[1]United States Patent Office 3@200,343 3,200,343 D.C. ARfl-iDLIFIER 'FAAVINTG FAST RECOVEil,',Y CH- All@ACTF,@r@ISTICS zer, PhiE?de@p!,Aa, Pa., assignor to Leegs P,ichn-rd A. ap,@q Norib-,zuD Comv-any9 Phil-,tdelphii, Pa., a corporation Cf pelinEYlva-lzlzl Fil--d Dec. 29, 74c. 103,232 11 Cia-@Ms. (C]. 330-17) This invention relates to amplifyin.- circuits and has for an object the provision of a high gain transistor amplifier which functions below the saturation range of its transistors and has a minimized value of internal high frcqliency feedback. Amplifiers usin- transistors as th--ir active elements are known by those skilled in the art 'Lo be limited in their applications becausp- they are Lnsuited for appl@@eations where the inpi-it signals vary over wide ranges in magnitude and where the speed of response to the input signals must b.-- quite high. In such applications, when the input sig.,ials are of a hi,-h forward biasing value, th-. tra@isistors become "ov.-rloaded" or saturated. A transistor may be defided as bein.- saturated when its collector-base jtjnction is forward biased at the same time that its emitter-base junction is forward biased. Upon removal of th-, hi.-h valtie input sigiial, many of the current carriers (commonly known as stoi-ed cbarge) will be trapped in the base reaion of the transistor. These trapped carriers will block the transistor and prevent amplification of subsequent input si,-nals iintil these carriers can be "swept out." Such "sweeving" time duration pre@er@ts an tindesirable tiine delay and results in a low speed of response and slow recovery from the high va'@tied input si,- nals. In addition, prior transistor amplifiers have been limited in t@eir applications because having a substantially low value of cutoff freqliency. Though atransistor itself may bave high gain capabilities (Beta) in the high frequency range, the fore,aoin.- low cutoff frequency results wben the transistor is included in an an-iplifier circuit which causes excessive i-Titernal high frequency feedback because of high values of collector resistance and hi,-h values of capacitance develop-,d b-,tween collectoi- and base of the transistor. Durin.- operation of known transistor amplitiers for diitering values of input signals, the conductivity of the transistor will vary prodi-icing a varying collector potential. NVith the transistor highly conductive, the potential diff'erenec between its collector and base will bc@ r--Iatively small and t large effective value of feedback capacitance is developed. In addition, conventional transistor amplifiers have required for operation a large value of collector resistance. As a result of the foregoing large value of collector resistance and lar.-e value of capacitance between collector and base, there will be developed a hi.-h Ievel of internal h:@gh frequency feedback resulting iii a low c,,itoff frequency for the transistor amplifier, i.e. low gain for frequencies above the low cutoi'l frequency. In accordapce with the present invention, there is provided a trapsistor amplifier (1) which is prevented from re,tchin- the saturation limits of its transistors and (2) has a minir.,iized value of hi,ah frequency feedback. In carryin.@ out the invention in one fo.-@n thereof, the transistor ami)lifier comprises a normally conductive output transistor and an input transistor having an input circuit for applyiiig thereto an i@ii)ut signal which may vary over a wide range of magnitude and may contain Iiigh frequency components. The emitter of the output transistor is connected to the collector of the input transistor and the inplt transistor has an olil,put circuit which inel'ades a resistor conn-,cted between its collector and 10, 1965 2 ground. A source of constant current is connected between @round and the cr@iitter of the input transistor. That constant current source, for a,selected maxin-lum conductivity of the inptit trans@'stor, limits the niaximum value of current supplied by it to the input transistor by way of its emitter to a value below the saturating range of that transistor for the wide range of input signal amplitude. A unidirectional device is connected in shtint with a constant current source for bypassing the dif10 ference between the ma,-Ilitude of the constant current from the constant currer,.t source and the emitter current of the input transistor for all values of conductivity below the aforesaid selected maximum value. For those lower values of conductivity, the emitter of the input 15 transistor will be maintained at a low impedance with respect to ground and thus the input transistor wi',l provide gain of a high order of magnitude for the lower amplitude signals. Further in acccrdance with the invention, there is pro20 vided a loop circuit for the output transistor which