CA1101537A - Facsimile receiver with improved fm demodulator - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
FM signals within a predetermined bandwidth representing light-dark variations in a document are transmitted over a communications network to a facsimile receiver. The FM signals are applied to differen-tiating and frequency doubling means so as to generate trigger signals with substantially equal amplitude and width for application to a single shot multivibrator. Reset means coupled directly to the input of the single shot multivibrator are responsive to the trigger pulses to reset the single shot multivibrator to the astable state when the frequency of the trigger pulses is sufficiently high that trigger pulses occur while the multivibrator is in a previously initiated astable state. An average DC voltage detector is connected to the output of the multivibrator such that the average DC voltage represents the ratio of the astable state duration to the stable state duration.

Description

53~

1 Uaclcground o[ ~I~e Inventlon

2 This invention relates to facsimile systems comprising a

3 transmitter, a receiver and a contmunications network therebetween.

4 More particularly, this invention relates to a system wherein a doc-ument is scanned in a facsimile transmitter to generate electrical 6 information-bearing signals representing the light-dark variations in 7 the document being scanned. These information-bearing signals are then 8 transmitted over the communications network to a facsimile receiver where 9 the information-bearing signals are converted to marks or images on a copy medium so as to form a copy which is a reasonable facsimile of the 11 original document.
12 In several commercially available facsimile systems, the in-13 formation-bearing signals which are transmitted over the communication network are FM (frequency modulated) signals. In general, the signals lj lie in a band between 1500 Hz. and 2400 Hz. which represents a portion 16 of the audio range which is transmitted over ordinary telephone lines.
17 Where this frequency range is utilized, the 1500 Hz. signal usually 18 represents a white level, the 2400 Hz. signal represents a black level 19 and signals in the frequency range between 1500 Hz. and 2400 Hz. rep-resent varying degrees o~ gray. In the alternative, the gray scale tnay 21 be eliminated or redttced such that frequencies in the lower portion of ~-22 the bandwidth toward 1700 Hz. and below may represent white and fre-23 quencies in the upper portion of the bandwidth toward 2200 Hz. and above ;~
24 may represent black.
One low cost, and extremely effective technique for demodula-26 tion of the FM signals, involves the use of one or more single shot 27 multivibrators. As shown in U.S. Patent Nos. 3,911,207 and 3,916,098, 28 a single shot multivibrator is triggered~ into its astàble state in re- -~
29 sponse to trigger pulses generated for each zero crossing of the FM
signals. When the received FM signals approach the lower portion of the 31 FM bandwidth (1500 Hz.), the trigger pulses corresponding to the zero 32 axis crossing of the FM signals triggers or sets the single shot .
.. ,, .: .: ~ .. , : ~

1 multivibr~tor to the astable state such tl)at the astable state duratlon 2 or duty cycle of the multivibrator represents a lesser portion of the 3 multivibrator cycle which includes the stable state duration. When the 4 frequency of the FM signals approaches the other end of the bandwidth (approximately or equal to 2400 Hz.), the astable state duration or 6 duty cycle of the multivibrator now represents a larger portion of the 7 multivibrator cycle. This variation in ratio in the duty cycle or 8 astable state duration to the stable state duration may be utilized to 9 control the writing at the facsimile recoiver by detecting the average DC value from the output of the multivibrator. When the average DC
11 value is relatively small, corresponding to the reception of 1500 Hz.
12 signals, the average DC voltage is utilized to control the writing 13 mechansim of the facsimile receiver so as to produce white on the copy 1~ medium. When the FM signal has a frequency of 2400 Hz. representing black, the average DC value which is relatively high is utilized to 16 control the writing mechanism in the receiver so as to produce black on 17 the copy medium.
18 Certain disturbances in the F~ carrier signal may produce 19 zero axis crossings at a rate corresponding to frequencies in excess of the 2400 Hz. which can produce errors on the copy medium. One particu-21 larly lik.ely source of such a disturbance is acoustic ringing where an 22 acoustic coupler is utilized between the communications network or tele-23 phone line and the receiver itself. In order to prevent the creation of 24 white holes in the copy during the reception of black signals, the af~re-said U.S. 3,916,098 includes means for resetting the single shot multi-26 vibrator in response to disturbances which occur while the multivibrator 27 is in the astable state thereby reinitiating the astable state so as 28 to prevent the formation of white holes in black copy.
~ As disclosed in the aforesaid U.S. Patent 3~916,098, the single shot multivibrator comprises a voltage comparator having one 31 input adapted to be coupled to a source of trigger si&nals and ano~her 32 input coupled to a first tap on a voltage divider connected between a S3~

