High Input-impedance Amplifier Circuits
Using bipolar transistors, integrated circuits and f.e.ts in a.c. amplifiers presenting low loading on the signal source�
by T. D. Towers,* M.B.E., M.I.E.E.
In the ordinary course, engineers do not ling and the emitter decoupling capacitors With germanium transistors, hje tended to find many requirements for low-frequency, present negligible impedance, three factors fall almost linearly with current below ImA; high input-impedance amplifiers. This is control the low-frequency impedance seen so that little use could be made of reducing fortunate, because the "ordinary" bipolar looking into the amplifier input terminal. the emitter current to put up input imped transistor, with which they have done most of These are the shunt impedances presented ance. Modern silicon transistors generally their circuitry for a decade now, is basically by the emitter circuit, by the base-bias resis hold up their current gain to much lower a low-impedance device. The "old-fashioned" tors and by the collector circuit, all in current levels than germanium. With devices thermionic valve had one big advantage; parallel across the input (both positive and of the common BC107, 8, 9 famlly, for ex with comparatively simple circuit arrange negative supply rails being a.c. grounded). ample, a current gain of 100 at 100,uA makes possible a transistor a.f. input resistance of ments you could easily achieve input imped . ances of over 100MQ. With bipolar transis 25-50kQ. Emitter circuit shunt impedance: So far tors, quite specialized circuits had to be as it is governed by the emitter circuit, the devised to get above even 100kQ. But things low frequency input impedance of the High input impedance with input trans� are changing again; we are moving into an transistor itself is given approximately by former or series resistor: Before we go on age where the f.e.t. and the monolithic inte Rin=1bb,+25hjelIE, where rhO' is the device to examine other high-impedance circuit grated circuit are displacing bipolar transis "base spreading resistance" (for transistors arrangements, we should not forget that you tors-just as transistors did valves. used in low-level circuits usually 30 to 300 Q), can use a transformer with a high step-down hj. is the common-emitter current gain (nowa turns ratio. Such a .transformer has the ad High input impedance with days usually 50 to 250), and lE is the emitter vantage of isolation, but, to get high imped bipolar transistors current in mA. For a typical low-level stage ance at low frequencies, its size and cost can with lE set 'about 1mA, we find Rin typically become prohibitive. The upper frequeney Consider the conventional common-emitter 1 to 2kQ. The formula for Rin shows that you limiimay also be limited by stray capaci amplifier stage shown in Fig. 1(a). At fre can increase the device input impedance by tances. Careful screening is called for, and it quencies where the input and output coup- reducing the emitter current, provided cur is sometimes difficult tQ provide simultan *Newmarker Transisrors Lrd. rent gain does not fall at the same time. eously minimum noise and high impedance. ,. Fig. 1. Arrangements for reducing shunting of emitter circuits on transistor input impedance. (a) Basic common-emitter amplifier operated ac low bias current. (b) Common-collector (emitter10llower) single transistor. (c) Compound two-transistor Darlington-pair common-collector. (d) Modified Darlington-pair common-collector. (e) Bootstrapped Darlington-pair. (f) and (g) Complementary n�p-n/p-n-p compound emitter10llower circuits. (h) Transistor-loaded emitter-follower.
