(Equation 1)

In terms of dB levels, the relationship is:

Vout(dB) = 0.5 x Vin(dB)

(Equation 2)

where 0 dB = 100 mVrms (see Figure 2 and 4).

The inputs and output are internally biased at VB (VCC/2),

and must therefore be capacitor coupled to external circuitry.

Pin 10 input impedance is nominally 10 k

Ω (± 20%), and the

maximum functional input signal is shown in Figure 18. Bias

currents required by the op amp and the variable gain cell are

internally supplied. Due to clamp diodes at the input (to VCC

and ground), the input signal must be maintained between

the supply rails. If the input signal goes more than 0.5 V

above VCC or below ground, excessive currents will flow and

distortion will show up at the output.

When no AC signals are present at the input, the variable

gain cell will attempt to set such a high gain that the circuit

may be come unstable. For this reason resistors R1 and R2,

and capacitor C1 are added to provide DC stability. The pole

formed by R1, R2 and C1 should have a pole frequency no

more than 1/10th of the lowest frequency of interest. The pole

frequency is calculated from:

f

+

R1

) R2

2

p xR1R2 C3

(Equation 3)

for the component values shown, the pole frequency is

≈ 16 Hz.

Likewise, the capacitor between Pins 11 and 8 should be

selected such that, in conjunction with the input impedance at

Pin 8 (

≈ 3200 Ω, ± 20%), the resulting pole frequency is no

more than 1/10 of the lowest frequency of interest. With the

components shown, the pole frequency is < 30 Hz. This pole

frequency is calculated from:

f

+

1

2

p x3.2 k x C

(Equation 4)

The output of the rectifier is filtered by the capacitor at

Pin 12, which, in conjunction with an internal 10 k resistor,

provides the time constant for the attack and decay times.

Figure 14 and 16 indicate how the times vary with the

capacitor value. The attack time for the compressor is always

faster than the decay time due to the fact that the rectifier is

fed from the output rather than the input. Since the output is

initially larger than expected (immediately after the input has

increased), the external capacitor is charged more quickly

during the initial part of the time constant. When the input is

decreased, the time constant is closer to that calculated by

t = RC. If the attack and decay times are decreased by using

a smaller capacitor, performance at low frequencies will

degrade.