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| UTC571 COMPANDER DESCRIPTION LI NEAR I NTEGRATED CI RCUI T The UTC571 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 fullwave rectifier to detect the average value of the signal, a linerarized temperature-compensated variable gain cell, and an operational amplifier. The UTC571 is well suited for use in cellular radio and radio communications systems, modems, telephone, and satellite broadcast/receive audio systems. FEATURES * Complete compressor and expandor in one IChip *Temperature compensated *Greater than 110dB dynamic range * Operates down to 6VDC * System levels adjustable with external components * Distortion may be trimmed out * Dynamic noise reduction systems * Voltage-controlled amplifier DIP-16 APPLICATIONS *Cellular radio *High level limiter *Low level expandor--noise gate *Dynamic filters *CD Player PIN CONFIGURATION RECT CAP 1 RECT IN 1 AG CELL IN 1 GND INV.IN 1 RES.R3 1 OUTPUT 1 THD TRIM 1 1 2 3 16 15 14 RECT CAP 2 RECT IN 2 AG CELL IN 2 VCC INV.IN 2 RES.R 3 2 OUTPUT 2 THD TRIM 2 4 5 6 7 8 UTC571 13 12 11 10 9 YOUW ANG ELECTRONI CO. CS LTD 1 UTC571 BLOCK DIAGRAM THD TRIM LI NEAR I NTEGRATED CI RCUI T R3 INVERTER IN R3 20K G IN R2 20K VARIABLE GAIN R4 20K VREF 1.8V OUTPUT R1 10K RECT IN RECTIFIER RECT CAP ABSOLUTE MAXIMUM RATINGS(Ta=25C ) Characteristic Maximum Operating Voltage Operating Temperature Power dissipation Symbol VCC TA PD Value 18 0~70 400 Unit V C mW AC ELECTRICAL CHARACTERISTICS(Ta=25C, Vcc=+6V,unless otherwise stated) Characteristic Supply Voltage Supply Current Output Current capability Output Slew Rate Gsin Cell Distortion Resister Tolerance Internal Reference Voltage Output DC Shift Expandor Output Noise Unity Gain Level Gain Change Reference Drift Resistor Drift Tracking Error(measured relative to value at unity gain) Equals [VOVO(unity gain)]dB-V2dBm Channel Separation Note: 1. Input to V 1 and V 2 grounded. 2. Measured at 0dBm, 1kHz. 3. Expandor AC input change from no signal to 0dBm. 4. Relative to value at T A = 2 C. 5. Electrical characteristics for the UTC571 only are specified over -40 to +8 C temperature range. 6. 0dBm = 775mV RMS . Symbol VCC ICC IOUT SR Test Condition No signal Min 6 Typ. 3.2 Max 18 4.8 Unit v mA mA V/s 20 Untrimmed Trimmed 1.7 Untrimmed No signal, 15Hz-20kHz 1kHz -1.5 .5 0.5 2.0 % 0.1 5 15 % 1.85 2.0 V 30 150 mV V 20 60 0 +1.5 dBm 0.1 dB +2,-25 +20,-50 mV +8,-0 % +0.2 +0.2 60 -1,+1.5 dB dB Rectifier input, V2=+6dBm,V1=0dB V2=-30dBm, V1=0dB YOUW ANG ELECTRONI CO. CS LTD 2 UTC571 FUNCTION DESCRIPTION CIRCUIT DESCRIPTION LI NEAR I NTEGRATED CI RCUI T The UTC571 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 V REF . 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 0.1A. G | VIN - VREF | avg R1 or G | VIN | avg R1 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. For step changes in amplitude, the change in gain with time is shown by this equation. G(t)=(Ginitial-Gfinal)e-t/+Gfinal; =10k x CRECT The variable gain cell is a current-in, current-out device with the ratio IOUT /IIN controlled by the rectifier. IIN is the current which flows from the DG 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. IIN = VIN - VREF R2 = VIN R2 A compensation scheme built into the DG 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. The operational amplifier (which is internally compensated) has the non-inverting input tied to V REF , and the inverting input connected to the DG cell output as well as brought out externally. A resistor, R 3 , is brought out from the summing node and allows compressor or expander gain to be determined only by internal components. The output stage is capable of 20mA output current. This allows a +13dBm (3.5V RMS ) output into a 300W load which, with a series resistor and proper transformer, can result in +13dBm with a 600 output impedance. A