March 10, 1964 J. DIJKSTRA 3,124,760 WITH VARIABLE PEAK clipping AND Age Filed Dec. 2, 1960

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NVENTOR JAN DJ KSTRA Y A& 2 tel-4 .AGENT 26 3,124,760 United States Patent Office Patented Mar. 10, 1964

Ads Sponds to the correct value required to obtain a sym 3,224,760 metrical clipping of the signals. In order to achieve this, TRANSJSTGER AMPL: FiR WETH WARABELE the resistors 9 to E in series with the load 7 are switched PEAK CPPNG AND AGC Jan Dijkstra, Eidovea, Netherlands, assignor to North over simultaneously with the switching-over of each of Ailerican Philips Coapay, Era.c., New York, N.Y., a the resistors 2 to 15 in parallel with the load 7. corporatica of Delaware A calculation proves that correct symmetrical clipping Eiled Dec. 12, 1950, Ser. No. 75,367 is obtained with a purely ohmic load, if the condition: Cains priority, application (Gerisianayan. 27, 1960 R20=0'R9-11--2a:'R, 12-15 is fulfilled, in which R20 desig 7 Cairns. (C. 330-29) nates the value of the resistor 29, R11 the value of the O Series resistors 9, 10, SE, respectively concerned, R., 12-15 This invention relates to a transistor amplifying cir the resistance value of the parallel combination of the cuit arrangement, particularly to a low-power amplifier, load 7 and of the parallel resistors 2, 13, 14, 15 respec for example, for use in hearing-aid apparatus. In such ively concerned and o' the collector-base current ampli an amplifier usually the peaks of the oscillations are fication factor of the transistor 6. It is achieved at the clipped so that the volume of the sound oscillations pro 5 same time that the base bias current is adjusted to a lower duced cannot exceed a given level. It is also known to value if the clipping level has been adjusted to a lower provide, moreover, automatic gain control in accordance value. with the sound volume. The last-mentioned control A similar effect is obtained, when the load 7 is con brings about at least partial suppression of the harmonics nected in parallel with the resistors 9-1 and the upper effected by the clipping, since the sound oscillations are 20 terminal of the resistor 259 is connected to the lower ter limited to a smalier range owing to this gain control. niinal instead of to the upper terminal of the load 7. In The invention is characterized in that the peaks are the case of a purely ohmic load 7 the condition clipped in known manner by a substantially symmetrical over-excitation of the last transistor of the amplifier and R20=(x'(RI, 9-11--R12-15) in that variable resistors are connected in series as well as 25 parallel with the load, for example, the telephone or the is to be fulfilled in order to obtain a symmetrical clipping in a manner such that the amplitude of the of the peaks. However, the results of the arrangement oscillations fed to a rectifier for producing the automatic shown in the figure are preferred owing to the lower cur gain control-voltage can exceed the clipping level to a rent consumption and to the smaller negative signal feed material extent, when the resistors are adjusted to pro 30 back. vide a lower value of this level. Use is furthermore made of the fact that, if the load Owing to the measure according to the invention it is 7 has an important reactive component, the direct voltage ensured that with the adjustment to a lower clipping level at the collector of the transistor 6 becomes lower (less the sensitivity of the automatic gain control remains sub negative) with an adjustment to a lower symmetrical stantially constant. clipping level, so that a lower threshcid voltage is ren The invention will now be described more fully with dered operative via the resistors 2A, 22 in the circuit of reference to the embodiment shown in the drawing. the rectifier 7. Consequently, the automatic gain control The hearing-aid apparatus comprises a microphone 1, starts at a lower signal level. a first transistor amplifier 2 with a tone-control 3, fur The aforesaid conditions are fulfilled approximately ther transistor 4, 5 and 6 and a load 7, for 40 within a given range by means of a single potentiometer, example, a telephone, a headphone, the earphone of of which the partial resistances between the variable tap hearing-aid glasses or the like. ping and the terminals of the resistor body take over The total resistance in the collector circuit of the last the function of the resistors 9-11 and 2-15 respectively. transistor 5 is proportioned so that excessively high signal An improved approximation may be obtained by a suit peaks are clipped symmetrically by a correct adjustment 45 able, non-linear variation of the resistance value with of the base bias current by means of a resistor 29. In the place of the tapping and/or by switching in suitable order to vary the peak clipping level provision is made bias resistors. of a switch 8 by means of which, in accordance with the In a practical embodiment the following circuit ele ments were used: invention, differently proportioned resistors 9, 10, E and 50 A 2, 3, 14, E5 can be switched in series and in parallel : respectively with the load 7. A rectifier 17 to produce an 2 ------0C58 automatic gain control of the first transistor 2 is con * ------0C57 nected via a blocking capacitor 18 to the collector of the 5 ------OC59 transistor 6. By means of resistors 21, 22 a threshold 6 ------0C60 voltage is introduced for the rectifier 7. 