1 first reference voltage and a second reference voltage. When the one 2 input to the comparator exceeds a predetermined level represented by 3 the voltage at the first tap of the voltage divider, a flip-flop coupled 4 to the output of the voltage comparator is reset thereby applying an S appropriate control voltage to the base of a transistor which is 6 rendered nonconductive so as to permit the timing capacitor of the single 7 shot multivibrator to ~harge during the astable state. When the ~harge ~
8 on the timing capacitor reaches a predetermined level as determined by ~:
9 a comparator which is connected to another tap on the voltage divider, the timing capacitor is discharged. By providing a reset means asso- -11 ciated with the single shot multivibrator having an input indirectly 12 connected to the input to the single shot multivibrator through a cap-l3 acitor, the transistor which controls the charging of the timing cap-14 acitor may be rendered conductive in response to any disturbance which appears to be or resembles a trigger pulse applied to the input o the 16 single shot multivibrator. This in turn reinitiates the charging of the 17 timing capacitor so as to eliminate the creation of any white holes in 18 black copy.
19 The trigger pulses which are applied to the input of the single shot multivibrator and the reset circuitry of the prior art 21 facsimile transceivers such as ~hat shown in the a~oresaid U.S. 3,916,098 22 are shown in the aforesaid U.S. 3,911,207 More particularly, the trig-23 ger pulses have been generated by RC circuitry for differentiating the 24 FM signals so as to create spikes of opposite polarity at axis cross~
ing times of the FM signals. The output from the RC circuitry is then 26 applied to a frequency doubling circuit comprising a single semicon-27 ductive path through the collector-emitter cirGuit of a transistor ~
28 connected in series with a yoltage dropping impedence between a first 29 re~erence voltage and a second reference voltage. The transistor is rendered conductive in response to spikes of opposite polarities by the 31 use of oppositely poled diodes with one connected to the base of the 32 transistor and the other com~ected to the emitter of the transistor.

.

as37 1 When a positive going spike is applied to the one diode connected to the 2 base of the transistor, the transistor is rendered conductive so as to 3 pull the collector of the transistor toward the second voltage reference 4 level. Similarly, a negative going spike applied to the other diode will lower the voltage at the emitter of the transistor so as to again b render the transistor conductive and thereby pull the collector and 7 input to the single shot multivibrator toward the second reference po-8 tential 9 Although the above-described frequency doubling circuit per-forms adequately, the trigger pulses which are generated at the output 11 and applied to the single shot multivibrator and the reset circuitry 12 are of somewhat different width and amplitude depending upon whether a 13 positive going spike or a negative going spike initiates the trigger 14 pulse. In addition, the edges of the pulses generated by the frequency doubling circuit do not have a particularly steep slope. These charac-16 teristics of the trigger pulses can lead to a failure in the reset cir-17 cuitry. It is for this reason that the capacitor has been provided for 18 the indirect connection between the input to the comparator and the 19 transistor of the reset circuitry.
U.S. Patent No. 3,467,772 - Crane discloses the use of a 21 pair of parallel single shot multivibrator circuits which are triggered 22 by pulses having a frequency double the frequency of the FM signals 23 However, there is no disclosure of the particular frequency doubler 24 circuit Moreover, there is no reset circuitry disclosed in the Crane patent which would require trigger pulses characterized by uniform width, 26 unifonn amplitude and fast or steep leading and tra~ling edges 27 Summary of the Invention 28 It is an object of this invèntion to provide an improved 2~ facsimile receiver capable of faithfully reproducing a document, In accordance with this object, the facsimile receiver com-31 prises means for generating trigger pulses having a frequency propor-32 tional to the frequency of received FM signals representing llght-dark '.

