TRANSISTORIZED AUDIO AGC AMPLIFIER

Nicholas J. Hudak, General Electric Company

Syracuse, New York

Introduction

Modern broadcasters are calling for tran-sistorized amplifiers in their studio installa-tions. The use of transistors has resulted insmall, compact amplifier designs, with theadded features of lower power consumption,less heating, and greater reliability than withtube-type equipment. Many studio amplifiershave been transistorized. The exception hasbeen the popular automatic Gain Control amp-lifier. Ever since the introduction of theUnilevel in 1954, the studio compression amp-lifier has found many applications in radio andtelevision broadcast applications. It hasproven to be highly useful in intercom service,as well as its major function of providing auto-matic control of program channel levels.

Program Amplifier Operation

The transistorized audio AGC amplifierconsists of a program amplifier and two con-trol circuits; as shown in a simplified schema-tic diagram, figure 1. With the Compress andExpand switches turned off, the unit functionsas a conventional program amplifier of 75 dbgain.

The input impedance is that of an unloadedinput transformer. It will bridge a source im-pedance of 150 or 600 ohms. High input gainfor optimum signal to noise is realized byconnecting Ql and Q2 in a Darlington compound.Q3 and Q4 are conventional Class A amplifica-tion stages, which feed the signal to a compli-mentary-symmetry pair, Q5 and Q6. Q5 andQ7 will conduct power to the load on negativehalf cycles, while Q6 and Q8 conduct on posi-tive half cycles. The output stages are biasedClass A to +14 dbm output, and then operateClass AB up to +30 dbm output.

Diffused silicon mesa transistors are usedas power amplifiers. They exhibit low leakageand high frequency characteristics whic_1maketheir use ideal in program amplifiers.

A transistor series regulator, zenerdiode controlled, is included in the amplifierto make the operation independent of powersupply fluctuations. My thanks go to colleagueRichard Taylor, who developed the programcircuits.

The interstage gain control, Rl, is of theAC shunt type. Gain of the amplifier is de-creased by merely "shunting out" part of thesignal to ground. When the control is turnedto zero resistance the base of Q3 has zero im-pedance to ground. D. C. bias conditions arenot affected because of the isolation providedby the two coupling capacitors.

Though not shown in the diagram, thepoints across Rl are taken through plug con-tacts to external circuits in an audio console.An external control is then used as MasterGain Control for the console. The alternatemethod of using a variable ladder attenuatorfor master gain control, ahead of a constantgain amplifier, had one distinct disadvantage -the noise output of the amplifier remained con-stant. When signal level is decreased by re-ducing gain with an interstage gain control, thenoise output is also decreased.

Choice of An Attenuating Device

In previous tube designs, compressionamplifiers utilize a variable mu tube as thegain reduction device. A portion of the audiooutput is rectified by a diode, and the resultingDC control voltage is fed back to the grid of avariable mu tube. The gain of the input stageis reduced by an amount proportional to thechange of DC control voltage.

Similar techniques are not feasible intransistor circuitry. A variable mu transistorhas not been developed. Even a small changein base bias results in distorted signals. Thelow impedance properties and leakage difficul-ties of transistors mean that a differentapproach is required.

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A device is -required whose dynamic im-pedance will change with a changing DC voltage.The following are some of the components thatwere investigated, but rejected because ofsmall dynamic range, poor frequency response,slow attack time, or feedback instabilities:signal diodes, varactor diodes, unijunction andtetrode transistors, thermistors, and tunneldiodes.

A device that does meet all of the re-quirements is the photoconductive cell. Oneproperty of cadmium sulfide and cadmiumselenide is such that its resistance willchangewith varying amounts of light falling upon itssurface. The resistance can be changed from10 megohms to 40 ohms. Increasing the lightintensity causes the resistance to decrease.The photoconductive cell works on the princi-ple that light energy causes the electrons tobe raised to higher orbits in the molecularstructure. Under light conditions more elec-trons are made available for conduction be-cause they are loosely held in their outer or-bits. Under dark conditions, conductivitydecreases, i. e. resistance increases; becausethe electrons fall back to lower orbits and aremore tightly held in their molecular structure.

The photoconductive cell is a suitable de-vice that can be placed in parallel with Rl toattenuate the gain of the amplifier. Since itsresistance range is so great, it can readilyattenuate over a 30 db range. It causes nochange in frequency response because it hasonly one to five picofarads of capacitanceassociated with it. The DC control voltageis applied to a lamp, which is coupled to thephotoconductive cell. Thus the feedbackcoupling medium is a ray of light. Distortionremains very low, even under conditions ofexcessive compression, because DC biasingin the amplifier is not affected.

The design was finalized using a photo-conductive cell that was selected for its fastresponse time, proper resistance range,physical size, and spectral response. Factorswhich were considered in the selection of thelamp were: voltage and current requirements,light inertia, -physical size, spectral range,and life hours.

The Compression Circuit

If a greater than normal signal level isapplied to the input of a compression ampli-fier, the gain is automatically reduced toprovide a normal output level. As shown inFigure 1, a portion of the output signal iscoupled from a feedback winding on T2, throughswitch SI, to the compression control circuit.The DC amplifier stages, Q12 and Q13, arenormally in a cutoff condition. Positive halfcycles will be conducted by diode CR1. Thispositive DC control voltage will turn on Q12which in turn causes Q13 to conduct. Theamount of current flowing through Q13 and itsload - - the lamp in RIA, will be proportionalto the peak amplitude of the rectified signalvoltage. The amount of light from the lamp,therefore, is proportional to the amplitude ofthe signal voltage. Resistance of the photo-conductive cell in RIA changes according tothe amount of light furnished to it by the lamp.This change of resistance in turn varies thegain of the amplifier to restore normal outputlevel. Typical attack time is 20 milliseconds;recovery is one second.