55 The arrangement operates as follows. The alternating Rectifier 17 ------0A47 voltage at the collector of the transistor 6 is capable of Resistors: increasing, in the case of an ohmic load, to about half 9 ------ohms-- 270 the supply voltage VB before the peak clipping becomes 60 ------do---- 680 operative. The clipping becomes operative at a value 11 ------do---- 180 which is the higher, the greater is the reactive compo 2 ------do---- 120 nent of the load impedance 7. The resistors 12 to 15 3 ------do---- 120 effect that the current then passing through the load 7 * ------do---- 270 amounts to only a fraction of the collector current of 65 5 ------do---- 1000 the transistor 6. Consequently, the peak clipping level 20 ------do---- 27-39K of the current passing through the load 7 can be varied ------do---- 2.2K by Switching over the parallel resistors 2 to 15. This 22 ------do---- 15K variation is accompanied, however, by a variation of the 23 ------do---- 1.8K total resistance measured across the terminals of the 70 28 ------do---- 2.2K load 7. An adjusting current supplied via the resistor 25 ------do---- 120K 29 to the base of the transistor 6 then no longer corre 26 ------do---- 22K 3,124,760 3 4. 5. A transistor peak clipping amplifier comprising a Capacitor 13 ------pf - 0.8 transistor having base, emitter and collector electrodes, a Telephone 7: source of input signals connected to said base electrode, Ohms at 1000 c./s. ------300 a source of operating potential having first and Second Ohms at 0 c./s. ------100 5 terminals, means connecting Said first terminal to said Supply voltage ------v-- 1.3 emitter electrode, first and second variable resistance means connected serially between said collector electrode Direct collector voltage ------V-- 1 and second terminal, reactive load means connected in What is claimed is: parallel with one of said resistance means, bias resistor 1. A transistor amplifier comprising a transistor hav LO means connected between said base electrode and the ing base, emitter and collector electrodes, collector cir junction of said first and second resistance means to cuit means connected to said collector electrode, a Source provide base bias for said transistor for Symmetrical clip of signals connected to said base electrode, means Con ping, whereby said base bias varies with variation of necting said emitter electrode to a point of constant po said first and second variable resistor means to main tential, said collector circuit comprising first variable re tain symmetrical clipping, rectifier means, and means con sistor means in series with the parallel combination of necting said rectifier means to said collector electrode for load means and second variable resistor means, rectifier producing an automatic gain control voltage. means, and means connecting said rectifier means to Said 6. The amplifier of claim 5, in which one end of Said collector electrode for producing an automatic gain con second resistance means is connected to said collector trol voltage. 20 electrode, and said load means is connected in parallel 2. The amplifier of claim 1, comprising bias resistor with said second resistance means. means connected between said base electrode and one 7. A transistor amplifier comprising a transistor hav end of said load means. ing base, emitter and collector electrodes, a Source of 3. The amplifier of claim 2, wherein the resistance of input signals connected to said base electrode, a Source said bias resistor is substantially equal to: 25 of operating potential having first and second terminals, means connecting said first terminal to said emitter elec trode, first and second variable resistor means serially where o' is the collector-base current amplification factor connected in that order between said second terminal and of the transistor, R1 is the resistance of Said first vari said collector electrode, reactive load means connected in able resistor means, and R2 is the resistance of the paral 30 parallel with said second resistor means, base bias re lel combination of said second variable resistor means sistor means connected between said base electrode and and said load means, whereby said signals are substan the junction of said first and second resistor means for tially symmetrically clipped by said transistor. providing a base bias for said transistor that varies with 4. A transistor amplifier comprising a transistor hav the values of said first and second resistor means, and ing base, emitter and collector electrodes, collector cir automatic control voltage producing means having a cuit means having one end connected to Said collector threshold that varies with the values of said first and electrode, a source of operating potential, means con Second resistors, said control voltage producing means necting said source of operating potential between the comprising third and fourth resistor means connected other end of said collector circuit means and said emit serially between said collector electrode and first terminal ter electrode, a source of signals connected to said base 40 in that order, capacitor means connected in parallel with electrode, said signals having sufficient amplitude to over said third resistor means, an automatic gain control volt drive said transistor whereby said signals are psak clipped age output terminal, and rectifier means having one elec by said transistor, said collector circuit means compris trode connected to said output terminal and the other ing serially-connected simultaneously variable first and electrode connected to the junction of said third and second resistance means and load means connected in fourth resistor means. parallel with said second resistance means, bias resistor means connected between said base electrode and the References Cited in the file of this patent junction of said first and second resistance means, recti fier means, and means connecting said rectifier means to UNITED STATES PATENTS said collector electrode to produce an automatic gain 50 2,898,411 Chow ------Aug 4, 1959 control voltage.