bandgap reference provides the reference voltage for all summing nodes, a regulated supply voltage for the YOUW ANG ELECTRONI CO. CS LTD 3 UTC571 stable biasing over a wide temperature range. LI NEAR I NTEGRATED CI RCUI T rectifier and DG cell, and a bias current for the DG cell. The low tempco of this type of reference provides very The typical performance characteristics illustration shows the basic input-output transfer curve for basic compressor or expander circuits. COMPRESSOR INPUT LEVEL OR EXPANDOR OUTPUT LEVEL(dBm) +20 +10 0 -10 -20 -30 -40 -50 -60 -70 -80 -40 -30 -20 -10 0 +10 COMPRESSOR OUTPUT LEVEL OR EXPANDOR INPUT LEVEL(dBm) Basic Input-Output Transfer Curve TYPICAL TEST CIRCUIT Vcc=15V 0.1F 10F 6.11 20k 2.2F 20k V1 3.14 G 7.10 V0 VREF 2.2F 10k 30k V2 2.15 4 1.16 2.2F 5.12 8.2k 8.9 200pF Typical Test Circuit YOUW ANG ELECTRONI CO. CS LTD 4 UTC571 INTRODUCTION LI NEAR I NTEGRATED CI RCUI T Much interest has been expressed in high performance electronic gain control circuits. For non-critical applications, an integrated circuit operational transconductance amplifier can be used, but when high-performance is required, one has to resort to complex discrete circuitry with many expensive, well-matched components. This paper describes an inexpensive integrated circuit, the UTC571 Compandor, which offers a pair of high performance gain control circuits featuring low distortion (<0.1%), high signal-to-noise ratio (90dB), and wide dynamic range (110dB). CIRCUIT BACKGROUND The UTC571 Compandor was originally designed to satisfy the requirements of the telephone system. When several telephone channels are multiplexed onto a common line, the resulting signal-to-noise ratio is poor and companding is used to allow a wider dynamic range to be passed through the channel. Figure 1 graphically shows what a compandor can do for the signal-to-noise ratio of a restricted dynamic range channel. The input level range of +20 to -80dB is shown undergoing a 2-to-1 compression where a 2dB input level change is compressed into a 1dB output level change by the compressor. The original 100dB of dynamic range is thus compressed to a 50dB range for transmission through a restricted dynamic range channel. A complementary expansion on the receiving end restores the original signal levels and reduces the channel noise by as much as 45dB. COMPRESSION INPUT LEVEL +20 0dB EXPANSION OUTPUT LEVEL +20 0d B -40 -40 NOISE -80 -80 Figure 1. Restricted Dynamic Range Channel The significant circuits in a compressor or expander are the rectifier and the gain control element. The phone system requires a simple full-wave averaging rectifier with good accuracy, since the rectifier accuracy determines the (input) output level tracking accuracy. The gain cell determines the distortion and noise characteristics, and the phone system specifications here are very loose. These specs could have been met with a simple operational transconductance multiplier, or OTA, but the gain of an OTA is proportional to temperature and this is very undesirable. Therefore, a linearized Tran conductance multiplier was designed which is insensitive to temperature and offers low noise and low distortion performance. These features make the circuit useful in audio and data systems as well as in telecommunications systems. YOUW ANG ELECTRONI CO. CS LTD 5 UTC571 THD TRIM 8.9 LI NEAR I NTEGRATED CI RCUI T R3 6.11 R3 20K R2 20K BASIC CIRCUIT HOOK-UP AND OPERATION INVIN 5.12 GIN 3.14 G R4 30K VREE 1.8V 7.10 OUTPUT IG R1 10K RECTIN 2.15 VCC PIN 13 GND PIN 4 1.16 CRECT Figure 2. Chip Block Diagram Figure 2 shows the block diagram of one half of the chip, (there are two identical channels on the IC). The fullwave averaging rectifier provides a gain control current, IG , for the variable gain (G) cell. The output of the DG cell is a current which is fed to the