dbx2150vca_AN 电路图.pdf

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1、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. Howe

2、ver, 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 va

3、ried by the varying gain of the VCA. The input port must be connected to a source impedance of less than 20k 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) t

4、he 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,

5、 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 600 loads are to be driv

6、en. 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

7、 impedance (1), Pin 4 should be sourced from 50 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. 1 RadioFans.CN 收音机爱 好者资料库 APPLICATION NO

8、TE 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 resis

9、tor is usually 3.3k. 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 descri

10、bed in the (II) CIRCUIT IMPLEMENTATION section of this note. Several VCAs may be served by one circuit. 2 RadioFans.CN 收音机爱 好者资料库 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-curre

11、nt conversion and AC coupling of the signal input to the VCA. Additionally, the source impedance is 20k 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

12、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 Amps RMS. B.OUTPUT The output terminal (pin 8), provides a high impedance current outpu

13、t 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

14、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. 47) 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

15、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

16、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

17、 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 -2.55

18、V 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 esta

19、blishing itself. 3 IBIAS 0.5mA 1.4 RadioFans.CN 收音机爱 好者资料库 APPLICATION NOTE 103 - THE dbx 2150 VOLTAGE CONTROLLED AMPLIFIER The anti-latch circuit may be shared between many VCA ICs. The circuit of Fig. 2 shows the necessary changes to the Q1 base bias network required to compensate for the added vo

20、ltage drop of the diodes D1 - Dn. D. GAIN CONTROL Fig. 1 shows the basic configuration with positive voltage causing negative gain. The point labeled EC accepts control voltage from a low impedance source (5 k) pins 3,4 to be sourced from 1 pin 2 to be sourced from 50 Tracking Accuracy1%, -80dB to +40dB gain Noise*-86dBV (CCIR, +15dB programmed gain) Frequency Response*20Hz to 20kHz (.5dB, -40dB to +40dB programmed gain) *Measured with RIN = 20k, ROUT = 20k, CIN = 1uF, COUT = 47pF 8