REV. B

–8–

SSM2018T

normal and actually disables that output amplifier ensuring that

it will not oscillate and cause interference problems. Shorting

the output to the negative supply does not cause the supply

current to increase. This amplifier is only used in the “OVCE”

application explained later.

The control port follows a –30 mV/dB control law. The applica-

tion circuit shows a 3 k

W and 1 kW resistor divider from a control

voltage. The choice of these resistors is arbitrary and could be

any values to properly scale the control voltage. In fact, these

resistors can be omitted if the control voltage has been properly

scaled. The 1

mF capacitor is in place to provide some filtering

of the control signal. Although the control feedthrough is trimmed

at the factory, the feedthrough increases with frequency (TPC

16). Thus, high frequency noise can feed through and add to

the noise of the VCA. Filtering the control signal helps minimize

this noise source.

Theory of Operation of the SSM2018T

The SSM2018T has the same internal circuitry as the original

SSM2018. The detailed diagram in Figure 2 shows the main

components of the VCA. The essence of the SSM2018T is

the gain core, which comprises two differential pairs (Q1–Q4).

gain core is steered to one side or the other of the two differential

pairs. The tail current for these differential pairs is set by the

current is steered (by the “Splitter”) to the left differential pair;

the opposite is true for a negative input.

To understand how the gain control works, a simple example is

best. Take the case of a positive control voltage on Pin 11. Notice

that the bases of Q2 and Q3 are connected to ground via a 200

W resistor. A positive control voltage produces a positive voltage

on the bases of Q1 and Q4. Concentrating on the left-most

differential pair, this raises the base voltage of Q1 above that of

Q2. Thus, more of the tail current is steered through Q1 than

through Q2. The current from the collector of Q2 flows through

the external 18 k

W feedback resistor around amplifier A3. When

this current is reduced, the output voltage is also reduced. Thus,

a positive control voltage results in an attenuation of the input

signal, which explains why the gain constant is negative.

The collector currents of Q2 and Q3 produce the output voltage.

The output of Q3 is mirrored by amplifier A1 to add to the

overall output voltage. On the other hand, the collector currents

of Q1 and Q4 are used for feedback to the differential inputs.

Because Pins 6 and 4 are shorted together, any input voltage

produces an input current which flows into Pin 4. The same is

true for the inverting input, which is connected to Pin 1. The

overall feedback ensures that the current flowing through the

input resistors is balanced by the collector currents in Q1 and Q4.

Compensating the SSM2018T

The SSM108 has a network that uses an adaptive compensation

scheme that adjusts the optimum compensation level for a given

gain. The control voltage not only adjusts the gain core steering,

it also adjusts the compensation. The SSM2018T has three

compensation pins: COMP1, COMP2, and COMP3. COMP3

is normally left open. Grounding this pin actually defeats the adap-

tive compensation circuitry, giving the VCA a fixed compensation

point. The only time this is desirable is when the VCA has fixed

feedback, such as the Voltage Controlled Panner (VCP) circuit

APPLICATIONS

The SSM2018T is a trimless Voltage Controlled Amplifier (VCA)

for volume control in audio systems. The SSM2018T is identi-

cal to the original SSM2018 in functionality and pinout; how-

ever, it is the first professional quality audio VCA in the

marketplace that does not require an external trimming potenti-

ometer to minimize distortion. Instead, the SSM2018T is laser

trimmed before it is packaged to ensure the specified THD and

control feedthrough performance. This has a significant savings

in not only the cost of external trimming potentiometers, but

also the manufacturing cost of performing the trimming during

production.

Basic VCA Configuration

The primary application circuit for the SSM2018T is the basic

VCA configuration, which is shown in Figure 1. This configura-

tion uses differential current feedback to realize the VCA. A

complete description of the internal circuitry of the VCA, and

this configuration, is given in the Theory of Operation section

below. The SSM2018T is trimmed at the factory for operation in the

basic VCA configuration with class AB biasing. Thus, for optimal

distortion and control feedthrough performance, the same con-

figuration and biasing should be used. All of the graphs for the

SSM2018T in the data sheet have been measured using the

circuit of Figure 1.

V+

1 F

150k

18k

V+

18k

1 F 18k

47pF

1 F

50pF

1k

3k

V–

SSM2018T

Figure 1. Basic VCA Application Circuit

In the simple VCA configuration, the SSM2018T inputs are at a

virtual ground. Thus, 18 k

W resistors are required to convert the

input voltages to input currents. The schematic also shows ac

coupling capacitors. These are inserted to minimize dc offsets

generated by bias current through the resistors. Without the

capacitors, the dc offset due to the input bias current is typically

5 mV. The input stage has the flexibility to run either inverting,

noninverting, or balanced. The most common configuration is

to run it in the noninverting single-ended mode. If either input

is unused, the associated 18 k

W resistor and coupling capacitor

should be removed to prevent any additional noise.

The common-mode rejection in balanced mode is typically

55 dB up to 1 kHz, decreasing at higher frequencies as shown in

TPC 21. To ensure good CMRR in the balanced configuration,

the input resistors must be balanced. For example, a 1% mis-

match results in a CMRR of 40 dB. To achieve 55 dB, these

resistors should have an absolute tolerance match of 0.1%.

The output of the basic VCA is taken from Pin 14, which is the

output of an internal amplifier. Notice that the second voltage

output (Pin 16) is connected to the negative supply. This is