L5;~7 1 variations in a c~ocument. Timing means are coupled to the trigger pulse2 generating means for timing out periods of predetermined duration where 3 the timing periods are initiated in response to the tri&ger pulses.
4 Detector means are coupled to the output of the timing means for gener-sting a writing control signal varylng as a function of the ratio of 6 timing periods to the time between timing periods. Writing means are 7 coupled to the output of the detector means for marking on a copy medium8 so as to reproduce the light-dark variations of the document on the copy9 medium in response to the writing control signal.
In further accordance with this object of the invention, the 11 receiver may comprise reset means coupled to the timing means in the 12 output of the trigger pulse generating means for resetting the timing 13 means in response to trigger pulses during the timing periods.
14 It is another specific objcct of this invention to provide trigger pulses which will reliably actuate the timing means and the ~;
16 reset means.
17 It is a more specific object to generate trigger pulses of 18 uniform width and amplitude and having fast or steep leading and trail-19 ing edges so as to reliably actuate ~he timing means and the reset means.
In accordance with this specific object of the invention, " `::
2~ the trigger pulse generating means comprises means for generating 22 pulses of opposite polarity having a frequency substantially proportional 23 to the received FM signals. A pair of parallel semiconductive paths are 24 connected in series with impedance means between a first reference voltage and a second reference voltage. The parallel semiconductive paths are 26 connected to the means for generating pulses of opposite polarity so 27 as to render the semiconductive paths alternately conductive in response ;~
28 to the pulses of opposite polarity therèby genera~ing trigger pulses of 29 the same polarity at the impedance means. ~
In the preferred embodiment, the parallel semiconductive ~;
31 paths comprise substantially the same impedance. This is accomplished 32 by a first transistor comprising a collector-emitter circuit connected .
- -between the impedance means and the second voltage reference level with a base connected to the output of the means for generating pulses of opposite polarity. A second transistor comprises a collector-emitter circuit connected between the impedance means and the means for generating pulses of opposite polarity and having a base connected to the second reference voltage level. A diode is connected between the collector-emitter circuit of the second transistor and the impedance means to assure substantially equal voltage across the parallel conductive paths.
In accordance with a particular er~odiment of the invention, there is provided, in a facsimile receiver comprising FM demodulator means including a first reference voltage, a second reference voltage, a voltage divider con-nected between said first reference voltage and said second reference voltage, a voltage comparator having one input adapted to be coupled to a source of trigger signals and another input adapted to be coupled to a first tap on said voltage divider between said first reference voltage and said second reference voltage, a flip-flop coupled to the output of said voltage comparator and adapted to change state in response to the output of said voltage comparator, a timing capacitor automatically charged to a predetermined level in response to the state of said flip-flop, reset switch means coupled to a second tap at said voltage divider between said first tap and said second reference voltage for discharging said capacitor, the improvement comprising: means for generating trigger pulses having a frequency substantially proportional to the frequency of FM
signals received by said receiver, said trigger pulses having a predetermined voltage excursion, and means for connecting IL537 `:

s~id rneans for generating trigger pulses directly to said one input o~ s~id compara-tor and said reset switch means, said voltage excursion of said trigger pulses assuring that said reset switch means is actuated in response to each of said trigger pul~es.
srief Descriptions of the Drawinqs Fig, 1 is a block diagram of a facsimile system wherein the invention is utilized, Fig. 2 is a schematic circuit diagram of a pre-f^rred embodiment of the invention, and Fig. 3 is a waveform diagram comparin~ the trigger pulses generated by the prior art circuitry and the trigger pulses generated by the circuitry of this invention.
Detailed Description of a Pre~erred Embodiment Referring now to the facsimile system shown in Fig. 1, a facsimile transmitter comprises a drum 12T rotated by a motor lOT 50 as to create a re]Lative scanning movement between a document carried by the drum 12~ and a scanning head not shown. As the scanning head is advanced axially along the drum 12T and the drum rotates about its axis, successive paths on the document are illuminated and vari-ations in light intensity due to the re~lectivity of the document are scanned by a photodetector 14. The photodetector 14 then converts these variations in light intensity which are a function of the reflectivity of the scanned document into electrical signals. m ese electrical signals are amplified in a preamplifier 16 and\utilized to control a VC0 (Voltage controlled oscillator) 18 to generate FM
(frequency modulated) signals representing the information content of the document carried by the drum 12T. The ,~
frequency modulated signals are then conditioned by a - 7a -:. . .