summing node of the operational amplifier. Resistors are provided to establish circuit gain and set the output DC bias. The circuit is intended for use in single power supply systems, so the internal summing nodes must be biased at some voltage above ground. An internal band gap voltage reference provides a very stable, low noise 1.8V reference denoted VREF . The non-inverting input of the op amp is tied to VREF , and the summing nodes of the rectifier and G cell (located at the right of R1 and R2) have the same potential. The THD trim pin is also the VREF potential. R3 CIN1 R2 G VOUT VIN CIN2 R1 R4 VREF NOTE: GAIN= 2R3VIN (avg) R1R2IB IB=140A CRECT Figure 3. Basic Expander Figure 3 shows how the circuit is hooked up to realize an expandor. The input signal, VIN is applied to the inputs of both the rectifier and the G cell. When the input signal drops by 6dB, the gain control current will drop by a factor of 2, and so the gain will drop 6dB. The output level at VOUT will thus drop 12dB, giving us the desired 2 to 1 expansion. YOUW ANG ELECTRONI CO. CS LTD 6 UTC571 LI NEAR I NTEGRATED CI RCUI T G R1 20K R1 10K CREC T CF RD C RDC CIN VIN R3 20K VOUT R4 30K VREF Figure 4. Basic Components Figure 4 shows the hook-up for a compressor. This is essentially an expandor placed in the feedback loop of the op amp . the G cell is setup to provide AC feedback only, so a separate DC feedback loop is provide by the two RDC and CDC. The values of RDC will determine the DC bias at the output of the amp. The output will bias to: VOUT DC = 1 + RDC1 + RDC2 R4 )1.8V 30K VREF = (1 + RDCTOT The output of the expander will bias up to: VOUT DC = 1 + R3 R4 )1.8V = 3.0V VREF VREF = (1 + 20K 30K The output will bias to 3.0V when the internal resistor are used. External resistor may be placed in series with r3, (which will affect the gain ), or in parallel with r4 to raise the DC bias to any desired value. YOUW ANG ELECTRONI CO. CS LTD 7 UTC571 CIRCUIT DETAILS--RECTIFIER LI NEAR I NTEGRATED CI RCUI T V+ I=VIN/R1 R1 VIN RS 10K IG CR Figure 5. Rectifier Concept Figure 5 shows the concept behind the full-wave averaging rectifier. The input current to the summing node of the op amp, V IN R 1 , is supplied by the output of the op amp. If we can mirror the op amp output current into a unipolar current, we will have an ideal rectifier. The output current is averaged by R5 , CR, which set the averaging time constant, and then mirrored with a gain of 2 to become IG , the gain control current. Q3 Q4 Q5 Q7 R1 10K Q1 Q2 D1 Q6 RS 10K VIN Q9 Q8 I1 I2 CR Figure 6. Simplified Rectifier Schematic Figure 6 shows the rectifier circuit in more detail. The op amp is a one-stage op amp, biased so that only one output device is on at a time. The non-inverting input, (the base of Q 1 ), which is shown grounded, is actually tied to the internal 1.8V V REF . The inverting input is tied to the op amp output, (the emitters of Q 5 and Q 6 ), and the input summing resistor R 1 . The single diode between the bases of Q 5 and Q 6 assures that only one device is on at a time. To detect the output current of the op amp, we simply use the collector currents of the output devices Q 5 and Q 6 . Q 6 will conduct when the input swings positive and Q 5 conducts when the input swings negative. The collector currents will be in error by the a of Q 5 or Q 6 on negative or positive signal swings, respectively. ICs such as this have typical NPN bs of 200 and PNP bs of 40. The a's of 0.995 and 0.975 will produce errors of 0.5% on negative swings and 2.5% on positive swings. The 1.5% average of these errors yields a mere 0.13dB gain error. YOUW ANG ELECTRONI CO. CS LTD 8 UTC571 LI NEAR I NTEGRATED CI RCUI T At very low input signal levels the bias current of Q 2 , (typically 50nA), will become significant as it must be supplied by Q 5 . Another low level error can be