Rc Rs Tr t--o Tr Cout o-J Cin 1---0 t--o � Rs Cout Co ut R E ' Cout C RE E RE RE RE
(a) (b) (c) (d)
T C r1 E Tr1
+---4--f r-o Cout t--o Cout
(e) (0 (g) (h)
Wireless World, July 1968 197 Reducing shunting effect of base bias resistors on input impedance: RS1 So far, we have ignored the effect on input impedance of RS . RS any base bias resistor networks shunted across Tr RS2 Tr Tr the transistor input. Fig. 2(a), showing an elementary single-resistor bias circuit, illus 0-1 trates the problem. You will see that Rs is Cin directly across the input (both supply rails I----<> I----<> being a.c. earthed). Obviously the total Rio Cout Cout RE cannot exceed RB. RE The conventional technique to raise the effective a.c. resistance of RB without raising (a) ( b) (c) its d.c. value is to boots trap it as shown in Fig. 2(b). RB is replaced by two resistances RBl and RB2 of the same total resistance. Pro vided RBl and RB2 are large compared with Rs RE, the transistor emitter-follower action makes the top end of RB2 move up and down with its bottom end. Thus its a.c. impedance is Tr Rx raised effectively by the current gain of the 0-1 0-1 transistor. Cin Cin The more thermally-stable conventional Rs, two-resistor bias network of Fig. 2(c) can also C I----<> be bootstrapped for high impedance. Fig. 2(d) s Cout ' shows an arrangement using a single bootstrap RS RE capacitor, CB, in which the resistance Rx in RS2 series with the base can be low value for ther mal stability, but have high effective imped Cd) (e) ance so as not to shunt the input a.c. signal. Fig. 2. Reducing the shunting of base bias resistors on input impedande. (a) Basic single-resistor Fig. 2(e) shows a two-capacitor arrangement transistor bias circuit. (b) Bootstrapping single bias resistor by capacitance feedback to its centre. where both top and bottom bias resistors are (c) Basic two-resistor base bias circuit. (d) Single-capacitor method of /)Qotslrapping two-resistor capacitor-bootstrapped by splitting each as bias network. (e) Two-capacitor method of bootstrapping two-resistor bias network. explained for Fig. 2(b) above. In general, the boots trap capacitance values must be large enough to present negligible One other simple approach is to accept out having to go to a higher rail voltage. impedance at the lowest frequency, fo, to CB the intrinsic low input resistance of the tran The emitter current of Tn, being the base be handled. A useful guide is to make greater than 1O/(foRB), where Rn is the bias sistor and achieve the required high input current of Tr2, is usually low: typically of resistor being boots trapped. resistance by using a series resistor to the the order of 1 - 5 pA only. Trt should be a input. The main objection to this is the high gain transistor with high at low current, Reducing shunting effect of transistor noise level likely to arise from the large value such as the 2N930 with hj. greater than 100 collector output resistance: From the con series resistor. at 1O,uA. Some designers raise the standing ventional low-frequency T-equivalent circuit Trt current in with a separate emitter bias of the transistor in the common-collector resistor, RE', in Fig. 1 (d); This shunts the Reducing shunting effect of emitter configuration with emitter resistor, RE, shown input impedance of Tn, but its shunt effect circuits on input impedance: Going back in Fig. 3(a), you can see that the internal collec on the overall input impedance may be more tor resistance, re, shunts the signal path. At to the common-emitter stage of Fig. lea), if hj" than offset by the increase in due to the low current levels with modern transistors, we remove the emitter bias resistor decoup RE' higher Tn bias current. Usually is made re usually lies between 1 and 5MQ, so that it ling capacitance, CE, the transistor input 5 to 20 times RE. is difficult to reach higher input impedances impedance becomes Rin=Tbb+hj, (25/lE+RE). To reduce the loading 'Of RE on the input, than re unless it too is bootstrapped. Fig. 3(b) This shows that we can increase the input you can use the bOotstrap feedback arrange shows the basic arrangement for this, where a impedance by using an undecoupled emitter ment of Fig. lee), where the bottom of RE' is . resistance, Rc, is inserted between the collec resistor. It would seem that you could in connected to the top of.RE. By the. emitter tor and h.t. (to isolate re from earth a.c.-wise) crease Rin indefinitely by increasing RE, but, of TT2, RE' CF, when RE rises above about one-tenth of the follower action the lower end of and a capacitor, to the collector (and thus moves with its top, 'and its resistance value is re) collector load resistance, Rc, the voltage gain the outer end of feeds back in phase signals by of the common-emitter stage approaches effectively multiplied th� current gain of from the top end of the emitter resistance. Tn usually RE re Rc/RE. The gain thus falls to unity when (provided, as is the case, is This raises the effective a.c. impedance of RE) RE=Rc. This is why designers looking for small compared with With silicon tran and reduces its shunting effect on the input high transistor input impedance tend to use sistors, the VBE (d.c.) of Tr2 is approximately signal. If possible, the collector bias resis RE' the alternative near-unity-gain common O.6V, so that is fixed by the selected bias tance; Rc, value should be at least ten times current, lE!, of Tn as RE,=0.6/lE' (ignoring RE, but, if only a limited d.c. supply voltage is collector arrangement of Fig. 1 Cb). By using silicon transistors with high gain at low cur the negligible base current into Tn). available, this may not be possible, unless we rent, it is possible to increase RE to a value Combinations of ri-p-nand p-n-p transistors are using a multiple emitter-follower, as giving input impedances of the order of permit other arrangements for bootstrapping explained previously in Figs. 1(c)-(e), where RE several thousand ohms with this simple up the effective value of the' emitter resistor is relatively small. arrangement. in an emitter follower. For example, in both Fig. 3(c) illustrates a circuit using a separate Unless you go to a very high rail voltage, Fig. l(f) and leg), it can be shown that Rio transistor, Tn, to bootstrap the Tn collector even when you resort to low bias currents has a high value give approximately by output resistance without excessive loading you are limited in how high you can rais� Rin = hid hj,2 RE. . of its emitter output. The a.c. voltage at the the value of RE. You can get round this with Another elegant circuit for providing a top of RE is transferred via CF and the base the Darlington compound-emitter-follower high-resistance emitter load in an emitter emitter of Tn to the collector of Tn. arrangement of Fig. l(c). Here, then; follower without high rail volt ages is shown It is also possible to bootstrap the base Rin=hjdhj,lRE approx., and you can see in Fig. l(h). Here the output resistance of bias resistor and the collector output resis that a.c.-wise by adding the extra transistor Tn (of the order of megohms) functions as an tance at the same time with a single capacitor you have effectively multiplied RE by hj,2 emitter load to Tn ano makes possiblean over as in the simple circuit arrangement of without raising its d.c. value, and thus with- all input resistance o�er lOM.!/. Fig. 3(d).