.537 1 signal conclitioning network 20 be~ore being applied to an acoustical 2 coupler 22 which is associated with a conventional telephone handset 24.
3 The FM carrier is transmitted by suitable means such as con-4 ventional telephone lines 25 to a facsimile receiver which is coupled to S another conventional handset 26 and an associated acoustical coupler 28.
6 In accordance with this invention, the FM carrier which is 7 amplified by the preamplifier 30 is applied to an improved circuitry 8 for generating trigger pulses characteri~ed by substantially uniform 9 pulse width and amplitude as well as fast or steep leading and trailing edges. The new and improved trigger pulse circuitry includes a differ-11 entiating circuit 32 which generates spikes of opposite polarity in 12 response to each zero axis crossing and a frequency doubling circuit 34.
13 In further accordance with this invention, the output of the 14 frequency doubling circuit 34 is applied to a single shot multivibrfltor 36 having an input directly connected to reset circuitry 38. The trig-16 ger pulses of uniform width and amplitude having steep leading and 17 trailin\g edges reliably reset the single shot multivibrator 36 in re-18 sponse to the output of the reset circultry 38 even though the single 19 shot multivibrator is in the astable state. In other words, the astable state of the single shot multivibrator 36 will continue at least a pre-, .
21 determined length of time after each trigger pulse generated at the out-22 put o~ the frequency doubler circuit 34.
23 The output from the single shot multivibrator 36 is applied to 24 a detector circult 40 for determining the average DC value of the single shot output. The writing control signal generated at the output of the 26 detector 40 is then applied to a stylus driver 42 for a stylus 44 assoc-27 iated with a movable head (not shown) juxtaposed to a copy medium carried 28 by a drum 12R. Relative movement between the copy medium and the head is achieved by rotating the drum 12R by means of a motor lOR and advanc-ing the head axially along the drum. The motor lOR is driven by a 31 scanning drive circuit 46 which initlates rotation of the drum lOR in 32 response to the output from the detector 40 by means of a control 3~ :

1 circuit 48.
2 The differentiatîng circuit 32, the frequency doubling cir-3 cuit 34, the single shot circuit 36 and the reset circuit 38 will now 4 be described in detail with reference to Fig. 2. As shown thereinJ the differentiating circuit 32 comprises an RC network including a capacitor 6 102 connected to the output of the preamplifier 30 and a resistor 104 7 connected to ground. The output from the differentiating circuit 32 at 8 the junction of the capacitor 102 and the resistor 104 is a series of 9 spikes which are of opposite alternating polarity and are coincident with the zero axis crossing of the FM carrier.
11 In accordance with this invention, the spikes from the differ-12 entiating circuit 32 are applied to the frequency doublin~ circuit 34 13 which comprises a pair of alternately conductive parallel semiconductive 14 paths which are connected through a voltage dropping impedance between `
a first reference voltage level and a second reference voltage level.
16 More particularly, the doubling circuit 34 comprises a first transistor 17 106 having a collector-emitter circuit connected in series with a volt-18 age dropping resistor 108 between a +8 volt power supply and ground. `~
19 The base of the transistor 106 is co~mected to the output from the differentiating circuit 32 so as to render the collector-emitter circuit ~ .
21 of the transistor 106 conductive in response to positive spikes. Con-22 duction of the collector-emltter circuit of the transistor 106 pulls the 23 ~unction between the impedance 108 and the collector of the transistor `~

24 106 to ground thereby generating a trigger pulse which is applied to the single shot multivibrator 36 and the reset circuit 38.

26 The second semiconductive path of the doubling circuit 34 com-27 prises the collector-emitter base circuit of a transistor 110 which is 28 connected in series with a compensating diode 112 between the voltage ~ dropping impedance 108 and ground. When a negative going spike from the 30 differentiating circuit 32 is applied to the emitter of the transistor ~`~

31 110, the base collector-emitter circuit is rendered conductive so as to 3~ pull the junction of the diode ll2 and the voltage dropping resistor 108 - . . , '' :; ' . ' .

.

