409:
877:
853:
389:
397:
989:
884:
500:
860:
515:
20:
105:
circuit is influenced by output impedance of the first circuit (as it is larger than the input impedance of the second circuit). In the ideal voltage buffer (Figure 1 top), the input impedance is infinite and the output impedance is zero. Other properties of the ideal buffer are: perfect linearity, regardless of signal amplitudes; and instant output response, regardless of the speed of the input signal.
818:
Some configurations of single-transistor amplifier can be used as a buffer to isolate the driver from the load. For most digital applications, an NMOS voltage follower (common drain) is the preferred configuration. These amplifiers have high input impedance, which means that the digital system will
1140:
A current buffer takes the input current which may have a relatively small Norton resistance and replicates the current at the output port, which has a high output resistance ... Input resistance is low ... Output resistance is high ... transform a current source with medium source resistance to an
451:
is expected. In this configuration, the entire output voltage (β = 1 in Fig. 2) is fed back into the inverting input. The difference between the non-inverting input voltage and the inverting input voltage is amplified by the op-amp. This connection forces the op-amp to adjust its output voltage to
1058:
Because the transistor output resistance connects input and output sides of the circuit, there is a (very small) backward voltage feedback from the output to the input so this circuit is not unilateral. In addition, for the same reason, the input resistance depends (slightly) upon the output load
951:
utilizes a voltage buffer to protect a very high impedance signal line by surrounding the line with a shield driven by a buffer to the same voltage as the line, the close voltage matching of the buffer prevents the shield from leaking significant current into the high impedance line while the low
248:
from a first circuit into an identical current with high impedance for a second circuit. The interposed buffer amplifier prevents the second circuit from loading the first circuit's current unacceptably and interfering with its desired operation. In the ideal current buffer (Figure 1 bottom), the
104:
from a first circuit into an identical voltage with low impedance for a second circuit. The interposed buffer amplifier prevents the second circuit from loading the first circuit unacceptably and interfering with its desired operation, since without the voltage buffer, the voltage of the second
249:
output impedance is infinite (an ideal current source) and the input impedance is zero (a short circuit). Again, other properties of the ideal buffer are: perfect linearity, regardless of signal amplitudes; and instant output response, regardless of the speed of the input signal.
130:
or tracks the input voltage. Although the voltage gain of a voltage buffer amplifier may be (approximately) unity, it usually provides considerable current gain and thus power gain. However, it is commonplace to say that it has a gain of 1 (or the equivalent
938:
The majority of amplifiers used to drive large speaker arrays, such as those used for rock concerts, are amplifiers with 26-36dB voltage gain capable of high amounts of current into low impedance speaker arrays where the speakers are wired in parallel.
718:
471:), meaning that the input of the op-amp does not load down the source and draws only minimal current from it. Because the output impedance of the op-amp is very low, it drives the load as if it were a perfect
463:
The impedance of this circuit does not come from any change in voltage, but from the input and output impedances of the op-amp. The input impedance of the op-amp is very high (1
589:
364:
to the load, again without current division because the output resistance of the buffer is infinite. A Norton equivalent circuit of the combined original Norton source
225:
to the load, again without voltage division because the output resistance of the buffer is zero. A Thévenin equivalent circuit of the combined original Thévenin source
569:), or other active devices. All such amplifiers actually have a gain of slightly less than unity (though the loss may be small and unimportant) and add a
906:
A non-linear buffer amplifier is sometimes used in digital circuits where a high current is required, perhaps for driving more gates than the normal
338:. However, if the Norton source drives a unity gain buffer such as that in Figure 1 (bottom, with unity gain), the current input to the amplifier is
199:. However, if the Thévenin source drives a unity gain buffer such as that in Figure 1 (top, with unity gain), the voltage input to the amplifier is
810:
As frequency is increased, the parasitic capacitances of the transistors come into play and the transformed input impedance drops with frequency.