198 www.keith-snook.info - Wireless World July 1968 Base rb One aspect of boots trapping with capacitors r", is that high input impedance circuits tend to rc be operated 'at low transistor bias currents, (�) Emitter and the leakage currents in ordinary alumin RE ium electrolytic capacitors can upset bias ColLector conditions drastically. For this reason, you will 'often find low-leakage tantalum capaci tors specified, where the total capacitance required is beyond the range of solid dielectric RF RC types. Practical high-impedance transistor amplifiers: Fig. 4(a) shows a simple single CF CF CF transistor, emitter-follower, high input imped <>--i ance amplifier operating at a low collector Cin current of about 100�, and with bootstrap � I----<' I--<> Cout Cout ping of base-bias resistors and collector out <::Out RE RE RE put resistance. These techniques produce an input impedance down to below 50Hz not less than O.5MQ. (b) Cc) (d) For input impedances greater than about Fig. 3. Reducing shunting rJtransistor collector output resistance on input impedance. (a) T 1MQ, usually two stages are necessary with equivalent circuit picom1l'!J)" collector transistor with external emitter resistor, RE. (b) Bootstrapping bipolar transistors. Fig. 4(b) is a typical cir r,via capacitor fr()mem�e:r.(c) Bootstrapping r,via buffer transistor. (d) Simultaneous cuit with an input resistance of 1.5MQ and an bootstrapping ofr, and I!s; output resistance of only 30Q. It employs an Fig. 4. High-inp1.ft�tmpedance amplifiers. (a) Single-transistor amplifier with Rin greater than n-p-n/p-n-p direct-coupled pair with boot 0.5M Qdown to 50 Jfz.(b) Two"stageamplifier with 1.5M Qinput and 30 Qoutput resistances. strapping of base bias resistors and emitter (c) Two-stage 10'1Qcircuit with boolslrapping of all three shunt circuits. (d) Three-stage 300M Q resistor. circuit with bandwidtltO:5 Hz-2,000 Hz. (e) Four-stage 1,000M Q circuit down to 10 Hz. Two stages can give an input impedance of +1 8V +9V the order of 10MQ, as, for example, the cas caded emitter follower circuit of Fig. 4(c) by 2M l80k General Electric Co, U.S.A. This employs bootstrapping of all three shunt elements. Cl and C2 values depend on the lowest fre quency, fo, to be passed by the amplifier and In� can be selected at the nearest preferred values ��. o--f ll---+-�""""'" O'l,u l,u to Cl=;=1!(150fo) and C2=1! (50fo!,), Output ...... ,...... -.-fI J---
-1 0V 33k l'5k 470
F . E . T. high input-impedance OA95 amplifiers 100,u The bipolar transistor has a low inherent Tr4 input resistance (1- 50kQ) and we have seen BFY51 that amplifier input impedances above 1MQ can be obtained only with quite complex tran sistor circuits. The f.e.t. on the other hand has 39k a high inhereent input resistance (from 109 Q 200,u upwards with modern devices). Consequently, (e) only simple f.e.t. amplifier circuits are needed 10k to get input impedances at a.f. from 10 to +10V 1,000MQ, while more complex circuits can
199 Wireless World July 1968 - www.keith-snook.info easily giv.e up to 1,000,000MD. The boot +12V +12V RC strapping techniques described earlier for 12k bipolar transistors apply equally to f.e.ts, NKT0112 whether p- or n-channel, junction- or insulated-gate, depletion- or enhancement� o-t o-t 0'01,u 0,01fL mode types. The n-channel, depletion, junc f--o tion f.e.t. is the commonest in use at the time of (a) 5M (b) 5M 10,u writing, and most circuitry will be illustrated 100,u in terms of this. oV oV