includes the resistor and the ground coniiection in the otitput circuit of the input transistor and also includes the base and emitter of t@e output transistor. A first source of supply is included in the loop cir-.uit and is disposed on 25 the resistor side of the ground connection. The first source of supply has a polarity wlich normally maintains the outp@at transistor conductive. A second source of supply is connected in the loop circuit b--tween the ground connection and the base of the output transistor 3o for establishin-. the collector of the input transistor at a relatively hiah potential w;th respect to groil-iid, thereby to produce a n-iinimum capacitance b@-tween said lastnamed col'iector and its bas@, to mip;mize hi.-h frequeiicy feedback to the input circuit of the input iransistor. The 35 part ot a loop circuit extending from the collector of the input transistor by way of the emitter and base of the norrlally conductive output transistor and by way of the second source of supply Drovides a low imp--dance path to ground for high frequencies thereby further to mini40 mize the high frequency feed back. Both the minimization of effective collector-emitter capacitance and the provision of said lo@v impedance path miintain high gain over an extended range of frequencies. The output circuit of the outpiit transistor includes a 45 source of supply which, for all o@,itputs of the input transistor, limits the operation of the output tra-@isistor to a ranae below saturation thereof. For a more detailed disclosurc of the itivention and for further objects and advantages tbereof, referenc-- is to be 50 had to the following description taken in conj;unction with the accompanyin- drawing in which there ;s scheiiiatically illtistrated a tr,,irsistor amplifier ernbodying the invention. 55 fe Referring now to the drawin,-, the invention in its prerred form has been shown as an amplifier co-mprising an input transistor 10 of the PNP type and an output transistor 11 of the NPN type. Signal input terminal 12 is connected to the base of input transistor 20, the emitter of 60 which is connected by way of resistor 14 to the positive side of battery 15 which has its negative side connected to ground. The resistor 14 and battery 15 are shunted by a diode 16 having its anode connected to the emitter of transistor 10 and its cathode to ground. The collector of 65 transistor 10 and the emitter of transistor 11 are connect,ed together and to one side of resistor 16'; the other side of resistor ,LS is connected to th-- negative side of battery 19 or equivalent D.C. voltage source. The positive side of battery 19 is connected to ground. It is to be un70 derstood that ground potential will be taken as zero while such ground potential may be at the potential of earth or at a rei'erence potential with respect to earth.
[2]In operation of the transistor amplifier, input signals are applied to the input terminal 12 and to the grounded i.,iput terminal 12a by the input source shown by the rectangle 12b, and output signals are produced at the output terminals 13 and 13a. When the input si.- nals applied to terminal 12 are of ne.-ative polarity with respect to the grounded terminal 12a, the emitter-base junction of input tra-iisistor 10 is forward biased and that transistor is rendered co@iductive. Diode 30 is connected between the base of the transistor 10 and ground and is effective when renc@ered fully conductive by ne.-ative input si.-nals to limit the maximum negative potential that may be developed at the base of transistor 10 with respect to ground. While the input signals may no', be of sufficient negative magnitude to render fully conductive diode 30, they still may be of suffici,-nt value to saturate transislor 10 except for the provision of the means which limits the maximum value of current supplied to the input transistor 10 by way of its emitter to a value below the saturat:on ran-,e of that transistor. Specif.;cally, for a se-jected maximum co@iduct@@Vity of transistor '@0, a constant current source formed by battery 15 and resistor 14 provides a constant current which limits the maximui-n value of current suppl.ied by way o'L conductor 14a to the emitter of transistor 10. In this manner, the emitter current of transistor 10 is limited to a value substantially equal to the constant current preselected by the voltage of battery 15 and the value of resistor 14. Since the collector current ol- transistor 10 is approximately equal to its emitter ciirrent, the collector current will be limited to a value approximately equal to the constant current. Thus, the emitter current and the colleclor current of transistor 10 are limit,-d in magnitude when input signals are applied to the input terminals 12 and 12a hav-ing values just sufficiently high to develop the preselected maximum conductivity of input transistor 10. For still hi.