573:. Only one transistor is shown as the active device in these schematics (however, the current source these circuits may require transistors too).
774:
without the buffer by a factor of (β + 1), which is substantial because β is large. The impedance is increased even more by the added
432:
1097:
980:
because the output current follows the input current). The current gain of a current buffer amplifier is (approximately) unity.
1059:
resistance, and the output resistance depends significantly on the input driver resistance. For more detail see the article on
439:
simply by connecting its output to its inverting input, and connecting the signal source to the non-inverting input (Fig. 3).
753:= β, which follows from the evaluation of these parameters in terms of the bias currents.) Assuming the usual case where
910:
of the logic family used, or for driving displays, or long wires, or other difficult loads. It is common for a single
210:
because the amplifier input resistance is infinite. At the output the dependent voltage source delivers voltage
1159:
349:
because the amplifier input resistance is zero. At the output the dependent current source delivers current
488:
1102:
961:
530:
800:
Using the small-signal circuit in Figure 5, the impedance seen looking into the circuit is no longer
713:{\displaystyle R_{\rm {in}}={\frac {v_{x}}{i_{x}}}=r_{\pi }+(\beta +1)({r_{\rm {O}}}||{R_{\rm {L}}})}
80:). This "buffers" the signal source in the first circuit against being affected by currents from the
421:
139:
546:
930:
effectively correspond with high-current capability single-input NOR or OR gates respectively.
1055:. This coupling capacitor is large enough to be a short circuit at frequencies of interest.
911:
581:
Using the small-signal circuit in Figure 4, the impedance seen looking into the circuit is
58:
8:
1031:
1016:
for DC emitter current) and driving another DC current source as active load (designated
952:
impedance of the shield can absorb any stray currents that could affect the signal line.
278:
561:
because the output voltage follows the input voltage); or similar configurations using
519:
504:
476:
253:
109:
54:
244:
Typically a current buffer amplifier is used to transform a current signal with a low
408:
1087:
566:
534:
303:
245:
164:
101:
65:
807:
but instead is infinite (at low frequencies) because the MOSFET draws no current.
1009:
Figure 6 shows a bipolar current buffer biased with a current source (designated
876:
554:
523:
508:
81:
901:
472:
576:
1153:
1107:
852:
795:
388:
1132:
988:
1092:
1072:
948:
870:
550:
518:
Figure 5: Top: MOSFET voltage follower Bottom: Small-signal, low-frequency
499:
417:
396:
252:
For a current buffer, if the current is transferred unchanged (the current
100:
A voltage buffer amplifier is used to transform a voltage signal with high
514:
306:(also referred to as "loading") the current delivered to the load is only
1112:
1082:
1077:
1060:
973:
965:
846:
842:
562:
35:
28:
503:
Figure 4: Top: BJT voltage follower Bottom: Small-signal, low-frequency
883:
480:
428:
413:
43:
570:
484:
46:
416:; also can be seen as the "ideal transistor" without a base-emitter
167:(also referred to as "loading") the voltage across the load is only
431:
gain buffer amplifier may be constructed by applying a full series
1133:"Lecture 20 - Transistor Amplifiers (II) - Other Amplifier Stages"
1048:
is delivered by the buffer via a large coupling capacitor to load
859:
907:
767:, the impedance looking into the buffer is larger than the load
557:
because the source voltage follows the gate voltage or, again, a
468:
132:
23:
Figure 1: Ideal voltage buffer (top) and current buffer (bottom)
19:
969:
436:
401:
50:
922:
is a single package containing 8 buffer amplifiers. The terms
914:
to contain several discrete buffer amplifiers. For example, a
475:. Both the connections to and from the buffer are therefore
992:
Figure 6: Bipolar current follower biased by current source
889:
Voltage gain is close to unity, used for voltage buffering.
