# NPN BJT Emitter Follower - supply at Collector Current (Ic).?

#### mrazldhamir

Joined Aug 4, 2016
2
Greetings everyone,

I am planning to design a current amplifier using an NPN BJT Transistor with an Emitter Follower configuration. On top of that, instead of using batteries, I am using 2 separate LDO voltage outputs as the input voltage(Vin/Vbb) to the BJT as well as the supply voltage (Vcc/Vsupply) to the BJT respectively.

LDO Vin = 3.3V ; 60mA
LDO Vsupply = 5V ; (=<200mA)
Amplified output needed = 2.6V ; 6A

So, my question is, in order to get that output current (6A) which is the Emitter Current (Ie), do I need to supply to collector current (Ic) with ~6A current supply of 5V LDO? Or can I supply to the collector current (Ic) with about the same as 60mA (Ib) because the function of the BJT(of this application configuration) is to apply gain (Beta) to the input currents themselves, (Ib & Ic) ?

Please do correct me if I understand the working principles wrongly.

And, thank you all in advance!

#### OBW0549

Joined Mar 2, 2015
3,566
So, my question is, in order to get that output current (6A) which is the Emitter Current (Ie), do I need to supply to collector current (Ic) with ~6A current supply of 5V LDO?
Of course you do. Transistors cannot create current all by themselves.

#### Alec_t

Joined Sep 17, 2013
13,155
Emitter current = base current plus collector current.

#### Delta Prime

Joined Nov 15, 2019
1,311
The Old Common collector Transistor circuit. That Emitter voltage always behind that Base voltage
.6 volts .7 volts come on
Geezzee kinda given an impression that the emitter is following the base?

#### dl324

Joined Mar 30, 2015
15,119
Welcome to AAC!
LDO Vin = 3.3V ; 60mA
LDO Vsupply = 5V ; (=<200mA)
Amplified output needed = 2.6V ; 6A
If I understand what you're saying, you can't get there. Where is the 6A supposed to come from?

If the 5V supply could supply more than 6A, you still wouldn't have a good 2.6V source because the base-emitter voltage will change with load current.

We prefer schematics for circuits over descriptions of them.

#### Papabravo

Joined Feb 24, 2006
19,319
Maybe we should start calling amplifiers copiers. What they do is use a power supply and the properties of semiconductors and vacuum tubes to make a big copy of a small signal. Now there is no suggestion that you can get something from nothing.

Common Emitter Copier -- has a nice ring. Doncha' think?

#### crutschow

Joined Mar 14, 2008
30,814
A basic rule is that the sum of the currents into a node must equal the currents out.

#### mrazldhamir

Joined Aug 4, 2016
2
Thank you guys for giving honest feedback. I really needed those.

I am sorry that I didn't attach the schematic design as I was just trying to judge the feasibility of the emitter follower as a current amplifier in my design application.

You guys have also confirmed my doubts about the current supply's requirement as I couldn't find any confirmation on my doubts on other forums nor other platforms. So, I thank you guys again.

Of course you do. Transistors cannot create current all by themselves.
Thank you, sir.

Emitter current = base current plus collector current.
Thanks, sir! Ie=Ib+Ic ; Ie=Ib+BIb

The Old Common collector Transistor circuit. That Emitter voltage always behind that Base voltage
.6 volts .7 volts come on
Geezzee kinda given an impression that the emitter is following the base?
You are totally right. That is why I wanted the output voltage at emitter to be 2.6V, with the Vin=3.3V, and minus Vbe of 0.7V (3.3V-0.7V=2.6V). But again, thank you for mentioning that. I really appreciate that.

Welcome to AAC!
If I understand what you're saying, you can't get there. Where is the 6A supposed to come from?

If the 5V supply could supply more than 6A, you still wouldn't have a good 2.6V source because the base-emitter voltage will change with load current.

We prefer schematics for circuits over descriptions of them.
Thanks for the welcome!
You do understand that correctly and I have the same doubt as you have, about not able to get there.

Maybe we should start calling amplifiers copiers. What they do is use a power supply and the properties of semiconductors and vacuum tubes to make a big copy of a small signal. Now there is no suggestion that you can get something from nothing.

Common Emitter Copier -- has a nice ring. Doncha' think?
OMG! That's exactly what I thought when I was trying to understand BJT amplifiers! They just copy from an external power supply and make a big copy of the small signals! We can't just amplify a power source without "injecting" an additional power Watts, power electronics just don't work that way. If we want additional power added efficiency, then we need an external power rather than from the thin air. haha.

And yes, I do think it has a nice ring, Common Emitter Copier a.k.a. CEC

#### Papabravo

Joined Feb 24, 2006
19,319
There is such a thing as a power amplifier, but the output power comes from the output power supply, not from the input power. Again it is just a copy of the input signal, but at a higher power level.