-her valu.-d negative indut signals, of insufficient magnitude to render diode 30 fully conductive, the foregoing limited magnitude emitter and collector ctirrents wiII increase but by negligibleamounts. For ne.-ative input signals which produce conductivity of transistor 10 below the aforesaid selected maximum value, the emitter current of transistor 10 will be of a lower order of magnitude than the magnitude of the con-stant current developed by battery 15 and resistor 14The difference between the magnitude of the constant current and the emitter current will render diode 16 conductive and will flow therethrough to ground and to the ne,,ative side of battery 15. Thus, it will be seen that the portio-.i of constant current in excess of the e@nitt er current is bypassed to grouiid. With diode 16 conductin,e, tho emittcr of transistor 10 will be maintained at a relatively low impedance Nvith respect to ground and thus that transistor will exhibit the characteristics of a comnion-emitter connected tra-iisistor. In its common-emitter coinection transistor 10 provides large current gain as described in detail in the "Handbook of Semiconductor Electronics," edited by Lloyd P. Hunter, McGraw-Hill 1956, at Chaper 11, page 20 et seq. Thus, for lower level negative input si.-nals, the diode 16 is effective t convert the input transistor circuit to one affording high gain and without possibility of saturation. It will be understood that the -ain of input transistor 10 is maintained at the foregoing high level as long as the diode 16 is conduct;vc to provide a low impedance path from tle emitter to ground. That high gain will only be reduced upon application of negative input signals which result in the conductivity of transistor 10 reaching its pr@.s--Iected maximum value. At slich input .level, the emitter current of transistor 10 will become substantially equal in value to 'Lhe constant current and the diode 16 will be rendered nonconductive. With diode 16 nonconductive, the path from the e@iiitter of transistor 10 to ground is at a hi.-h impedance because the path includes the resistor 14. As a result of that high 3,200,343 4 impedance in its emitter circuit, transistor 10 is bighly degenerative and thus operates in manner similar to that of an emitter-follower. When the input signals applied to input terminal 12 are of positive polarity with respect to the grounded input terminal 12a, the input transistor 10 is rendered nonconductive, the emitter-base junction of that transistor is thus reverse-biased apd the emitter and collector currents are reduced to zero as a limit. At that tirne, all of 10 the constant current from the constant current source will ilow by way of conductor 14a through the diode 16 to ground and to the negative side of battery 15. With reverse-biasing input signals, the potential at the emitter of transislor 10 will be maintained at a low fixed 15 value of potential with respect to ground. With forwardb'iasiig negative input signals, the emitter-base junction thereof will provide a relatively small potential drop of substantially constant magnitude and thas th-- potential developed at the eniitter of transistor 10 with respect to 20 ground will vary directly with the input signal potential applied to its base. Since diode 39 is connected between that base and ground, the maximum polenti-,l therebetween is held to relatively low order of ma.-nitude with respect to ground. Therefore, the excursion of the base 25 to .-round potential is held within a relatively small range oi- amplitudes, as for example 0 1/2 volt, which range is sufficient to produce the desired operation of transistor 10. As a result of the small range of variations in potential at the base of transistor 10, the emitter po',ential will 30 al.,o vary over a small range. The battery 15 is selected to have a potential substantially high with respect to the small range of potential variations at the emitter and with that value of battery 15, the value of resistor 14 is selected to be of suitably high magnitude to provide a 35 constant current having the above-described limited value. In this manner, as the potentials at the base and at the emitter of transistor 10 vary within their limited range, the current as supplied by the relatively high-voltage battery 15 and as limited by the resistor 14 will reiiiain sub40 stantially constant. The battery 21 connected between th-. base of outpu, transistor 11 and grourd is included in a loop c,'@rcuit for that transistor which comprises the bas-@ and emitter thereof, resistor 13 included in the collector circuit of 45 i@pui@ trar@sistor 10 and batteries 19 a-iid 21. The loop circuit h,-ts a Pround connection bet@veen batteries 19 and 1. The battery 19 is coinected in the loop circuit in @a diree-tio-@l corresponding v,,ith forward bias for transistor 11 Nvhile the battery 21 is connected iii a direction correspondiii.