918:
is a single package containing 6 buffer amplifiers, and an
577:
Impedance transformation using the bipolar voltage follower
545:
because the output voltage follows the input voltage); the
541:
because the emitter voltage follows the base voltage, or a
479:
connections, which reduce power consumption in the source,
1030:
is applied to the emitter node of the transistor by an AC
796:
Impedance transformation using the MOSFET voltage follower
464:
813:
1023:
for DC collector current). The AC input signal current
592:
108:
If the voltage is transferred unchanged (the voltage
712:
460:so the circuit is named op-amp voltage follower).
420:on the input signal. This is the basic circuit of
84:of the second circuit and may simply be called a
1151:
960:Simple unity gain buffer amplifiers include the
792:, so the addition does not make much difference
983:
529:Other unity gain buffer amplifiers include the
834:BJT (npn)
494:
933:
955:
895:
882:
875:
858:
851:
378:
1141:equal current with high source resistance
16:Electronic amplifier, a circuit component
1113:Voltage controlled voltage source filter
987:
513:
498:
407:
395:
387:
383:
18:
1098:Current differencing buffered amplifier
392:Figure 2: A negative feedback amplifier
61:to provide a more ideal source (with a
1152:
368:the buffer is an ideal current source
229:the buffer is an ideal voltage source
865:Typically used for current buffering
814:Chart of single-transistor amplifiers
819:not need to supply a large current.
447:of one (i.e. 0 dB), but significant
13:
700:
676:
602:
599:
404:–based unity gain buffer amplifier
135:), referring to the voltage gain.
57:to another while transforming its
14:
1171:
452:simply equal the input voltage (V
375:with infinite Norton resistance.
239:
95:
942:
725:(The analysis uses the relation
412:A voltage follower boosted by a
262:is 1), the amplifier is again a
236:with zero Thévenin resistance.
1125:
707:
689:
684:
666:
663:
651:
1:
1118:
274:or tracks the input current.
489:electromagnetic interference
7:
1103:Negative feedback amplifier
1066:
962:bipolar junction transistor
821:
531:bipolar junction transistor
270:because the output current
126:because the output voltage
10:
1176:
984:Simple transistor circuits
899:
495:Simple transistor circuits
295:) driving a resistor load
277:As an example, consider a
156:) driving a resistor load
138:As an example, consider a
118:is 1), the amplifier is a
26:
537:configuration (called an
422:linear voltage regulators
418:forward bias voltage drop
1041:. The AC output current
976:configuration (called a
934:Speaker array amplifiers
553:configuration (called a
27:Not to be confused with
1034:with Norton resistance
956:Current buffer examples
896:Logic buffer amplifiers
547:field effect transistor
379:Voltage buffer examples
266:; this time known as a
92:when context is clear.
76:output impedance for a
1006:
968:configuration, or the
871:Common drain/collector
714:
526:
511:
424:
405:
393:
288:, parallel resistance
24:
1160:Electronic amplifiers
1061:common base amplifier