L53~

l to gro~lnd thereby generating a trigger pulse which is appli~d to the 2 single shot muLtivibrator 36 and the reset circuit 3B. The compensat-3 ing diode 112 assures that amplitude of the trigger pulses are sub-4 stantially equal by providing a voltage drop whichJ when algebraically added to the voltage drop across the base-emitter circuit of the tran-6 sistor 110, substantially equals the voltage drop across the collector-7 emitter circuit of the transistor 106.
8 In a particularly preferred embodiment of the invention, the 9 transistors 106 and llO comprise 2N4123 transistors which, when satu-rated, have a collector-emitter voltage drop substantially equal to .2 ll volts. When the transistor llO is saturated with the base tied to 12 ground and the emitter driven negative, the collector voltage is sub-13 stantially -.6 volts with respect to ground and the emitter voltage is 14 substantially -.8 volts with respect to ground. By providing a voltage drop of 0 6 volts from the anode to the cathode of the diode 112, the 16 voltage at the ~unction of the diode 112 and the voltage dropping im-l7 pedance 108 is approximately 0 volts with respect to grolmd. Thus, the l8 trigger pulses which are generated in response to oppositely poled 19 spikes from the differentiating circuit 32 are of substantially equal ~ 20 amplltude. The pulses generated by the doubler circuit 34 are also of ~' .
21 substantially equal width with steep leading and trailing edges.
22 The single shot multivibrator circuit as shown in Fig. 2 com-23 prises a comparator 114 having an output coupled to a flip-flop 116, 24 one input coupled to the output of the frequency doubling circuit 34 and the other input coupled to a tap 118 in a resistive voltage divider 26 120 so as to provide a voltage reference for comparison with the input 27 trigger pulses. ~hen a trigger pulse is applied to the first input of 28 the comparator 114, th~output from the comparator sets the flip-flop 29 116. Simultaneously, thè flip-flop 116 removes a positive voltage from the base of a switching transistor 122 so as to permit a timing capaci-31 tor 124 coupled between the collector of the transistor 122 and ground 32 to charge.

- ~0 -1 The single shot multlvibrator further comprises a comparator 2 126 having one input coupled to the capacito} 124 and another input 3 suitably referenced by the voltage divider 120 at a tap 12B. When the 4 charge on the timing capacitor 124 reaches a predetermined level, the comparator 126 will produce an output so as to reset the flip-flop 116 6 and thereby apply a positive voltage to the base of the switching tran-7 sistor 122 discharging the capacitor 124. The output from the flip-flop 8 116 is applied to an output stage 130 which produces a voltage which is 9 detected by the average DC voltage detector 40 In accordance with another important aspect of this invention~
11 the input to the reset circuitry at base of a transistor 132 is con-12 nected directly to the input of the single shot multivibrator circuit 13 36. The emitter of the transistor 132 is coupled to a tap 134 on a 14 voltage divider 120 such that a negative voltage applied to the base of '~
lS the transistor 132 will cause the base-collector junction of the tran-16 sistor 132 to conduct and this in turn will remove the forward bias on 17 the base of the transistor 122 so as to cause the transistor 122 to 18 become nonconductive thus initiating charging of the timing capacitor 19 124 during presence of a trigger pulse. It will be observed that the trigger pulses which substantially approach ground, i.e.7 within .2 volts 21 of ground, are capable of rendering the transistor 132 conductive even 22 though the emitter of the transistor is biased relatively close to 23 ground by the tap 134 on the voltage divider 120. After the trigger 24 pulses have passed, the transistors 122 and 132 are rendered nonconductive and the capacitor 124 begins to charge again through resistors 136 and 26 138. The resistor 138 comprises a potentiometer which is adjustable so 27 as to permit an adjustment in the timing period of the multivibrator.
28 Assuming a detector circuit designed to demodulate FM signals 29 within the bandwidth from 1500 to, but not exceeding 2400 Hz., the astable state duration or astable pulse width may not exceed 1/4800 31 where 4800 is the ~requency (in Hz.) of the zero ax~s crossing and 32 trigger pulses for an FM carrier frequency of 2400 Hz., i.e., the ;37 astable pulse width may not exceed 209 microseconds, In the circuit of Fig. 2, the astable pulse width is established .-by -the component values of the resistor 136, the resistor 138, the capacitor 12~ and the tap setting on the resistor 138. :
For further details concerning the multivibrator, reference is made to U. S. Patent ~o. 3,916,098, ~ote that the multivibrator shown in Fig. 2 in block formed by broken lines may comprise an integrated circuit such as an NE555 chip manufactured by Signetics Corporation, In such a chip, the emitter of the transistor 132 would be connected to a voltage source other than that provided by the voltage divider 120.
In order to provide a more graphic explanation of the advantages to be derived from this invention, reference will now be made to the waveforms shown in Fig, 3.
Waveform a depicts the pulses of opposite polarity generated at axis crossing times of the FM carrier by the differentiating circuit 32. Waveform b depicts the trigger pulses which are generated by the prior art frequency doub-ling circuit disclosed in U, S. Patent No. 3,911,207 in response to the pulses of waveform a. It will be noted that the trigger pulses are of different amplitude and width, The trigger pulses generated at times t~, t2, and t4 more closely approach 0 volts than the trigger pulses at times tl, t3 and t5. However, none of the trigger pulses reach a sufficiently low voltage so as to reset the multivibrator as shown in Fig. 2 since the input\to the reset circuitry, -~
namely the base of the transistor 132, never becomes more negative than the emitter of the transistor 132 which is connected to the tap 134 of the voltage divider 120. More- ;
over, the leading and trailing edges of the pulses are slow.

