SSM2164

REV. 0

–7–

0

–20

–100

10

100

1M

100K

10k

1k

–40

–60

–80

FREQUENCY – Hz

+PSRR

–PSRR

Figure 20. PSRR vs. Frequency

25

20

0

1k

10k

1M

100k

15

10

5

+ISY

–ISY

Figure 21. Supply Current vs. R

BIAS

–45

–20

100

–35

–25

–25

–30

–50

–40

75

50

25

0

TEMPERATURE –

°C

CLASS A AND

CLASS AB

Figure 22. Gain Constant vs. Temperature

APPLICATIONS INFORMATION

Circuit Description

The SSM2164 is a quad Voltage Controlled Amplifier (VCA)

with 120 dB of gain control range. Each VCA is a current-in,

current-out device with a separate –33 mV/dB voltage input

control port. The class of operation (either Class A or Class

AB) is set by a single external resistor allowing optimization of

the distortion versus noise tradeoff for a particular application.

The four independent VCAs in a single 16-pin package make

the SSM2164 ideal for applications where multiple volume

control elements are needed.

Figure 23. Simplified Schematic (One Channel)

The simplified schematic in Figure 23 shows the basic structure

of one of the four VCAs in the device. The gain core is com-

prised of the matched differential pairs Q1-Q4 and the current

mirrors of Q5, Q6 and Q7, Q8. The current input pin, I

connected to the collectors of Q1 and Q7, and the difference in

current between these two transistors is equivalent to I

For

example, if 100

µA is flowing into the input, Q1’s collector

current will be 100

µA higher than Q7’s collector current.

Varying the control voltage V

one side of each differential pair to the other, resulting in either

gain or attenuation. For example, a positive voltage on V

C

steers more current through Q1 and Q4 and decreases the

current in Q2 and Q3. The current output pin, I

nected to the collector of Q3 and the current mirror (Q6) from

Q2. With less current flowing through these two transistors, less

current is available at the output. Thus, a positive V

the input and a negative V

The VCA has

unity gain for a control voltage of 0.0 V where the signal current

is divided equally between the gain core differential pairs.

The MODE pin allows the setting of the quiescent current in

the gain core of the VCA to trade off the SSM2164’s THD and

noise performance to an optimal level for a particular applica-

tion. Higher current through the core results in lower distortion

Q6

Q5

Q8

Q7

Q2

Q1

Q4

Q3

4.5k

Ω

500

Ω

450

Ω

MODE

V–

V+