SSM2018

Data Sheet

Rev. C | Page 10 of 16

THEORY OF OPERATION

The SSM2018 has the same internal circuitry as the original

SSM2018. The detailed diagram in Figure 28 shows the main

components of the VCA. The essence of the SSM2018 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. Note

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

Ω 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 kΩ 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 SSM2018

The SSM2018 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 SSM2018 has three

compensation pins: COMP1, COMP2, and COMP3. COMP3 is

normally left open. Grounding this pin actually defeats the adaptive

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

shown later in the data sheet. Thus, for the Basic VCA circuit or

the OVCE circuit, COMP3 should be left open.

Figure 28. Detailed Functional Diagram

15

4

16

11

13

12

9

10

6

7

2

8

5

14

3

1

BAL

GND

1.8kΩ

A4

A2

A1

A3

COMP 2

COMP 3

COMP 1 V

G

V+

V–

+IN

–IN

COMPENSATION

NETWORK

Im

SPLITTER

GAIN CORE

SSM2018

200Ω

200Ω

Q3 Q4

Q1

1–G

1–G

GG

Q2

MODE

Im – (

)

Is

2

Im + (

)

Is

2