- 12 ~

In contras-t, the trigger pulses generated by the frequency doublin~ circuit of this invention are of substan-tially equal amplitude and width, i.e., the pulses at times tol t2 and t4 are within .2 volts of 0 volts while the pulses at times tl, t3 and t5 actually reach 0 volts. It will also be noted that the leading and trailing edges of the pulses in waveform c are substantially steeper than those of waveform b, As a resul~, the pulses of waveorm c assure that, even though the reset circuitry is connected directly to the input of the multivibrator circuit, the multivibrator will be reset in response to any disturbance on the line so as to avoid any white holes in black copy.
Although portions of the transmitter and receiving circuitry depicted in Fig. 1 have not been shown in detail herein, such circuitry is shown in detail in U. S. Patent ~o. 3,911,207, Although a particular embodiment of the invention has been shown and described and various modifications have been suggested, it will be understood that the true spirit and scope of the invention as set forth in the appended claims embrace other modifications and embodiments which will occur to those of ordinary skill in the art. `~
This application is a division of Application Ser. No. 257,726, filed July 26, 1976.

;~ "

Claims (6)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. In a facsimile receiver comprising FM demodulator means including a first reference voltage, a second reference voltage, a voltage divider connected between said first reference voltage and said second reference voltage, a vol-tage comparator having one input adapted to be coupled to a source of trigger signals and another input adapted to be coupled to a first tap on said voltage divider between said first reference voltage and said second reference voltage, a flip-flop coupled to the output of said voltage comparator and adapted to change state in response to the output of said voltage comparator, a timing capacitor automatically charged to a predetermined level in response to the state of said flip-flop, reset switch means coupled to a second tap at said voltage divider between said first tap and said second reference voltage for discharging said capacitor, the improve-ment comprising: means for generating trigger pulses having a frequency substantially proportional to the frequency of FM signals received by said receiver, said trigger pulses having a predetermined voltage excursion, and means for connecting said means for generating trigger pulses directly to said one input of said comparator and said reset switch means, said voltage excursion of said trigger pulses assur-ing that said reset switch means is actuated in response to each of said trigger pulses.

2. The facsimile receiver of claim 1 wherein said voltage excursion of said trigger pulses approaches said second reference voltage so as to be substantially equal thereto.

3. The facsimile receiver of claim 1 wherein said means for generating trigger pulses comprises: means for differentiating said FM signals; a voltage dropping impedance connected to said first reference voltage; a first semi-conductive switching means connected between said voltage dropping impedance and said second reference voltage; a second semiconductive switching means connected between said voltage dropping impedance and said second reference voltage; and said first and second semiconductive switching means being alternately responsive to signals from said differentiating means for generating said trigger pulses.

4. The facsimile receiver of claim 3 wherein said first semiconductive switching means comprises a first transistor and said second semiconductive switching means comprises a second transistor.

5. The facsimile receiver of claim 4 wherein said first transistor comprises a collector-emitter circuit connected between said voltage dropping impedance and said second reference voltage and a base connected to said differentiating means and said second transistor comprises a collector-emitter circuit connected between said voltage dropping impedance and said differentiating means and a base connected to said second reference voltage.

6. The facsimile receiver of claim 5 comprising a diode connected between the collector-emitter circuit of said second transistor and said voltage dropping impedance.