caused by DC coupling into the rectifier. If an offset voltage exists between the V IN input pin and the base of Q 2 , an error current of V OS /R 1 will be generated. A mere 1mV of offset will cause an input current of 100nA which will produce twice the error of the input bias current. For highest accuracy, the rectifier should be coupled into capacitively. At high input levels the of the PNP Q 6 will begin to suffer, and there will be an increasing error until the circuit saturates. Saturation can be avoided by limiting the current into the rectifier input to 250mA. If necessary, an external resistor may be placed in series with R 1 to limit the current to this value. Figure 7 shows the rectifier accuracy vs input level at a frequency of 1kHz. ERROR GAIN dBm +1 0 -1 -40 -20 0 RECTIFIER INPUT dBm Figure 7. Rectifier Accuracy At very high frequencies, the response of the rectifier will fall off. The roll-off will be more pronounced at lower input levels due to the increasing amount of gain required to switch between Q 5 or Q 6 conducting. The rectifier frequency response for input levels of 0dBm, -20dBm, and -40dBm is shown in Figure 8. The response at all three levels is flat to well above the audio range. GAIN ERROR (dB) INPUT=0dBm 0 -40dBm -20dBm 3 10K FREQUENCY(Hz) 1MEG Figure 8. Simplified G Cell Schematic YOUW ANG ELECTRONI CO. CS LTD 9 UTC571 VARIABLE GAIN CELL LI NEAR I NTEGRATED CI RCUI T V+ I1 140A VIN R2 20K Q1 Q2 I2(=2I1) 280A Q3 Q4 IG V- IIN Note: LOUT = LG L1 IIN = IGVIN I2R2 Figure 9. Simplified G Cell Schematic Figure 9 is a diagram of the variable gain cell. This is a linearized two-quadrant transconductance multiplier. Q 1 , Q 2 and the op amp provide a predistorted drive signal for the gain control pair, Q 3 and Q 4 . The gain is controlled by I G and a current mirror provides the output current. The op amp maintains the base and collector of Q 1 at ground potential (V REF ) by controlling the base of Q 2 . The input current I IN (=V IN /R 2 ) is thus forced to flow through Q 1 along with the current I 1 , so I I2 - (I1 +IIN ) = I1 - IIN = IC2. C1 =I 1 +I IN . Since I 2 has been set at twice the value of I 1 , the current through Q 2 is: The op amp has thus forced a linear current swing between Q 1 and Q 2 by providing the proper drive to the base of Q 2 . This drive signal will be linear for small signals, but very non-linear for large signals, since it is compensating for the non-linearity of the differential pair, Q 1 and Q 2 , under large signal conditions. The key to the circuit is that same predistorted drive signal is applied to the gain control pair, Q3 and Q4. When two differential pairs of transistors have the same signal applied, their collector current ratios will be identical regardless of the magnitude of the currents. This gives us : IC1 IC2 IC4 IC3 I1 + IIN I1 - IIN = = plus the relationships IG = IC3 + IC4 and IOUT = IC4 - IC3 will yield the multiplier transfer function, IG I1 VINIG R2I1 IOUT = IIN = This equation is liner and temperature-insenstive, but it assumes ideal transistor. YOUW ANG ELECTRONI CO. CS LTD 10 UTC571 4 LI NEAR I NTEGRATED CI RCUI T VOS=5mV 3 4mV % THD 2 3mV 2mV 1 1mV 0.34 -6 0 +6 INPUT LEVEL(dBm) Figure 10. G Cell Distortion vs Offset Voltage If the transistors are not perfectly matched, a parabolic, non-linearity is generated, which results in second harmonic distortion. Figure 10 gives an indication of the magnitude of the distortion caused by a given input level and offset voltage. The distortion is linearly proportional to the magnitude of the offset and the input level. Saturation of the gain cell occurs at a +8dBm level. At a nominal operating level of 0dBm, a 1mV offset will yield 0.34% of second harmonic distortion. Most circuits are somewhat better than this, which means our overall offsets are typically about mV. The distortion