Single stage f.e.t. high input impedance circuits: If you want +12V +12V an input impedance of only a few megohms, you can use an n-channel 1M depletion f.e.t. in the common source arrange o-t o-t ment for which a typical circuit is given in 01,u 0'1,u Fig. 5(a). The NKTOl12 here has a typical RG Vp= 1.0V, IDss= lmA, and gmo= 1.5mA/V, 1M r-o f--o (c) 10,u (d) 10,u and is set up at a bias current of O.5mA, giving 10k a voltage gain of about 15 times with an a.f. 150k input impedance of 5MD shunted by about OV oV 30pF. In the common-drain or source-follower circuit of Fig. 5(b), while the input resistance is Fig.S. Single-stageJ.e.t.high-input-impedance amplifiers. (a) Common-source amplifierwith input still only 5MD, the shunt capacitance has been impedanceof SM D shunted by30 pF. (b) Common..drainamplifier with input impedanceof S M D reduces to about 2pF at the expense of volt shunted by 2pF. (c ) Common..drain temperature-stable amplifier with ISM D, SpF input imped age gain, A.=gmRs/(1 +gmRs) which has been ance. (d) Capacitor-bootstrapped amplifier with input impedanceof SM D, 3pF. reduced to 0.7 times. The 5M D input-resis tance-controlling gate bias resistor can be increased to 10 or even 15MD, but gate leak age currents then make the circuit sensitive +12V +12V to temperature. Tr 15k 2 The source-follower circuit of Fig. 5(c) 1M ut 2N718 � 2'2M with the gate-bias resistor bootstrapped gives 5,u I Tr1 high input impedance with better high Inp� I 2N2606 . I temperature stability, because the gate resis 0'01,u I 1 tor network has a relatively low d.c. resistance. I With RG at IMD as shown in the diagram, J, 22M Zin is 15MD shunted by Cin=5pF. Increasing 45k �RL � 10k RG to lOMD raises Rin to 150MD without 100,u I affecting Cin, but the circuit becomes tempera I ture sensitive. 150k 1 An alternative arrangement sometimes 2001 2'2k 1-£ I used is shown in Fig. 5(d), where the 20!E 10V oV capacitor bootstraps the IMD gate bias resis (a) (b) tor to give an overall input impedance of approximately 5MD shunted by 3pF.
+12V +5V Multistage f.e.t. high input-impedance amplifiers: For better temperature stability and lower input shunt capacitance, the input f.e.t. can be cascaded with bipolar transistor Input Input stages, using also bootstrap feedback techniques . input resistance of 150MD shunted by 1M . An 1M Output 5pF is obtained with the circuit of Fig 6(a), which utilizes a common-source f.e.t., Tr!, RC-coupled to a common-emitter transistor,
2'2k Tn, with overall feedback from transistor 8'2k 0'01,u 39k 5'6k Tn collector to f.e.t. source to provide the oV -19V necessary bootstrapping to reduce the shunt (C) (d) ing effect of the 50MD gate bias resistance across the input. With a voltage gain of 26dB, the amplifier has a bandwidth down to 10Hz for any signal source resistance. The top end 6'8k 1M Fig. 6. Multi-stageJ.e.t. high-input-impedance Inp� amplifiers. (a) 150MD, SpF input impedance I-�__ Hlh �ut with RC-coupledJ.e.t. and bipolar transistor. 220p 10,u (b) 1,200MD, 3.SpF input impedance with 100.000 d.c.-coupledJ.e.t. and transistor. (c) Low input M capacitance (OApF), high impedance (SMD) (e) d.c.-coupledJ.e.t. and transistor. (d) I,OOOMD, O.lpF input impedance four-stage m.o.s.f.e.t 1M 3'3k biological amplifier. (e) I,OOO,OOOMD input oV impedance m.o.s.f.e.t. amplifier.
