SSM2018 데이터 시트보기 (PDF) - Analog Devices

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SSM2018

Trimless Voltage Controlled Amplifier

Analog Devices 

Data Sheet

A compensation capacitor must be added between COMP1 and

COMP2. Because the VCA operates over such a wide gain

range, the compensation should ideally be optimized for each

gain. When the VCA is in high attenuation, there is very high

loop gain, and the part needs to have high compensation. On

the other hand, at high gain, the same compensation capacitor

would overcompensate the part and roll off the high frequency

performance. Thus, the SSM2018 employs a patented adaptive

compensation circuit. The compensation capacitor is Miller

connected between the base and collector of an internal

transistor. By changing the gain of this transistor via the control

voltage, the compensation is changed.

Increasing the compensation capacitor causes the frequency

response and slew rate to decrease, which tends to cause high

frequency distortion to increase. For the basic VCA circuit, 47 pF

was chosen as the optimal value. The OVCE circuit described

later uses a 220 pF capacitor. The reason for the increase is to

compensate for the extra phase shift from the additional output

amplifier used in the OVCE configuration. The compensation

capacitor can be adjusted over a practical range from 47 pF to

220 pF if desired. Below 47 pF, the parts may oscillate; above 220

pF the frequency response is significantly degraded.

CONTROL SECTION

As noted above, the control voltage on Pin 11 steers the current

through the gain core transistors to set the gain. The unity gain

(0 dB) condition occurs at VC = 0. Attenuation occurs in the

VCA for positive voltages (0 V to 3 V, typ), and gain occurs for

negative voltage (0 V to −1.3 V, typ). From –1.3 V to +3.0 V,

140 dB of gain range is obtainable. The output gain formula is

as follows:

VOUT = VIN × e(−aVC)

(1)

The exponential term arises from the standard Ebers-Moll

equation describing the relationship of a transistor’s collector

current as a function of the base-emitter voltage:

SSM2018

IC = IS × e(VBE /VT)

(2)

The factor a is a function not only of VT but also the scaling due

to the resistor divider of the 200 Ω and 1.8 kΩ resistors shown

in Figure 2. The resulting expression for a is as follows: a =

1/(10 × VT), which is approximately equal to 4 at room

temperature. Substituting a = 4 in the above equation results in

a −28.8 mV/dB control law at room temperature.

The −28.8 mV/dB number is slightly different from the data

sheet specification of −30 mV/dB. The difference arises from

the temperature dependency of the control law. The term VT is

known as the thermal voltage, and it has a direct dependency

on temperature:

VT = kT/q

where

k = Boltzmann’s constant = 1.38 E − 23

q = electron charge = 1.6 E − 19

T = absolute temperature in Kelvin)

This temperature dependency leads to the −3500 ppm/°C drift

of the control law. It also means that the control law changes as

the part warms up. Thus, our specification for the control law

states that the part has been powered up for 60 seconds.

When the part is initially turned on, the temperature of the die

is still at the ambient temperature (25°C for example), but the

power dissipation causes the die to warm up. With ±15 V

supplies and a supply current of 11 mA, 330 mW is dissipated.

This number is multiplied by θJA to determine the rise in the

die’s temperature. In this case, the die increases from 25°C to

approximately 50°C. A 25°C temperature change causes a 8.25%

increase in the gain constant, resulting in a gain constant of

30 mV/dB. The graph in Figure 22 shows how the gain constant

varies over the full temperature range.

Rev. C | Page 11 of 16

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