# Voltage amplifier with 2N3904

#### yoannwyffels

Joined Mar 23, 2016
16
Hello,

I've found a pretty good example:
http://www.instructables.com/id/Voltage-Amplifier/

For what I'm trying to do, I'm on a +5V DC Power supply (not a +15V), and need to have a gain x3
My input signal have a range between 0.5v and 1v

Does this schematic will be working well ?

Thank you for your help !

#### Alec_t

Joined Sep 17, 2013
14,205
My input signal have a range between 0.5v and 1v
Are you talking about a DC signal or an AC one. If AC, do you mean a) it has an amplitude within the 0.5v to 1V range or b) it has a positive peak of 1V and a negative peak of 0.5V? If AC, what frequency?
Here's the result using your circuit with a 1V amplitude 1kHz AC signal. Do you see the problem and know the cure?

#### yoannwyffels

Joined Mar 23, 2016
16
Are you talking about a DC signal or an AC one. If AC, do you mean a) it has an amplitude within the 0.5v to 1V range or b) it has a positive peak of 1V and a negative peak of 0.5V? If AC, what frequency?
Here's the result using your circuit with a 1V amplitude 1kHz AC signal. Do you see the problem and know the cure?
View attachment 104240
Hello Alec_t,

I'm talking about a DC signal which go from +0.5v to +1v. Maybe this amplifier (with 2n3904) only work with AC signal ?

#### dl324

Joined Mar 30, 2015
16,665
I'm talking about a DC signal which go from +0.5v to +1v. Maybe this amplifier (with 2n3904) only work with AC signal ?
Would C1 pass any DC component of the input signal?

#### yoannwyffels

Joined Mar 23, 2016
16
Would C1 pass any DC component of the input signal?
I think no, but even if I remove capacitor....

#### yoannwyffels

Joined Mar 23, 2016
16
Forget it, I will use any LM741 opamp to do what I want to do !

#### Alec_t

Joined Sep 17, 2013
14,205
Even if you remove the capacitor the circuit shown won't work as you want with a ground-referenced 0.5V-1V input.
a) 0.5V would switch the transistor off,
b) taking the output at the transistor emitter gives no gain at all.
c) if you take the output from the transistor collector you get gain, but there is inversion, i.e. as the input voltage rises the output voltage falls.
When the input goes from 0.5V to 1V do you want an output going from 1.5V to 3V?
I will use any LM741 opamp to do what I want to do !
Good luck with that

#### dl324

Joined Mar 30, 2015
16,665
Forget it, I will use any LM741 opamp to do what I want to do !
With a 5V supply, you'll need a rail to rail opamp.

#### Dodgydave

Joined Jun 22, 2012
11,234
Use an opamp like lm358, lm324

#### dl324

Joined Mar 30, 2015
16,665
lm741 opamp isn't a good choice ?
Read the datasheet. Input and output voltage are only guaranteed to within 3V of the supplies and minimum supply voltage is spec'ed at 10V. It will likely operate at a lower supply voltage and, if you're lucky, you'll be able to find a unit that gives better than typical voltage swings.

#### Alec_t

Joined Sep 17, 2013
14,205

#### AnalogKid

Joined Aug 1, 2013
10,943
For what I'm trying to do, I'm on a +5V DC Power supply (not a +15V), and need to have a gain x3
My input signal have a range between 0.5v and 1v
Does this schematic will be working well ?
No, for two reasons. First, this is not a voltage amplifier, it is an emitter follower. This is a circuit configuration that has current gain but no voltage gain.

Second, while R1 and R2 might be the correct ratio of values for biasing the circuit, their absolute values are way too small and will load down the input signal.

ak

#### AnalogKid

Joined Aug 1, 2013
10,943
Forget it, I will use any LM741 opamp to do what I want to do !
Well, that didn't take long.

ak

#### yoannwyffels

Joined Mar 23, 2016
16
Yes AnalogKid, was trying to recycle some components I have in stock to do what I want, but when it's impossible,... it's impossible

#### Bordodynov

Joined May 20, 2015
3,165
Maximum input signal (Vcc-Vce_sat)/2/Gain=(5V-0.4V)/2/3=0,766V