- with reverse bias for trans:stor 11. Bat50 tery J19 is selected to hav-. a polent:l al larger than that of battery 21 to provide a resultant forward bias for transistor 11 for all valucs of collector cirrent of transistor 10 includ@'ng zero thereby to maintain that transistor 55 normally conductive, i.e., :cond-active at all times. In this 1-pan@-ier, the output transistor 11 is conductively biased at a predetermined po;nt on its operating cliaract@-ristic in the absciice of any col@le@tor current of iiiput trans.islor 10. 60 With transistor 11 maintained normally conductive, its forward-biased emitter-base junction provides a relatively small resistance. cir@ce point 31 of the loop circuit is connected by way of the low valued emitter-base resistance of tran@.istor 11 to the Pegative side of battery 21 65 aiid the positive side thcr--of is connected to -round, point 31 is maintained at approximately the ne.-ative potential of batt,-ry 2i'L v;ith resp--ct to ground. In this manner, since poi-iit 31 is conn-,cted to tiie collector of trqnsistor 10, that collector is maintained at a substan70 tially constant negative poten@Lial with respect to ground which is a necessary requirement for proper operation of transistor 10. A positive polarity at that collector wou'@d result in saturalion of transistor 10. In addition battery 21 is sel-.cted to have a relatively Iii.-h potentiai 75 So that the aforesaid collec'Lor is maintained at a rela-
[3]tively high constait D.C. poter@tial with respect to ground. It will now be shown that as a result of that hi.-h negative potent,@'al establisbed between the collector of transistor 10 and ground, the effective capacitance produced between that collector and its base is minimizedWith input transistor 10 conductive, it will be remembered that the potential that may be developed between its base and ground varies over a smllt range and is of relative'iy low order of ma-nitude. Since the co-Ilector of transistor 10 is maintained at a stibstantially hi-,h constant ne@ative D.C. poten@tial with respect to ground and since the base is mailitaitied at a low order of potential vvith Tesoect to ground, the D.C. potential difference betvveen t@at collector and its base will be maintained at a substan-lially hi,-h constant value. As described in the above-cit--d "Handbook of Semiconductor Electronics" at Chapter 4, page 25 et seq., the effective capacitance between colleetor and base of a transistor is inversely proportional to the potential developed therebetween. As a result of the foregoing, the capacitance between the collector and base of transistor 10 will be of a substantially low constant value thereby to mi-@iimize internal high 'frequercy ieedback from point 31 to the iiiput circuit ol. transislor 10. It will now be shown that the a@'Oresaid feedback is further minimized by the provision of a low impedance path to grotind for high frequencies. As above described, the forward-biased emitter-base junc,tion of transistor 11 provides a relatively sniall resistar.ce. Accordingly, -for high frequencies or high frequency components, the po@int 31 is connecte4 to ground by way of a low impedance path which may be traced through the emitterbase junct@@o@n of transistor 11 and throu,@h the battery 21 (having a low :lnlpedance at high free,tiencies) to ground. The collector of transistor 10 is connected to point 31 and thus the collector resistance with respect to ground is also of a low order of magnittide ind is maintt-ined constant. As a result of t@his small collector resistance, the high frequency poitential e,rop developed across that resistance, i.e., the high frequ-,ncy potential drop between point 31 and ground, will also b-, of a relatively low order of magnitude. Since there is only a relatively small value of high frequency si.@nal available all Doint 31 for feedback from the collector of transistor 10 to its base, the small value of collector resistance is effective further to minimize the high frequency feedback. It will now be unde@rstood how the foregoing low effective value of collector to base capacitance in conjunction with the low value of collector resistance oftransistor -10 .ar@-atly miiimize the h,.,-h frequency feedback from point 3-,', to the iiiput circiiit of transistor 10. It is in this raanner that the 1-.igh .-ain capabilities of transistor 10 are extended for a wide ran.-e of frequencies extendin.