is not affected by the magnitude of the gain control current, and it does not increase as the gain is changed. This second harmonic distortion could be eliminated by making perfect transistors, but since that would be difficult, we have had to resort to other methods. A trim pin has been provided to allow trimming of the internal offsets to zero, which effectively eliminated second harmonic distortion. Figure 11 shows the simple trim network required. VCC R 3.6V 6.2K TO THD TRIM 20K 200PF Figure 11. THD Trim Network YOUW ANG ELECTRONI CO. CS LTD 11 UTC571 +20 0 -20 OUTPUT (dBm) -40 -60 -80 LI NEAR I NTEGRATED CI RCUI T MAXIMUM SIGNAL LEVEL 90dB 110dB NOISE IN 20kHz BW -100 -40 -20 0 VCA GAIN (0dB) Figure 12. Dynamic Range of UTC571 Figure 12 shows the noise performance of the DG cell. The maximum output level before clipping occurs in the gain cell is plotted along with the output noise in a 20kHz bandwidth. Note that the noise drops as the gain is reduced for the first 20dB of gain reduction. At high gains, the signal to noise ratio is 90dB, and the total dynamic range from maximum signal to minimum noise is 110dB. Control signal feedthrough is generated in the gain cell by imperfect device matching and mismatches in the current sources, I 1 and I 2 . When no input signal is present, changing I G will cause a small output signal. The distortion trim is effective in nulling out any control signal feedthrough, but in general, the null for minimum feedthrough will be different than the null in distortion. The control signal feedthrough can be trimmed independently of distortion by tying a current source to the DG input pin. This effectively trims I . Figure 17 shows such a trim network. Vcc R-SELECT FOR 3.6V 470k 100k TO PIN 3 OR 14 Figure 13. control Signal Feedthrough YOUW ANG ELECTRONI CO. CS LTD 12 UTC571 OPERATIONAL AMPLIFIER LI NEAR I NTEGRATED CI RCUI T The main op amp shown in the chip block diagram is equivalent to a 741 with a 1MHz bandwidth. Figure 18 shows the basic circuit. Split collectors are used in the input pair to reduce g M , so that a small compensation capacitor of just 10pF may be used. The output stage, although capable of output currents in excess of 20mA, is biased for a low quiescent current to conserve power. When driving heavy loads, this leads to a small amount of crossover distortion. I1 I2 Q6 D1 OUT Q7 -IN Q1 Q2 +IN D2 Q5 Q3 Q4 Figure 14. Operational Amplifier RESISTORS Inspection of the gain equations in Figures 3 and 4 will show that the basic compressor and expander circuit gains may be set entirely by resistor ratios and the internal voltage reference. Thus, any form of resistors that match well would suffice for these simple hook-ups, and absolute accuracy and temperature coefficient would be of no importance. However, as one starts to modify the gain equation with external resistors, the internal resistor accuracy and tempco become very significant. Figure 15 shows the effects of temperature on the diffused resistors which are normally used in integrated circuits, and the ion-implanted resistors which are used in this circuit. Over the critical 0C to +70C temperature range, there is a 10-to-1 improvement in drift from a 5% change for the diffused resistors, to a 0.5% change for the implemented resistors. The implanted resistors have another advantage in that they can be made the size of the diffused resistors due to the higher resistivity. This saves a significant amount of chip area. 1.15 NORMALIZED RESISTANCE 140/ 1.10 DIFFUSED RESISTOR 1k/ 1.05 LOW TC IMPLANTED RESISTOR 1% ERROR BAND 1.00 0.95 -40 0 40 80 120 TEMPERATURE Figure 15. Resistance vs temperature YOUW ANG ELECTRONI CO. CS LTD 13 UTC571 PACKAGE OUTLINE LI NEAR I NTEGRATED CI RCUI T UNIT: mm 16-DIP-P-300 GHJUHH YOUW ANG ELECTRONI CO. CS LTD 14 |
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