200 www.keith-snook.info - Wirless World July [968 Industrial Instruments Ltd, the Bromley, Kent,manu R up for posltlve feedback from the output 2 to the non-inverting input pin (3). The facturers of Transipack inverters, have been appointed f.J.A U.K. agents for Vam-Atlas, the American company approximate expression for the overall low 2 702A whose interests lie in the military static inverter field. frequency input impedance is Ri.=AfIORA!A" 6 7 C Output where AfIO is the open loop voltage gain, The Venner Group have recently announced an agree 1 3 ment with Control Logic Inc., of Massachusetts, which AJ is the gain with feedback and RA is the. In� gives Venner manufacturing and sales rights for a R3 input impedance into pin (3). By general family of integrated circuit logic cards and accessories. C2 R4 operational amplifier theory, AJ can be This agreement applies exclusively in the U.K., the shown to be approximately (RI +R2)!RI. Commonwealth and all E.F.T.A. countries. R1 C4 For the pA702A, typically AfIO�2,000, and S. Davall & Son Ltd, of 4 Wadsworth�Road, Greenford, RA=10k,Q. Two sets of circuit component Middlesex, have appointed agents I in France and values are given below to produce input North America for the precision relays and associated Fig. 7. Connecting up an integrated circuit components made by their manufacturing division, resistances of 2 and 6MQ. monolithic operational amplifier to give high in Perivale Controls Co. Ltd. The agents are Crouzet S.A., Cl C2 C4 RI of Paris, for Common Market countries and Eastern put resistance (2-6MQ ). 1,uF 1,uF 910pF 10kQ 91kQ Electric Company, of Montreal, for Icanada and the United States. 1,uF 4.7 f.J.F 1000pF 51kQ 100kQ Standard Telephones & Cables Ltd, have been awarded of the bandpass ranges from 500Hz for a Rl R4 A. Ri. a contract by Hawker Siddeley Aviation, worth approxi lOM source up to 50kHz for a 10k source. mately £250,000, to supply aerial systems for the Q Q 82kQ 220kQ 10 2MQ The Siliconix circuit of Fig. 6(b) shows a Hawker Harrier V /STOL aircraft destined for service 51kQ 3 6MQ with the RAF. p-channel f.e.t. d.c.-coupled to a bipolar tran sistor to give Zi. =1 ,200M Q shunted by The phase-shift network, R4, C4, provides A £430,000 contract has been placed with the M.EL Equipment Company Ltd, by the Ministry of Techno 3.5pF, with an overall voltage gain of 0.98, frequency stability. For best thermal stability, logy on behalf of the Ministry of Defence (Army and an output resistance of 600 Q. Here both RI + Rl should be made equal to R2. Department), for the supply of military radio equip the gate bias resistor and the source resistor When f.e.t.-input operational amplifiers ment. The equipment comprises single sideband hJ. are bootstrapped. become commonly available, they will have an transmitters L556 and radio telegraph adaptors L607. An input impedance of 5MQ shunted only input terminal resistance, RA, of the order of Racal Communications Ltd, of Western Road, Brack by OApF is provided by the d.c.-coupled 10MQ. Looking back at the formula for boot nell, Berks., have received contracts to the value of £lM f.e.t./bipolar pair in the Ferranti circuit of strapped input resistance given for the bi for new submarine h.f. communication equipment for use by the Royal Navy. Fig. 6(c). It features overall bootstrapping of polar j.J.A702A, it will be seen that very high both the gate bias resistor and the gate-drain overall input resistances can be expected in An order worth approximately £400,000 has been capacitance to achieve the low input shunt the future with f.e.t. operational amplifiers. awarded to Standard Telephones and Cables Ltd, by the capacitance. Italian Ministry o£ Defence, for instrument landing The four-stage circuit of Fig. 6(d) given by systems to be installed at the airports of Rome, Turin, Milan, Genoa, Venice and Naples. R. E. Webb in "Field Effect Transistor for
Biological Amplifier" in Electronic Engineering, Cossor Electronics Ltd, of Harlow, Essex, have December, 1965, pp .. 