HTCSEMI HT571ARWZ

The HT571 is a versatile low cost dual gain control circuit in which either channel may be used as a dynamic range compressor or expandor. Each channel has a full−wave rectifier to detect the average value of the signal, a linerarized temperature−compensated variable gain cell, and an operational amplifier. The HT571 is well suited for use in cellular radio and radio communications systems, modems, telephone, and satellite broadcast/receive audio systems. The HT571 compandor building blocks, as shown in the block diagram, are a full−wave rectifier, a variable gain cell, an operational amplifier and a bias system. The arrangement of these blocks in the IC result in a circuit which can perform well with few external components, yet can be adapted to many diverse applications. The full−wave rectifier rectifies the input current which flows from the rectifier input, to an internal summing node which is biased at VREF. The rectified current is averaged on an external filter capacitor tied to the CRECT terminal, and the average value of the input current controls the gain of the variable gain cell. The gain will thus be proportional to the average value of the input signal for capacitively−coupled voltage inputs as shown in the following equation. Note that for capacitively−coupled inputs there is no offset voltage capable of producing a gain error. The only error will come from the bias current of the rectifier (supplied internally) which is less than 0.1 μA. The output stage is capable of ± 20 mA output current. This allows a +13 dBm (3.5 VRMS) output into a 300 Ω load which, with a series resistor and proper transformer, can result in +13 dBm with a 600 Ω output impedance. A bandgap reference provides the reference voltage for all summing nodes, a regulated supply voltage for the rectifier and ΔG cell, and a bias current for the ΔG cell. The low tempco of this type of reference provides very stable biasing over a wide temperature range. The speed with which gain changes to follow changes in input signal levels is determined by the rectifier filter capacitor. A small capacitor will yield rapid response but will not fully filter low frequency signals. Any ripple on the gain control signal will modulate the signal passing through the variable gain cell. In an expander or compressor application, this would lead to third harmonic distortion, so there is a trade−off to be made between fast attack and decay times and distortion. The variable gain cell is a current−in, current−out device with the ratio IoUT/IN controlled by the rectifier. IN is the current which flows from the ΔG input to an internal summing node biased at VREF. The following equation applies for capacitively−coupled inputs. The output current, IOUT, is fed to the summing node of the op amp. A compensation scheme built into the G cell compensates for temperature and cancels out odd harmonic distortion. The only distortion which remains is even harmonics, and they exist only because of internal offset voltages. The THD trim terminal provides a means for nulling the internal offsets for low distortion operation.

• Complete Compressor and Expandor in one IChip
• Temperature Compensated
• Greater than 110 dB Dynamic Range
• Operates Down to 6.0 VDC
• System Levels Adjustable with External Components
• Distortion may be Trimmed Out
• Dynamic Noise Reduction Systems
• Voltage Controlled Amplifier
• Pb−Free Packages are Available*

• Cellular Radio
• High Level Limiter
• Low Level Expandor − Noise Gate
• Dynamic Filters
• CD Player