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Can I use voltage follower and emitter follower for same purpose

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Can I use voltage follower and emitter follower for same purpose.

Yes and no depending on your requirements.

A voltage follower uses an op-amp to set the voltage on the output to be the same as the voltage on the input. It does however have limited drive capability.

An emitter follower on the other hand drives or sources current to the output so that said output is a base emitter voltage different from the base voltage. However, emitter followers only drive in one direction. An NPN emitter follower can only source current, a PNP can only sink current. As such if the output voltage is too high (or low for the PNP) for some reason, the follower is not able to pull it back.

Emitter followers can however pass much more current than an op-amp.

So "Can you use voltage follower and emitter follower for same purpose?" If designed correctly to deal with the voltage differences one can be substituted for the other if the load requirements overlap the capabilities of both methods.

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    \$\begingroup\$ Or you could use both together if you want the advantages of both :) \$\endgroup\$ – Peter Smith Jan 7 '18 at 14:58
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Voltage Follower, using an OpAmp, may have a rather precise copying of Vin onto Vout, with ability to handle varying loads while still performing that precise copying.

The emitter follower will have the standard Vbe (maybe 0.6v at 1ma, or at 0.1mA) with that 0.6 volt offset between Vin and Vout.

Additionally the 0.6volt will vary with temperature, with load, etc.

But the emitter follower can be very fast (and easy to have oscillate).

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  • \$\begingroup\$ And an emitter follower can usually deliver more current than an op-amp voltage follower. \$\endgroup\$ – Peter Bennett Jan 7 '18 at 7:34
  • \$\begingroup\$ There are two important pros of the opamp based follower: (1) the required unity gain is much more precise if compared with an emitter follower and (2) the output resistance is much more smaller. \$\endgroup\$ – LvW Jan 7 '18 at 9:38
  • \$\begingroup\$ Zout strongly depends on frequency, for an OPAMP. Near unity gain bandwidth, the Zout becomes the inherent Rout of the two output devices; two bipolars running at 1mA, thus 26 ohms reach or 13 ohms combined, are what you will see; thus a UA741 near 1MHz has Rout (Zout) of 13 ohms. \$\endgroup\$ – analogsystemsrf Jan 8 '18 at 3:25

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