803-805, produces an received an order, worth approximately £250,000, for input impedance greater than 1,000MQ . the supply and installation, at Stornoway, Isle of Lewis, with a shunt capacitance lessthanO.1pF. Using and Burrington, Devon, of two d ual�hannel and two single�hannel SSR 700 interrogatorsystems. an n-channel depletion mosfet with a gate Announcements leakage current of less than 10-11 A, it features The series of Philips colour television courses for Henry's Radio Ltd, have opened an additional elec unity voltage gain with an output resistance of service engineers, started in July 1967 by Combined tronics centre at 309 Edgware Road, London W.2. only 25 Q. The amplifierand input connections Electronic Services Ltd, have ceased for the time being. have to be doubly screened. The internal Over 1,000 people attended. Plans are now under way for a new series scheduled for the Autumn. The A licensing agreement has been completed between screen is connected to the amplifier output courses will be of 4 days' duration and take place at the the Decca Navigator C01l1'any, London, and Inter and the external screen to ground, thus re colour television school at C.E.S. Ltd, Waddon, Croy national Standard Electric Corporation, a subsidiary ducing the stray capacitance to ground by the don, Surrey. of International Telephone and Telegraph of U.SA, to same amount as the input resistance is in co-<>perate in the field of marine and commercial aviation A one-day symposium on "The Numerical Solution electronics. creased by the large overall feedback from . of Laplace's Equation" will be held at the John Dalton the top of the 5.6kQ last emitter load resis College of Technology, Chester Street, Manchester l, on Thorn Electrical Industries (London) and International tance. 8th July. Fee £4 15s. Rectifier Corporation (California) have signed a 12-year As the final f.e.t. example, the relatively At the recent annual general meeting of the Radio & agreement to continue as joint ventures the six jointly simple Mullard circuit of Fig. 6(e) gives an Electronic Component Manufacturers' Federation, owned semiconductor companies in Europe. Inter input resistance of 1,000,000MQ with a volt the following result of the postal ballot for the election national Rectifiers will exercise management control age gain of about 0.98 and an output resis of the Council was announced:- Belling & Lee (N. D. (receiving a management fee) and will provide tech Bryce), A: F. Bulgin & Co. (R. A. Bulgin), Colvern (R. nical assistance and licences relating to semiconductor tance of only 50Q. The f.e.t. 3.3kSJ source F. Collinson), A. H. Hunt (S. H. Brewell), McMurdo products. resistor is bootstrapped by the BCY72 tran Instrument Co. (F. W. Irons), Mullard (Dr. F. E. Jones, I I sistor, as is also the 10 Q gate bias resistor elected chairman), Multicore Solders (R. Arbib), Painton Texas Instruments and Sony announced that the via the 2� capacitor. & Co. (C. ¥. Benham), and Standard Telephones & Japanese Government has approved the establishment Cables (E. E. Bivand). of Texas Instruments Japan Ltd. Fifty per cent of the capital is to be furnished by each of the companies, The Institution of Electrical and Electronics Tech and semiconductor devices including integrated circuits High input impedance with nician Engineers has decided to accept the City and and certain electrical control devices are to be-manu i.c. operational amplifier Guilds of London Institute's Electrical Technicians' factured. Certificate with two endorsement certificates as satisfying " the technical education requirements for election to the Amphenol and Plessey recently signed an agreement Engineers are now beginning to use inte class of Grad uate. whereby the new Astro 348 connector range developed grated circuit monolithic operational ampli by the Amphenol Corporation of Illinois is to be manu- The Munich Fair and Exhibition Co. have announced . factured by the Plessey Company Ltd. fiers as complete circuit elements instead of that the fourth Electronica-the international trade building up circuits with discrete bipolar exhibition of electronic components and related measur transistors or f.e.ts. High input impedance ing and production equipment-will be held from 22nd GEC-ABI Telecommunications Ltd. of Coventry, to 28th October, 1970. The third Electronica exhibition have won orders worth £250,000 for completely solid can be obtained with these by the simple takes place this year from 7th to 13th November. state 2 GHz and 7 GHz radio links and multiplex expedient of putting the required input equipment in Hong Kong and Bahrain. The contracts resistance in series with the amplifier input, An agreement has been reached, subject to contract, have been placed by Cable and Wireless Ltd. which is a' virtual earth point. However, between RedifTusion Ltd and the Rank Organisation Ltd, for the purchase by Rediffusion of the w hole of bootstrapping techniques can also be used Rank's wired sound and television networks and the Marconi Company, of Chelmsford, have won a contract as shown in Fig. 7. Here a standard Fair television set rental business associated with these relay valued at £700,000 to supply a completely new tele child pA702A operational amplifier is set undertakings. vision system to the state ofBahia inBrazil.
Wireless World July 1968 - www.keith-snook.info 201