- into the high frequency range. There w,.11 now be described the effects on the abovementioned loop circuit as the coilector current of transistor 10 varies for varying input signals. As @hat collector current increases, the current flowing throu,-h resistor 1,i increases and the potential @drop across that resistor also increases. However, at that time the potential at the point 31 will change in a positive direction and the output transistor 11 will be less conductively biased resulting in a decrease in the flow of its emitter current. Thus, as the collector current of transistor 10 increases, the -.mitter current of transistor 11 decreases so that the total @current flowing through resistor 18 is maintained at approximal,ely a fixed level. In this manner, the collector curr@-nt of transistor 10 adds to the emitter current of transistor 11 to produce the resultant current flowing through resistor .18. The emitter current of transistor It will be approximately caual to its collector current and that collector current will flow f-rom the positiv-- side of battery 24 t.liroi-,,Prh resistor 25 to the collector thereof. The bittery 24 ard resistor 25 are selected so that for all values of collector current of transistor 10 including zero and for 3,2@00,343 all values of cdrrent flowin.- through resistor 12, thetransistor 11 is not operated within its saturation range. Thus, not only @input transistor 10 blit also output transistor 11 are prenented from operating in their saturation range. With the foregoing connection to the collector of transistor 11 iii conjunction with the connections of the loop circiiit to the base and em@'tter thereof, the output transistor 11 is operated in the common-base coniiguration. 10 It will now be understood that tipon application of a negative inplt si-nal producing the selected maximum condtictivity of transistor 10, the collector current of that t.,ansistor will be at its maximum value and t' ius the emilter curreit of output transistor Il- wihl be at its lowest 15 order of magnitude. Thi-is, there will be developed a collector current for trprsistor II of lowest order of iiiagnitude. As a reslilt, the output terminal '-,3 which is coniiected to the collector of transistor 11 will be at its higliest positive potential with respect to its grounded output 20 terminal 13a. 1-ii similar wa-,iner, tipon a!)rjlication of an inplit signal producing a small collector current of transistor 10, the emitter current of transistor 11 will b.- of a hi-h order of magnitude. Thiis, there w@ll b-, deveioi)ed a relatively 25 high collector current for transistor 11. As a result, the potential drop across the resistor 25 will be of a bi,-h order of ni-,tgnitud-. apd the otitput terminal 13 will be at a high negative potential with resp-.ct to its @rounded output te@-ininal 13cr. 20 It will Pow be understood how the transistor ariplifier comprisiiig input transistor 10 and output transistor 11 reverses in phase or inv,-rts the output signal relative to the input. By way of example of siiitable coniponents and circuit 35 parameters, when the input transistor 10 is of the PNP type 2N504 of the Phileo Corporation a-.id the output transistor 11 is of the NPN type iN-101 of Texas Ilistruiient Corp., the other suit-able coiiiponents may be: Diode 16 ---------------------- S669G Transitrc)n. 40 Diode 30 --------- ------------- IN 625. Resisto r 14 --------- ----------- 15K ohms. Resisto r 18 --------- ----------- I IK ohms. Resisto r 25 --------- ----------- 2K ohms. Battery 15 --------- ------------ 18 volts. 45 Battery 19 --------------------- 30 volts. Battery 21 --------------------- 7 volts. Battery 24 --------------------- 7 volts. Although the principles of th-- invention have been explained by referenpe to a preferred amplifier circuit, it 50 will be understood that the invelit@'oi comprehends many modifications of si-ich circuit. For example, outi3ut trans;stor 11 may be of the PNP type and with suc@ change the polarity of the battery 19 will be reversed to maintain transistor 11 normally conductive. I.-i addition, the 55 polarity of the battery '14 will also be reversed to provide a source of ne.-ative potential for the collector of transistor 11. With stich changes, resistor @'S will be of a higher value than above ind;cated and it will be understood that the current flow through that resistor will be 60 of low order of niagnitude. In further modifications of the invention, both transistors 10 and 11 may be of the NPN type or transistor 10 may be of the NPN type while transistor 11 m@ ay be of the PNP type. With the fore.-Oin.@ changes ;n the types of 65 transistors 10 and 11, correspondin- chanpes will be @nade in the polarity of the sources of supply 15, 19, 21 and 24, in the value of resistor lu@' and in the connections of the terminals of the diodes 16 and 30 to establish the conduc70 tive states of the transistors and diodes above described to preclude saturation Lor a wide ran,-e of input signal levels alid to miniiiiize high freouency feedback. I