The level at which gain reduction occurscan be set by means of the Compress Thres-hold Adjust control R2. Threshold can beselected so that the output can be held constantanywhere from +4 to +30 dbm output. Thecompression ratio is 10 to 1, i.e. with a tendb increase in input signal above threshold,the output increases only one db. A 30 dboverload results in only a 3 db rise in output,which is within the range of a standard VUmeter.

The voltage across the lamp is a directindication of the amount of gain reduction;therefore, this voltage has been wired out toa pin on the connector. A standard VU meter,with a suitable series resistor, can be calibra-ted to read gain reduction. The maximumvoltage present across the lamp is half itsrated voltage, thus giving it exceptional re-liability. The computed life time of the lampis in excess of 10 years.

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The Noise Reduction Circuit

When a compression amplifier is normallyoperated with some gain reduction, and thesignal is removed, the gain increases to maxi-mum. At this time the noise at the output alsoincreases. An expand-gate circuit is incorpDr-ated to reduce gain, thereby reducing noise,when the signal is removed.

The expand-gate circuit, Q10 and Qll, isshown in Figure 1. Under no signal conditions,with the Expand switch S2 in the ON position apositive voltage is applied through RIO and Rllto the base of Qll. The current through Qlland the lamp in RIB produces light to fall onthe surface of the photoconductive cell in RIB.The decreased resistance results in reducedgain of the amplifier. The amount of gain re-duction, and also noise reduction, is set bythe Noise Reduction Adjust control Ri1. Atypical setting is 10 db of noise reduction.

When a signal is applied to the amplifierit is presented to the expand circuit from thefeedback winding on T2. Diode CR2 conductson positive half cycles; and this DC positivevoltage on the base of Q10 is amplified causingthe collector to go more negative. The baseof Qll, which was biased on by the voltagefrom R10 and Rll, also goes negative and cutsoff the current to the lamp in RIB. Thus thegain increases to normal when a signal isapplied to the amplifier.

The threshold of expansion is chosen sothat there is a 20 db range of linear operationbefore the threshold of compression is reached.The dynamic linear range was chosen to be 20db because this is the readable limits on astandard VU meter. Both the Expander andthe Compression operation may be disabledby means of front panel switches.

Physical Considerations

The Transistorized AGC amplifier hasbeen constructed so that it is interchangeablewith the program amplifier in an audio console.It is a plug-in unit contained in a steel chassisapproximately 10 1/2" x 3 1/2" x 2 1/2". Itweighs four pounds. Very little power is

wasted in producing heat. The amplifier con-sumes only one-tenth the power of a similartube-type equipment. Because of its smallsize and low heating, six of the units may bemounted on a shelf of 3 1/2 inches of verticalrack space.

Operational Features

In systems applications, the transistorizedaudio AGC amplifier is a versatile tool. Whenthe unit is used in program amplifier service,output levels are precisely controlled. As anextreme example, a 30 db increase in inputwill not exceed the limits of the VU meter be-cause of the 10 to 1 compression ratio. Thisfeature can be a valuable asset to the stationat which the announcers do their own audiomixing. The automatic gain control relievesthem of riding levels closely, so that theymay concentrate on their announcing talents.

Tight audio control is made available atno sacrifice in performance. Frequency re-sponse is rated + 1 db, 30 to 15, 000 cyclesunder any condition of gain reduction up to 30db. Distortion rating is 1% or less 50 to 15, 000cycles and 1 1/2% or less 30 to 15, 000 cycles,under any condition of gain reduction up to 30db. Signal to noise is 65 db or better. Thereare no balancing controls to adjust. Amplifica-tion characteristics of transistors, unlike tubes,do not change with time. After the thresholdcontrols are set to the desired levels, yearsof maintenance-free operation should be ex-pected.

Another application is the use of the AGCamplifier as an automatic fader control. Amusic input source, such as from a turntableor tape, is set to operate at or slightly abovethreshold. The music is automatically fadedinto the background when an announcer talks,but is automatically restored to proper levelwhen the announcement has ceased. If theAGC amplifier is used ahead of a peak limitingamplifier, then it is possible to raise theaverage signal level sent to the transmitter byoperating with a fixed amount of compression.

The transistorized audio AGC amplifier isideally suited for unattended operation from

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remote locations. The unit may serve as aline amplifier at the studio, or it may be usedat the remote location. Since the input im-pedance is that of an unloaded input transfor-mer, a microphone may be connected directlyto the input. The unloaded-input gain is 81 dI;therefore the unit will replace a pre-amp andline amplifier to feed the telephone line direc-tly. A similar application in intercom servicewill improve and simplify the systems facili-ties. Intercom and paging systems are usuallysubjected to a variety of input levels, becauseof differences in voice intensities presentedfrom different distances to the microphone.The AGC amplifier will adjust for these input

level changes to provide a constant outputlevel. The transistorized audio AGC ampli-fier can be used to prevent excessive audiovariations in many particular applications.

References

(1) Dwight V. Jones, ?"Efficient High QualityProgram Amplifier Circuits Using theIndustrial Silicon Series - 2N2107 and2N2108. " Application Notes #90. 3 12/61.General Electric Company, SemiconductorProducts Department, Syracuse, NewYork

Fig. 1 - Simplified Electrical Diagram.

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Fig. 2 - AGG Amplifier, Front View.

Fig. 3 - AGC Amplifier, Side View, Cover Off.

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