2 The TTL Inverter. (i) An input transistor, T 1, which performs a current steering function, can be thought of as a back-to-back diode arrangement.

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1 The TTL Inverter.1 Crcut Structure The crcut dagram of the Transstor Transstor Logc nverter s shown n Fg..1. Ths crcut overcomes the lmtatons of the sngle transstor nverter crcut. Some of the notable features of the crcut structure of the TTL Logc famly are as follows: () An nput transstor, T 1, whch performs a current steerng functon, can be thought of as a back-to-back dode arrangement. CC R B R B T 1 Fg.. Equvalent of Input Current-Steerng Transstor The transstor can operate n ether forward or reverse mode to steer current to or from T. Snce t has a forward current gan, t provdes a hgher dscharge current to dscharge the base of T when turnng t off. () The output transstor par, T and T shown n Fg.. and referred to as a totem-pole output, can actvely source or snk current to or from capactve loads and allows the output voltage to be defned ndependently of the load connected to the gate. Resstor, R, serves to lmt current. Under steady-state condtons, only one transstor s N at a tme. T FF R L T N T N T FF R L Fg.. utput Current Drvng Transstors 1

2 10Ω R kω R B 1.6kΩ R 1 T Input T 1 T utput T 1kΩ R H (.86) A B C T 1 T FF T FF T N T 1 T N T FF T N 1 T 1 T N T N T N FF T 1 RE N T T T FF L (0.) D E 5 IL (1.) IH (1.) Fg..1 Crcut Dagram & Transfer Characterstc of a TTL Inverter

3 () The dode, D, serves to ncrease the effectve BE of T whch allows T to be turned FF before T turns N fully. Ths prevents large surge currents from flowng when both transstors conduct durng transtons between logc states. The dsadvantage s that the hgh logc voltage s reduced by an amount of the dode drop as shown n Fg... T D T Fg.. Use of Dode n Totem-Pole utput () Fnally, T s a phase spltter drvng transstor to drve the output stage. It allows the logc condton to be phase-spltted n opposte drectons so that the output transstors can be drven n ant-phase. Ths allows T to be N when T s FF and vce versa as shown n Fg..5. R C L T FF 1 HI L T 1 HI T N 1 L HI R E Fgure.5 The Phase Splttng Stage

4 . Logcal peraton The logcal functonng of the crcut can be establshed by determnng the state of conducton of each transstor n turn from nput to output for all possble combnatons of nput states. Transstors can be taken as ether N or FF. Note that the nput transstor, T 1, may conduct n ether forward or reverse mode. Drawng up a table of conducton states accordngly wth reference to Fg..1 gves: INPUT T 1 T T T D UPUT L N FR FF FF N CUT-IN N CUT-IN HI HI N RE N N FF FF L L n - HI out and HI n - L out acton Ths s logc nverter. Transfer Characterstc The transfer characterstc can be deduced by applyng a slowly ncreasng nput voltage and determnng the sequence of events whch takes place wth regard to changes n the states of conducton of each transstor and the crtcal ponts at whch the onset of these changes occur. Consder the crcut and transfer characterstc of Fg..1. Pont A Wth the nput L and the base current suppled to T 1, ths transstor can conduct n the forward mode. Snce the only source of collector current s the leakage of T then T 1 s drven nto saturaton. Ths ensures that T s FF whch, n turn, means that T s FF. Whle there s no load present, there are leakage currents flowng n the output stage whch allow the transstor T and the dode D to be barely conductng at the pont of cut-n. CUT IN CUT CC BE D IN Pont A : 0,

5 Pont B As the nput voltage s slowly ncreased, the above condton prevals untl, wth T 1 N n saturaton, the voltage at the base of T rses to reach the pont of conducton. Then: CUT BE IN CE Pont B : 0.5 Pont C As the nput voltage s further ncreased, T becomes more conductng, turnng fully N. Base current to T s suppled by the forward based base-collector juncton of T 1 whch s stll n saturaton (.e both junctons of T 1 are forward based). Eventually, T reaches the pont of conducton. Ths happens when: + BE N BE CUT IN CE Note that wth transstor T at cut-n, BE 0.6 whch means that the current through R s 0.6/1kΩ 0.6mA. Wth operaton n the lnear actve regon, the collector current n T s α F I E mA. The voltage drop across R 1 s then R1 0.58mA 1.6 kω 0.9. Under ths condton the voltage drop across T s: CE CC R1 R CE Ths confrms that T s stll operatng n the forward actve mode. Wth T begnnng to conduct there s a conducton path for current through T and the dode, D, whch then turns fully N. In ths case: CC R1 BE N D N Pont C :

6 Pont D As the nput voltage s further ncreased, T conducts more heavly, eventually saturatng. T also conducts more heavly and eventually reaches the pont of saturaton. As T becomes more conductng, ts collector current ncreases. Ths n turn ncreases the voltage drop across R 1 whch n turn means that the voltage across T,.e. CE, decreases. Ths falls below the requrement for conducton n T and the dode, D, so that both of these turn FF pror to the saturaton of T. When T reaches the edge of saturaton: BE + BE N CE CE 0. Pont D : 1., 0.. Nose Margns Usng ponts C and D on the transfer characterstc n Fg..1 to dentfy the crtcal ponts, we have: L 1. L 0. L 1.0 H 1. H.8 H 1. The manufacturer s specfcaton guarantees: L 0.8 L 0. L 0. H.0 H. H 0. 6