1032:Norton current source
999:and with active load
991:
715:
517:
502:
411:
399:
391:
384:Op-amp implementation
22:
928:non-inverting buffer
590:
149:, series resistance
59:electrical impedance
788:<< (β + 1) R
347:no current division
208:no voltage division
1007:
710:
527:
520:equivalent circuit
512:
505:equivalent circuit
483:from overloading,
425:
406:
394:
122:; also known as a
25:
893:
892:
830:
633:
433:negative feedback
264:unity gain buffer
120:unity gain buffer
1167:
1144:
1143:
1137:
1129:
1088:Common collector
978:current follower
924:inverting buffer
886:
879:
862:
855:
828:
822:
719:
717:
716:
711:
706:
705:
704:
703:
692:
687:
682:
681:
680:
679:
647:
646:
634:
632:
631:
622:
621:
612:
607:
606:
605:
567:cathode follower
559:voltage follower
543:voltage follower
539:emitter follower
535:common-collector
336:
334:
333:
323:
320:
304:current division
268:current follower
246:output impedance
197:
195:
194:
184:
181:
165:voltage division
124:voltage follower
102:output impedance
66:output impedance
40:buffer amplifier
1175:
1174:
1170:
1169:
1168:
1166:
1165:
1164:
1150:
1149:
1148:
1147:
1135:
1131:
1130:
1126:
1121:
1069:
1053:
1046:
1039:
1028:
1021:
1014:
1004:
997:
986:
958:
945:
936:
904:
898:
825:Amplifier type
816:
805:
798:
791:
786:
779:
772:
765:
758:
750:
746:
742:
738:
734:
730:
699:
698:
694:
693:
688:
683:
675:
674:
670:
669:
642:
638:
627:
623:
617:
613:
611:
598:
597:
593:
591:
588:
587:
579:
555:source follower
524:hybrid-pi model
509:hybrid-pi model
497:
459:
455:
443:here implies a
435:(Fig. 2) to an
386:
381:
373:
362:
358:
354:
343:
332:
328:
324:
321:
319:
315:
311:
310:
308:
300:
293:
286:
260:
242:
234:
223:
219:
215:
204:
193:
189:
185:
182:
180:
176:
172:
171:
169:
161:
154:
147:
140:Thévenin source
116:
98:
82:electrical load
32:
17:
12:
11:
5:
1173:
1163:
1162:
1146:
1145:
1123:
1122:
1120:
1117:
1116:
1115:
1110:
1105:
1100:
1095:
1090:
1085:
1080:
1075:
1068:
1065:
1051:
1044:
1037:
1026:
1019:
1012:
1002:
995:
985:
982:
957:
954:
944:
941:
935:
932:
902:Digital buffer
900:Main article:
897:
894:
891:
890:
887:
880:
873:
867:
866:
863:
856:
849:
839:
838:
835:
832:
826:
815:
812:
803:
797:
794:
789:
784:
777:
770:
763:
756:
748:
744:
740:
736:
732:
728:
723:
722:
721:
720:
709:
702:
697:
691:
686:
678:
673:
668:
665:
662:
659:
656:
653:
650:
645:
641:
637:
630:
626:
620:
616:
610:
604:
601:
596:
578:
575:
496:
493:
473:voltage source
457:
453:
385:
382:
380:
377:
371:
360:
356:
352:
341:
330:
326:
317:
313:
298:
291:
284:
258:
241:
240:Current buffer
238:
232:
221:
217:
213:
202:
191:
187:
178:
174:
159:
152:
145:
114:
97:
96:Voltage buffer
94:
78:current buffer
70:voltage buffer
49:that copies a
15:
9:
6:
4:
3:
2:
1172:
1161:
1158:
1157:
1155:
1142:
1134:
1128:
1124:
1114:
1111:
1109:
1108:Driven shield
1106:
1104:
1101:
1099:
1096:
1094:
1091:
1089:
1086:
1084:
1081:
1079:
1076:
1074:
1071:
1070:
1064:
1062:
1056:
1054:
1047:
1040:
1033:
1029:
1022:
1015:
1005:
998:
990:
981:
979:
975:
971:
967:
963:
953:
950:
943:Driven guards
940:
931:
929:
925:
921:
917:
913:
909:
903:
888:
885:
881:
878:
874:
872:
869:
868:
864:
861:
857:
854:
850:
848:
844:
841:
840:
836:
833:
827:
824:
823:
820:
811:
808:
806:
793:
787:
780:
773:
766:
759:
752:
695:
671:
660:
657:
654:
648:
643:
639:
635:
628:
624:
618:
614:
608:
594:
586:
585:
584:
583:
582:
574:
572:
568:
564:
560:
556:
552:
548:
544:
540:
536:
532:
525:
521:
516:
510:
506:
501:
492:
490:
486:
482:
478:
474:
470:
466:
461:
450:
446:
442:
438:
434:
430:
423:
419:
415:
410:
403:
400:Figure 3. An
398:
390:
376:
374:
367:
363:
348:
344:
337:
305:
302:. Because of
301:
294:
287:
280:
279:Norton source
275:
273:
269:
265:
261:
255:
250:
247:
237:
235:
228:
224:
209:
205:
198:
166:
163:. Because of
162:
155:
148:
141:
136:
134:
129:
125:
121:
117:
111:
106:
103:
93:
91:
87:
83:
79:
75:
71:
67:
64:
60:
56:
52:
48:
45:
41:
37:
30:
21:
1139:
1127:
1093:Common drain
1073:Preamplifier
1057:
1049:
1042:
1035:
1024:
1017:
1010:
1008:
1000:
993:
977:
959:
949:driven guard
946:
937:
927:
923:
920:octal buffer
919:
915:
905:
817:
809:
801:
799:
782:
781:, but often
775:
768:
761:
754:
726:
724:
580:
563:vacuum tubes
558:
551:common-drain
542:
538:
528:
462:
449:current gain
448:
445:voltage gain
444:
440:
426:
369:
365:
350:
346:
339:
307:
296:
289:
282:
276:
271:
267:
263:
256:
251:
243:
230:
226:
211:
207:
200:
168:
157:
150:
143:
137:
127:
123:
119:
112:
107:
99:
89:
85:
77:
73:
69:
62:
39:
33:
1083:Common gate
1078:Common base
974:common-gate
966:common-base
843:Common gate
206:, and with
36:electronics
29:Data buffer
1119:References
916:hex buffer
487:and other
481:distortion
441:Unity gain
414:transistor
44:unity gain
760:>>
655:β
644:π
571:DC offset
485:crosstalk
456:follows V
281:(current
142:(voltage
53:from one
47:amplifier
1154:Category
1067:See also
477:bridging
90:follower
912:package
908:fan-out
831:(NMOS)
345:, with
335:
309:
272:follows
196:
170:
131:0
128:follows
55:circuit
970:MOSFET
837:Notes
829:MOSFET
522:using
507:using
467:to 10
437:op-amp
402:op-amp
86:buffer
74:higher
68:for a
51:signal
1136:(PDF)
429:unity
72:or a
63:lower
42:is a
926:and
847:base
743:) (V
735:= (I
254:gain
110:gain
38:, a
1045:out
972:in
964:in
549:in
533:in
454:out
366:and
359:= I
329:+ R
227:and
220:= V
190:+ R
88:or
34:In
1156::
1138:.
1063:.
1027:in
947:A
747:/I
739:/V
491:.
469:TΩ
465:MΩ
458:in
427:A
133:dB
1052:L
1050:R
1043:i
1038:S
1036:R
1025:i
1020:C
1018:I
1013:E
1011:I
1003:C
1001:I
996:E
994:I
845:/
804:L
802:R
790:L
785:π
783:r
778:π
776:r
771:L
769:R
764:L
762:R
757:O
755:r
751:)
749:B
745:T
741:T
737:C
733:π
731:r
729:m
727:g
708:)
701:L
696:R
690:|
685:|
677:O
672:r
667:(
664:)
661:1
658:+
652:(
649:+
640:r
636:=
629:x
625:i
619:x
615:v
609:=
603:n
600:i
595:R
565:(
372:A
370:I
361:A
357:A
355:I
353:i
351:β
342:A
340:I
331:A
327:L
325:R
322:/
318:A
316:R
314:A
312:I
299:L
297:R
292:A
290:R
285:A
283:I
259:i
257:β
233:A
231:V
222:A
218:A
216:V
214:v
212:A
203:A
201:V
192:A
188:L
186:R
183:/
179:L
177:R
175:A
173:V
160:L
158:R
153:A
151:R
146:A
144:V
115:v
113:A
31:.
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