Studio Sdbx2150vca-AN User Manual

APPLICATION NOTE 103 - THE dbx 2150 VOLTAGE CONTROLLED AMPLIFIER

I. DESCRIPTION

A. GENERAL

The dbx 2150 integrated circuit VCA is a current in, current out device with a voltage sensitive control port.

Both positive and negative control ports are available. The recommended supply voltage is ±12V. However, non-

symmetrical supplies and other voltages may be accommodated.

B. INPUT

The input port of the VCA (pin 1) is a virtual ground which accepts current determined via an external resistor.

The input port must be AC coupled in order to insure that DC offsets of the preceding stages will not be varied by

the varying gain of the VCA. The input port must be connected to a source impedance of less than 20kΩ at high

frequencies (above 100 kHz) in order to insure stability. Maximum input current varies with the gain setting and

the bias current programmed. At the recommended biasing levels (3mA) the maximum input current should not

exceed 1.5 mA instantaneous.

C. OUTPUT

The output of the VCA (pin 8) must be connected to a virtual ground for proper operation. An op-amp is used

to convert the VCA output current into a voltage. The feedback network of this op-amp must contain a small

capacitor, connected directly between the VCA output and the op-amp output for stability.

Any op-amp of convenience may be used as the current-to-voltage converter; a bi-fet type is suggested where

only high impedance loads are to be driven, a 5534 type (unity gain compensation) where 600Ω loads are to be

driven.

D. CONTROL

The control port takes the form of three pins (2,3,4) which allow positive gain with positive voltage (pins 2 and

4), negative gain with positive voltage (pin 3), and symmetry adjustment (applying a voltage, differential between

pins 2 and 4). Pins 2 and 3 should be sourced from a low impedance (<1Ω), Pin 4 should be sourced from 50Ω

impedance. The control ports present a high impedance load to the driving circuit.

The control constant is +6mV/dB on pins 2 and 4, and -6 mV/dB on pin 3. This constant has a temperature

coefficient of 0.33%/oC.

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APPLICATION NOTE 103 - THE dbx 2150 VOLTAGE CONTROLLED AMPLIFIER

E. CURRENT PROGRAMMING

The bias current in the VCA IC is set by a resistor from pin 5 to the negative supply. The voltage at pin 5 is

normally four diode drops below ground. Nominal bias is 3mA, so with a -12V negative supply, the bias setting

resistor is usually 3.3kΩ.

The VCA is capable of going into a latch-up mode if the negative supply voltage is removed when positive

supply is present. To prevent this latch up,a circuit is attached to pin 5 which prevents the voltage at pin 5 from

going above its nominal -4VBE level. The circuit is described in the (II) CIRCUIT IMPLEMENTATION section of

this note. Several VCAs may be served by one circuit.

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APPLICATION NOTE 103 - THE dbx 2150 VOLTAGE CONTROLLED AMPLIFIER

II. CIRCUIT IMPLEMENTATION

Fig. 1 shows the basic VCA IC configuration. R1 and C1 provide voltage-to-current conversion and AC

coupling of the signal input to the VCA. Additionally, the source impedance is 20kΩ at high frequencies, as

required for stability. The input node, pin 1, is a virtual ground, accepting an input current only.

The maximum input current is determined by the input impedance and the input voltages expected. Since

output clipping may take place for small input currents with large gains, care must be taken when scaling the

input network that total input plus output currents are no greater than

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Amps RMS.

B. OUTPUT

The output terminal (pin 8), provides a high impedance current output equal to the desired gain times the

input current. To convert this signal to a voltage, pin 8 is connected to the summing junction of an op-amp, as

shown in Fig. 1. The feedback resistor around this op-amp, R3, determines the constant of current-to-voltage

conversion taking place.

C3 is necessary to prevent oscillations of the output op-amp due to the capacitance between pin 8 of the VCA

IC and ground. C3 is normally 33pF, as shown. A small resistor (e.g. 47Ω) may be added between C3 and R3 as

drawn, and the output taken from the junction of R3 and C4. This connection provides isolation from capacitive

loads at the output of the op-amp.

C. SUPPLY CONNECTIONS

Pin 7 should be connected directly to the positive supply (nominally +12 V) as shown in Fig. 1. Pin 5, the

negative supply terminal for the IC, must be supplied with a current to program the internal bias network. The

nominal value of this current is 3mA, but if reduced signal handling capacity is acceptable this current may be

reduced proportionate to the allowable reduction in headroom.

Since the maximum voltage which may be applied across the IC is 30V, supply voltages of up to ±24V can be

accommodated by appropriate scaling of R5, R8, and the R6/R7 voltage divider.

The action of the anti-latch circuit is to draw current from pin 5 when the voltage at pin 5 rises high enough to

forward bias the Base-Emitter junction of Q1. Since the normal voltage at pin 5 is ~-2.2V, and the R6, R7 network

establishes a voltage of -2.55V at the base of Q1, normally Q1 and Q2 are turned off. If, however, the VCA begins

to latch, the pin 5 voltage rises as current is internally diverted into the output (pin 8). Q1 senses this voltage rise

and responds by turning on Q2, drawing enough current from pin 5 to prevent the latch from establishing itself.

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I

– 0.5mA

BIAS

1.4